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AIM3A
Analog Input Module

Introduction tionalresistorlocationsprovidedon themodule.Theinput
signals are connected to removable screw terminal ships.
The AIM3A analog input module accepts input signals of Alternately, a user-installed mass termination connector is
flOOmV full scale through klOV full scale, and outputs a available for single-ended channels0-7 and 16-32, or differ-
signal of flOV full scale to the A/D converter module in ential channels O-7. The mass termination connector uses
slot1 ofthedataacquisitionsystem.TheAIM3Acanaccept the pin out format of the Analog Devices 3B mainframe
32 single-ended (common ground) inputs, or 16 differen- ribbon cable connector to permit direct connection of the
tial (floating) inputs, or a mixture of both. All AEvl3A AlM3A to a 3B signal conditioning subsystem.
functions,includinggainofxl,x10,andx100,areprogram-
mable, permitting single-ended and differential inputs to
be intermixed with a different gain setting for each chan- Hardware Compatibility
nel. Full 16-bit precision results from the AIM3A's low-
noise instrumentation amplifier, linearity of 0.005%, and The AIM3A can be used in slots 2 through 10 of the 500A
attention to design, signal routing, and shielding. When and 5OOP. Up to 9 AJM3A modules can be used in these
used with an AMMlA (AMMZ) A/D module, the AIM3A [email protected](32SE
will support 5OkHz (62.51d-12) measurements at any gain channels per AIM3A plus 16 SE channels on the AMMx in
and channel mix. slot 1). For best performance, analog modules should be
mounted in the lowest-numbered available slots. The
Alh43A can also be used in the option slot of the Model 575
The AIM3A can be configured to accept current inputs by or576forupto48analoginputchannels(32SEchannelson
installing appropriate current sensing resistors in the op. the AlM3A plus 16 SE channels on the AMMx).




Document Number: 501-918-01 Rev. C / 3-92
Copyright 0 1989 Keithley Instruments, Inc., Cleveland, OH 44139 (216)248-0400 AIM3A-1
SH 0 0 SH
0 I 0 I 0 o-
1 0 0 l-
2 0 o 2-
3 I 0 I 0 3-
qo 4-
0 5-
0 6-


LJ 0
0
7-
NC
AlM3A
Analog Input Module




Software Compatibility terminal block, first strip 3/16 of insulation from the end of
the wire which you want to attach. Loosen the desired
Full control of the AIM3A requires software specifically terminal screw on the block and insert the bare end of the
designed for this module. Keithley's KDAC500 software wire into the corresponding hole. Tighten the screw se-
fully supports the AlM3A. If you are using third-party curely to compress the tab against the wire.
software, be certain that the software fully supports the
AIM3A.
After you have attached all the desired signal wires to a
terminal block, replace the terminal block by lining it up
AIM3A Specifications with the mating pins on the AIM3A and pressing it back
into place.
(Specifications apply for 18" to 28"C, 1 year)

Input channels: 16 differential or 32 single-ended, software Alternate connections are available through the optional
selectable cable mass termination connector Jll. Note that Jll con-
nects only to channels 0 through 7 and 16 through 23 in
Input characteristics:
single-ended mode, or 0 through 7 in differential mode.
Input Protection: lt3OV max (powered), i-15V max
This connector is available from Scotch/3M as Part Num-
(unpowered)
ber 3429, and can be easily field-installed if it is needed.
Input Resistance: IOOMR
Input Bias Current: 5OnA

Software selectable gains: xl X10 xl00 Installation
Input range (+V): 10 1 0.1
Gain accuracy (% rdg): 0.05 0.01 0.01 All features and operating modes of the AlM3A module
Gain Nonlinearity (%): 0.005 0.005 0.005 are programmable with the exception of jumpers Wl and
Gain Tempco (%/"C): 0.002 0.002 0.002 W2 and resistor sockets J5 JlO. Jumpers Wl and W2
Input offset @IV): 250 25 25 connect input low to module ground, and are normally left
Offset Tempco QV/"C): 50 6 2 in place. They may be removed for measurements where
Input noise remote ground sensing is needed. A jumper may be plain
@V RMS, 1OOKHz BW): 24 2.7 1.3 wire, or may resemble a resistor.
Settling time to .Ol%: 15ps 15ps 15la
CMRR (DC to 60Hz): 70db 90db 100db
Sockets J5 JlO are used for current sensing resistors, or for
Power up conditions: gain = Xl, input channels = differ- ground reference resistors for some types of differential
ential, channel selected = channel 0. measurements. These are special situations which are
covered under the "Applications" topic later in this man-
ual.
Standard Connections
CAUTION
The input connections to the AIM3A are made through
Turn off power to the data acquisition system
quick-disconnect terminal blocks Jl, J2, J3, and J4,or through
before you insert or remove any module. To
mass termination Jll.
minimize the possibility of EM1 radiation,
always operate the data acquisition system
with the cover in place and properly secured.
A quick-disconnect terminal block can be removed from
the AlM3A to facilitate making connections. Pull the block
straight off the board with a firm, even pressure. Do not pry
CAUTION
the terminals with a screwdriver or sharp object, or you
Make sure you have discharged any static
may damage the circuit board.
charges on your body before handling the
AIM3A. You can do this most easily by simply
touching the chassis of a computer or data
Note that each individual terminal on a terminal block
acquisition mainframe which is plugged into
consists of a small metal block with a hole and metal
compression tab within the hole. To make connections to a

AIM3A-3
AIM3A
Analog Input Module




a grounded, 3-wire outlet. Avoid touching The AIM3A has a single, differential instrumentation am-
components or the card edge connector of the plifier that amplifies all input signals, whether single-
module. ended or differential. A differential amplifier responds
only to the difference in voltage between two signals
connected to its non-inverting (+) and inverting (-) inputs.
NOTE The common mode voltage is the average of the two input
Analog input modules should be placed in the voltages. The actual voltage at each input is not important
lowest-numbered availableslots. Thisposition- as long as the common voltage is within the common mode
ing will provide the shortest distance to the range of the amplifier. The common mode range of thi
Analog Master Measurement module in slot 1, AIM3Awillnotbeexceededaslongasallinputsarewithin
and will also minimize any possibility of noise the range -lOV to +lOV referenced to module ground.
pick-up from the power supply or other mod-
l&S.
For example, if the input to the (+) input is 9 Volts and the
input to the (-) input is 8 Volts, the amplifier will amplify
For a compatible multi-slot data acquisition system (e.g. the +1 Volt difference signal and reject (ignore) the 8.5 volt
Model 5OOA or 5OOP), remove the top cover of the system common mode signal at both inputs. The ability to reject
by loosening the cover retaining screws located in the the8.5Volt commonsignal isreferred toasCommonMode
upper corners of the rear panel. Slide the cover back about Rejection.
one inch and then lift it off. Insert the AIM3A module in the
desired slot with the component side facing the system
power supply. Replace the system cover. When the AIM3A amplifies a differential input signal, the
(+) input of the amplifier is connected to the selected (+)
channel on terminal block Jl or J3, and the (-) input of the
In a single-slot system such as the Model 575 or 576, first amplifier is connected to the corresponding (-) channel
attach the supplied right-angle bracket to the AIM3A (see terminal on J2 or J4. For example, if channel 2 and differen-
Figure 2). Install the AIM3A in the option slot, with the tial mode are selected, the (+) input of the amplifier is
component side of the board facing upward. Secure the connected to pin 7 of Jl, and the (-1 input of the amplifier is
bracket to the rear panel of the system. Close and secure the connected to pin 4 of JZ. As long as the voltage at either pin
cover. is within +lO to -10 volts as measured to the module
ground (pin 10 of Jl and J3, or pin 1 of J2 and J4), the
amplifier will amplify the difference between the inputs.



End View Top View When the AIM3A amplifies a single-ended input signal,
the (+) input is connected to the selected channel on termi-
I nalblockJ1, J2, J3,or J4,and the (-1 inputisconnected toone
of common input low connections available on Jl, JZ, J3, or
-e screws 14.
screw
Bracket
Threaded Hole
The jumpers Wl and W2 provide a user-removable con-
nection between the input low connections and module
ground,andshouldnormallybeleftlnposition.TheAIM3A
`@we 2. Model 575 Mounting Bracket amplifier will now amplify the voltage difference between
the selected (+) input terminal and module ground.



Applications As an example, if channel 30 and single-ended mode are
selected, the (+) input of the amplifier is connected to pin
The following brief description of the AIM3A's amplifier 8 of J4, and the (-) input of the amplifier is connected to pin
circuitry will help you make the most effective use of the 10 of J4 (or pin 1 of J3, since these two pins are internally
module. connected). Pin 10 of J4 and pin 1 of J3 are jumpered to
module ground by WZ. The amplifier will amplify the
difference between pin 8 of J4 and pin 10 of J4.

AIM3A-4
AlM3A
Analog Input
Module




There are two groups of 16 single-ended input channels, Single-ended Input, Remote Ground Sensing, Using
and each group has its own (-) input connection. Single- Terminal Blocks
ended channels 0 through 7 and 16 through 23 form one
group, with their (-) input connection brought to pin 1 of Jl Figure 4 shows four single-ended inputs connected to the
and pin 10 of J2. Jumper Wl connects this (-) input connec- AIM3A using a remote ground sense line. All 16 single-
tion to the module ground as supplied from the factory. ended inputs on Jl and J2 use a single ground sense line.
Single-ended channels 8 through 15 and 24 through 31 This method is useful when up to 16 voltage sources to be
form the second group, with their (-) input connection measured are all located at a single remote location, with
brought to pin 1 of J3 and pin 10 of J4. Jumper WZ connects one side of each signal source connected to ground at the
this (-1 input connection to the module ground as supplied remote location. There may be a voltage difference be-
from the factory. tween the ground at the remote location and the ground on
theAlM3A. This will cause an error in the voltage readings
if the connection in Figure 3 is used.
CAUTION
To minimize the possibility of EMI radiation,
use shielded cable for input signals. Connect In Figure 4, the ground connection on theAIM3A is discon-
the shield to module ground, but do not con- nected by removing jumper Wl. The ground at the remote
nect the shield at the opposite cable end to location is carried to the AlM3A by the remote ground
anything.Themaximuminputvoltageallowed sense wire. The AIM3A will measure the difference be-
with module power on is i30 Volts, or 225 tween the remote signal, and the remote ground, and
Volts with module power off. If any input eliminate the error caused by the difference in ground
exceeds f10 Volts, all inputs will be lnopera- voltage between it and the remote location. Two groups of
tive. 16 single-ended inputs can be wired this way, with the first
group using Jl, J2, and Wl, and the second group using J3,
J4, and W2 Refer also to Figures 10 and 11 later in this
There are several choices for the method of connecting manual.
inputs to the AIM3A. You must decide which method will
yield the best results for a given application. To achieve 16-
bit performance, care must be taken to minimize error Differential Inputs, Using Terminal Blocks
sources caused by ground loops and shielding problems.
The guidelines for choosing connection methods are as When there are multiple inputs with different ground
follows: points, or the actual ground points are not known, the
differential input wiring can be used to prevent ground
loop errors and reject common mode noise. Figure 5 shows
Single-ended Input, Local Ground Sensing. Using two inputs connected as differential input channels 2 and
Terminal Blocks 3. Either the (+) or the (-) input on any differential input
maybeconnected toground,orbothmay bediiferent from
For input sources which can be grounded, but are not ground. A restriction does exist, however. No input termi-
connected to a ground of their own, the connections shown nal on the AIM3A can be connected to a voltage beyond
in Figure 3 will give good results. The AIM3A provides the +lOVor-10Vwithrespecttothe.4IM3Agroundreference.
ground connection for the input sources. Two possible If this happens, reading errors may occur on the channel
connectionsareshown.SourcesVOandVl haveindividual which is greater than 10 volts. At about +15 or -15 volts,
cables going to each source. Up to 32 sources can be other channels will begin to read in error, and above +3OV
connected to a single AIh43A in this way. Four input or -3OV, the AIM3A will be damaged.
common terminals are provided, one on each connector.
Connectall thewiresfromasingleinputsource to thesame
connector, asshownfor Jl inFigure3. SourcesV16andV17 When the AtM3A is used to measure a floating source, the
show a cable with a single input common wire. This readings may appear noisy, or may fluctuate or rise until
connection method can be used for up to 16 sources in a they reach the maximum input permitted by the pro-
group where all the sources have one wire in common. The grammed gains. This problem results where the floating
sources in a single group must be connected to channels sourcehasnolow-impedancecurrentretumpathtoground.
which all use the same input common connection. There Over a period of seconds to minutes, the input of the
are two input common connections, one for channels 0 AIM3A charges off the input signal until the common
through 7 and 16 through 23, and a second input common mode voltage present at the input exceeds the allowable
connection for channels 8 through 15 and 24 through 31. level, and the module no longer functions properly. The

AIM3A-5
AIM3A
Analon Inaut Module




solution is to install a resistor for the affected channel from and should be determined empirically. These resistors can
inputlow toground.Thevalueof theresistormaybein the be mounted on headers which are plugged into the avail-
range of 5k-lOOk, depending on the signal characteristics, able resistor sockets on the AEvf3A.




Notes:

1. Jumpers Wl and W2 must be in position. Wl connects input common for signals connected to Jl
and J2 to module ground. W2 connects input common of J3 and J4 to module ground.

2. All shields for singleended signals connected to Jl are attached to pin 10 of Jl, The shield input
on any connector is the right-most pin (Le. closest to the strain relief block of the AIM3A).

3. All input common lines for single-ended signals connected to Jl are attached to pin 1 of Jl, The
input common on any connector is the left-most pin (i.e. furthest from the strain relief block of the
AIM3A).


`igure 3. Single-ended, Local Ground Sense, Using Terminal Blocks




AIM3A-6
AIM3A
Analog Input Module




Wl Removed for remote I--- A+
Ground sense with Jl & J2




Remote Ground
Sense




Notes:

1. Jumpers Wi and or W2 must be removed for this mode of operation: Remove Wl if connections
Ji and J2 are used, or W2 if J3 or J4 are used.

2. The input common on any connector is the left-most pin (Le. furfhest from the strain relief block
of the AIM3A.

3. The shield input on any connector is the right-most pin (i.e. closest to the strain relief block of
the AIM3A.


`igure 4. Single-ended, Remote Ground Sense, Using Terminals




AIM3A-7
AIM3A
Analog Input Module




Notes:

1. Jumpers Wl and or W2 may be in or out for this setup.

2. Input Common terminals have no effect for differential measurements.

3. The input common on any connector is the left-most pin (i.e. furthest from the strain relief
block of the AIM3A.

4. The shield input on any connector is the right-most pin (i.e. closest to the strain relief block of
the AIM3A.
igure 5. Differential Inputs, Using Terminal Blocks




Single-ended Inputs, Using Mass-termination The pin out of Jll when used for single-ended inputs is
shown in Figure 6. The pin out of Jll when used with
A location is provided near the strain relief block of the differential inputs is shown in Figure 7. AU the input signal
AIh43A module for an optional mass termination connec- connections previously described in Figures 3,4, and 5 can
tor Jl 1. Connector Jl 1 has its pins wired in parallel with the be implemented with Jll. When Jll is used with the
connections on Jl and JZ, and provides a method for AnalogDevices3B subsystem, the single-ended input with
quickly connecting up to 16 single-ended or 8 differential remote ground sensing configuration is used, and Wl
signals to the AIM3A. The pin out of Jll is identical to that must be removed.
of the Analog Devices 38 subsystem connector. When Jll
is used, the remaining input terminals on J3 and J4 of the
AIh43A may still be used in the usual fashion.




AIh43A-8
AlM3A
Analog Input Module




Remove Wl if remote ground is used
Pinout of user-installed connector


( ~$$?$zd




Typical Input Wiring :

To Input 6



Remote ground,
if used
To Input Common




Notes:

1. Connector is user-installable. ScotchWM Part Number 3429.

2. Connectors J3 and J4 can be wired for other operating modes.

3. Jumper Wl connects input common to module ground. Remove Wl if remote ground sensing is
used
`igure 6. Single-ended Hook-up, Using Mass Termination




AIM3A-9
AIM3A
Analog Input Module




Wl and W2 have no effect on
differential measurements
Pinout of user-installed connector
/




Typical Input Wiring :



To Input 5+
To Input 5-

To Shield




Notes:

I, Connector is user-installable, Scotchl3M Part Number 3429.

2. Connectors J3 and J4 can be wired for other operating modes.

3. Jumpers Wl and W2 have no effect on differential measurements.

`igure 7. Differential Hook-up, Using Mass Termination



Current Measurement Using Terminal Blocks common mode voltage limitation applies as described
before; that is, no input terminal can be more than 10 volts
TheAIM3A can measure currents by using current sensing positive or negative with respect to the module ground.
resistors to convert the current to a voltage. The voltage
drop across the current sensing resistor may then be meas-
ured and converted to a current. DIP sockets and DIP The example in Figure 8 shows two 4-20 ma current loop
headers are provided on the AIM3A to mount the current sensors being connected to single-ended channels 0 and 1.
sensing resistors. If all the currents being measured have a An external power supply is needed to power the sensors.
common rehzrn path, the connection method shown in Up to 16 current loop sensors can be connected to Jl and J2
Figure 8 can be used. If the currents being measured have and use the common current return path as shown. 16
different return paths, the connection method shown in additional current loop sensors can be wired to J3 and J4,
Figure 9 must be used. In both connection methods, the and use the second common current return path. When the


AIM3A-10
AIM3A
Analog Input Module




Remove Wl for single-ended
current loop senscrs
connected to Jl & J2
I, \



I+




4 20mA
current
senscrs

External
voltage source
to power senscm
Sensc return path for
channels 0 - 7
and16-23




1. Up to 16 senscrs can be connected to Jl and J2

2. Precision 250 ohm 0.01% O.iW resistors are available from Keithley DAC as part number 500.
RES-250. These resistors can be soldered to the DIP header plugs which are supplied in J5-JlO.

3. Remove jumper Wl if connectors Ji and/or J2 are used. Remove W2 if J3 and/or J4 are used

4. The same configuration can be repeated for J3 and J4.

`igure 8. Measuring Currents Using Single-ended Inputs



common current retcrn is used as shown for single-ended 5OOn or the resulting input voltage will be greater than
current inputs, single-ended voltage inputs in that group 1OV. Ohm's Law can be used to calculate the value as
of 16cannot beused. Figure9showsasingle4-ZOmasensor fellows:
connected up for current sensing in the differential mode.
In this connection method, either input terminal on the Voltagedrop = maximumexpectedcurrentxresistorvalue
AIM3A may be connected to ground either at the external
power supply or at the sensor. Generally, you may choose any resistor value which gives
a suitable voltage drop. To avoid heating which may affect
measurement accuracy, make sure the maximum power
The resistor value used for current measurements should dissipated in the resistor is well within the power rating of
be chosen such that the resulting voltage drop does not the resistor. Calculate this dissipation with another Ohm's
exceed the voltage input range for the channel being used. law equation,
Thus, if a maximum cuTTent of 20 mA is expected, and if the
input range is HOV, the resistor cannot be any larger than I'd2 x R


AlM3A-11
AIM3A
Analog Input Module




WI and W2 have no
on differential current
affect

measurements

.Eb m


4 - 20mA
sensor




1
nnnlti ' I-- 4 v
+ External
voltage supply
to power sensor
+7
Each sensor
has its own
return current
path




1. Up to 8 sensors can be connected to Jl and J2

2. Precision 250 ohm 0.01% 0.1 W resistors are available from Kelthley DAC as part number 500.
RES-250. These resistors can be soldered to the DIP header plugs which are supplied in J5-JlO.

3. Jumpers Wl and W2 have no effect on differential current measurements.

4. The same configuration can be repeated for J3 and J4.

`igure 9. Measuring Currents Using Differential Inputs



There is one case where you most install a specific value Grounding
resistor. Keithley's software includes an engineering units
flag which facilitates measurement of 4-2OmA current Care must be taken in wiring test circuits to minimize
loops. This EUF rehnns a reading of current
in inilliamps. errors due to the resistance of connection wire, especially
It equates 4mA as 1V measured across the resistor, and where the wire must carry an appreciable current. Figure
2OmA as 5V. Ohm's law requires a resistor value of 250 10 shows a circuit which has an inherent problem resulting
ohms for this relationship to hold. Single-ended or differ- in a measurement error of 26%. The 1.28 ohm and 50 ohm
entialmcdecanbeused. Resistorswith avalueof250ohms resistors are not actually discrete resistors in the circuitry,
@ 0.01% are available from DALE resistors; part number but instead represent the equivalent resistances of wire or
RN55E25008, or from Kaithley in packs of eight resistors as other components. Figure 11 shows how the jumper Wl
500~RES-250. may be removed to overcome this measurement problem.




AIM3A-12
AIM3A
Analog Input Module




Amplifies 0.626V




50 ft. of #24 awg



50R = 1OOmA load
Voltage drop in ground wire = (Sensor amp circuit
E=IR=0.10~1.26=0.126~ power, for example)

Error in measured voltage =
0.126VIO.5V n 26%




`igure 10. Errors Due to Ground Loop




(0.626V - 0.126V) - 0.5V




50 ft. of #24 awg



50R - 1 OOmA load
(Sensor amp circuit
Using remote ground sensing power. for example)
subtracts the ermr due to
ground wire resistance




`igure 11. Avoiding Ground Loop Errors with Remote Ground Sensing
AIM3A
Analog Input Module




Selecting gains or more inputs of the AIM3A. A single-pole input filter
may be easily constructed using one resistor and one
The AIM3A module must be used with a data acquisition capacitor (see Figure 12). The relative RC values will
system containing an AMMlA, AMM2, or other A/D depend on a variety of factors, including the frequency
module. AllKeithleymasteranalogandA/Dmodulesalso of the noise, the required attenuation, and the neces-
include a programmable gain ("global") amplifier which sary response time. The RC values can be computed
can be used to amplify any analog signal before it is from the formula:
digitizedbytheA/Dconverter.TheAIM3Aalsohasanon-
board ("local") instnxmentation amplifier which amplifies f&m =&
only those signals connected to the AIM3A. Thus, total
gains of xl, x2, x5, x10, x20, x50, x100, x500, and xlOW can
be applied to signals connected to the AIM3A. The follow-
ing guidelines should be used in selecting gains: Where f is in Hz, C is farads, and R is ohms. The resulting
system response time within 0.01% is then equal to 9.2xRC.
Where possible, the amplifier on the AlM3A should be
used. The AIM3A amplifier processes the signal before
it passes through the remaining multiplexer and ampli- As an example, assume that 10 counts of 60Hz noise is
fier circuits, so the amount of noise which is amplified present in the signal. To reduce the noise to one count (i.e.
along with the signal will be minimized. to reduce the noise voltage by a factor of lo), an attenuator
The maximum signal presented to the A/D converter factor of 10 (20dB) will be necessary at 60Hz. A single-pole
as a result of all the applied gains cannot exceed the filter will roll off at a rate of 20dB per decade. Thus, a 3dB
A/D converter range. point of 6Hz would be chosen to attenuate to 6OHz noise by
20dB. Rearranging the above equation to solve for R we
have:
Input filtering
R= 1
2n x c x f(s*)
Some types of measurements may be susceptible to noise
from the environment. Examples include measurement of
high-impedancesignals,ormeasurementsperformednear
Picking a nominal value of 0.5pF for C, the necessary resis-
motors, SCR controllers, etc. You should routinely take
tance is:
care to minimize noise in your measurement setups. Typi-
cal steps include:
R= 1
1. Use short cable runs and shielded connecting cables. 2rr x (0.5 x 10-6) x 6
2. Properly use shielding. Only one differential signal
should be carried in a shielded cable. The shield should
R = 53,OOOohms
be terminated only on one end of the cable; generally to
a shield terminal on the AIM3A.
3. Minimize external sources of noise, re-orient equip-
The resulting response time (Tr) is:
ment, or use a power outlet for the data acquisition
system which is not shared by any other equipment.
Tr = 9.2xRC
4. Take several readings and average them. If the noise is
random, it will be filtered out mathematically through
Tr = 9.2 x 53000 x 0.5
averaging.
5. Use any filtration available in the data acquisition sys-
Tr = 0.24~~
tem. The AMM2 module includes a programmable
filter with pass bands of 1OOkHz or ZkHz. The reading
rate available with the 1OOkHz filter is 50kHz, while the
Note that there are a number of RC values that can be used
reading rate available with the 2kHz filter is 1kHz.
in a given application. To minimize the effects of the series
6. Consider external conditioning or filtration. In some
resistance, however, keep the value of R as low as possible.
casesyoumaywant toimplementafilterdirectlyatone




AIM3A-14
AIM3A
Analog Input Module




Where signals of low to moderate impedance are mixed on
one AIM3A, sufficient settling time or other measures
must be taken to assure accurate measurements. When the
AIM3A multiplexer selects any channel, a small charge
flows into or out of the AIM3A. If the next channel selected
by the multiplexer is a low impedance source, the AIM3A
and source will rapidly settle for the next measurement. If
the multiplexer instead switches to a high impedance
signal source, more time will be required for the AlM3A
input to settle at the new voltage.


igure 12. Single-pole Input Filter Where high and low impedance signals are mixed on one
AIM3A, it is thus important that sufficient settling time be
provided per channel. It may also be helpful to short one
High-Impedance Measurements AIM3A channel to ground, and to scan that channel be-
tween the actual signal channels. Alternately, an external
buffer amplifier may be inserted between a high imped-
In measuring an analog signal, the measurement instru-
ance signal and the AIM3A input. Such an amplifier pres-
ment places a small load on the signal and absorbs a small
ents a low impedance to the AIM3A, and may also be
amount of current from the signal source. Ideally, this load
designed with some gain as well. A buffer amplifier can be
should be zero, but will actually be some finite value. The
built using an SET-input op amp and a few components.
input impedance of the AIM3A is 100 Megohms, which is
See an analog IC applications guide for more information.
comparable to a high-quality digital multimeter. Under
most measurement situations, the AIM3A will present a
negligible load to the signal source. However, measure-
ment of signals from very high-impedance transducers User-Configured Components
such as pH meter electrodes requires input impedance
several orders of magnitude higher. The user-configurable components on the AIh43A are J5,
J6, J7, J8, J9, JlO, Jll, Wl and W2.




Table 1. User-configured Components on the AIM3A Module

Application

Used for current sense resistors for single-ended inputs 0 through 7, or for ground reference resistors for
differential (+) input channels 0 through 7.
Used for current sense resistors for single-ended inputs 8 through 15, or for ground reference resistors
for differential (+) input channels 8 through 15.
Used for current sense resistors for single-ended inputs 16 through 23, or for ground reference resistors
for differential (-) input channels 0 through 7.
Used for current sense resistors for single-ended inputs 24 through 31, or for ground reference resistors
for differential f-1 input channels 8 through 15.
Used for current sense resistors for differential inputs 0 through 7.
Used for current sense resistors for differential inputs 8 through 15.
Jumper which connects the input common of single-ended input channels 0 through 7 and 16 through
23 to module ground.
Jumper which connects the input common of single-ended input channels 8 through 15 and 24 through
31 to module ground.




AIM3A-15
AlM3A
Amlo,q Input Module




Table 2. Pin Outs for Single-ended Input Current Sense Resistors


Single-ended Differential Input Input
Connector Channel Channel TWIlliII~l Common

;z 1
0 +I
co 7
8 10
9

; 3
2 +3
+2 6
5 11
12

J5 4 +4 4 13

;z 6
5 +6
c5 2
3 15
14
J5 7 +7 1 16

J6 8 +8 8 9

fi 10
9 +I0
+9 7
6 10
11

:z 11
12 +12
+11 5
4 12
13

; 13
14 +I4
+13 3
2 15
14
J6 15 +15 1 16



;: 17
16 -1
-0 9
10 7
8
J7 18 -2 11 6
:: 20
19 -4
-3 12
13 4
5



:; 22
21 -5
-6 14
15 2
3
J7 23 -7 16 1


:: 25
24 -8
-9 10
9 8
7

; 27
26 -11
-10 12
11 6
5

:: 28
29 -12
-13 14
13 4
3
30 -14 15 2
31 -15 16 1




AIh43A-16
AIM3A
Analog Input Module




Table 3. Pin Outs for Differential Current Sense Resistors



1 Differential
connector Channel
I
0
1
11 6
;i 2
3 12 I 5
13 4
; 4
5 14 3
15 2
:; 6
7 16 1

Jl'J 8 9 I 8
JlO 9 10 7
JlO 10 11 6
JlO 11 12 5
JlO 12 13 4
JlO 13 14 3
Jl'J 14 15 2
Jl'J 15 16 1




Theory of Operation The input multiplexer allows either single-ended or differ-
ential signals to be selected as inputs to the instnunenta-
The AlM3A circuitry is divided into three sections. The tion amplifier. When a differential input is selected, the (+)
input multiplexer consists of Ul, U2, U3, U4, and U5. The input of the selected channel is connected to the non-
instrumentation amplifier consists of U6, U7, U8, and the inverting input of the instrumentation amplifier by Ul (for
associated resistors. ThedigitalcontrollogicconsistsofU9, channels 0 through 7) or U3 (for channels 8 through 15).
UlO, Ull, U12, U13, and U14. The (-1 input of the selected channel is routed through U2
(Ch O-7) or U4 (Ch 8-15) to pin 14 of U5B. U5B is enabled,
connecting the (-) input of the selected channel to the
The programmable gain instrumentation amplifier is con- inverting input of the instrumentation amplifier. The 1K
structed using an AD625JN amplifier (U7). The inputs of resistor networks Rl, R2, R3, R4 combine with clamping
U7 are connected to the input multiplexer through protec- diodes inside Ul, U2, U3, U4 to provide input over voltage
tion resistors R25 and R26. The output of Ui' at pin 10 is protection. The diodes are arranged to conduct whenever
connected to J12 pin 3. The ANA-COM signal at pins 2 and the input voltages either go above the +15 volt supply, or
43ofJ12isthesystemzerovoltagereferencepoint,grounded go below the -15 volt supply.
at the A/D converter input on the Ah4MlA or [email protected] U8
buffers this ground reference, and feeds the U7 ground
reference input at pin 7. Whenasingle-endedinputisselected,oneofthefourinput
multiplexers,Ul,U2,U3,orU4,connects theselectedinput
to its output. U5A is enabled, so that the outputs of all four
U6 and resistors Ii11 through R24 form the programmable input multiplexers are connected to the non-inverting
gain control for amplifier U7. Pins 5 and 12 of U7 drive the input of theinsmunentationamplifier. If the selected input
ends of a balanced voltage divider, and U6 selects two taps is on either Jl or J2, then U5C is enabled, connecting the
on the divider based on the gain setting programmed. The inverting input of the instrumentation amplifier to input
selected voltage divider taps are connected to U7 pins 2 low reference connections on Jl and J2. If the selected input
and 15. is on J3 or J4, then U5D is enabled, connectig the inverting



AlM3A-17
AIM3A
Analog Input Module




input of the instrumentation amplifier to input low refer- portionsofU9andUlO.LatcheddatabitsD6andD7switch
ence connections on J3 and J4. the gain of the instrumentation amplifier by controlling
U6.

ThedigitalcontrollogicrespondsonlytoawritetoCMDA.
A write to CMDA occurs when CMDA and R/W are both AIM3A Commands and Command Loca-
pulsed low, causing U9D to pulse high. U14E inverts the tions
output of U9D, and will cause U12 to latch the data on DO
through D4 at the trailing edge of the pulse. The input
The AlM3A is controlled by writing to the Command A
channel selection is controlled by these latched data bits,
(CMDA) address for the slot in which the module is
which are further decoded by U9A, U9B, U9C, UlOA, mounted. Programmable parameters include selection of
UlOB, UlOC, U14A, and U14B. When a write pulse occurs,
channel, single-ended or differential mode, and gain.
data is not latched by Ull if data bits D6 and D7 are both Command B is not implemented on the AlM3A, and there
low at the time of the write pulse. This is accomplished by
are also no READ modes for the AIM3A.
AND gate UlOD, which combines the write pulse from
U9D with the output of U13A. U13A is high if either D6 or
D7 or both are high. When a write is allowed to Ull, it Keithley's KDAC500 software offers complete program-
latches data bits D5, D6, and D7. Latched data bit D5 selects mability of the AIM3A.
single-ended or differential input mode by altering the
decoding of the channel selection data bits D3 and D4 in




Table 4. AIM3A Command Locations and Functions


Read Functions:

COMMAND FUNCTION
CMDA NOIW
CMDB NOW2

Write Functions:

COMMAND FUNCTION
CMDA Select channel, gain, and single-ended or differential input.
CMDB NOIVZ




AlM3A-18
AIM3A
Analog Input Module




Table 4. AIM3A Command Locations and Functions (Cont.)




TO
AMMlA

At%2
16 Diff Multiplexer

32% Multiplexer

Inputs




Channel select
(CMDA write)


Programmable gain
(CMDA write)




D7 D6 D5 D4 D3 D2 Dl DO




T -II- s!g$y;:




lo-x10Gain
11 - x100 Gain



* A simultaneous write of 000 or 001 to bits D7. D6, and D5 will have no effect on the last write to
D5, D6, or D7.




AIh43A-19
AlM3A
Analog Input Module




Calibration Procedure than CAFE, use that address instead of CFFB in the DEF
SEG statement.
There are four adjustments on the AIM3A: Xl0 gain, Xl00
gain,andfwooffsetvoltageadjustments.Theoffsetadjust- 6. Using the BASICA immediate mode, execute the fol-
ments can be made with equipment of average accuracy, lowing statement:
whereas the gain adjustments require equipment meeting
or exceeding the performance of the equipment specified POKE 4,96
for the procedures. The gain calibrations will degrade by a
maximum of 0.01% per year. If the gains require readjust- This will select the Xl gain and single-ended channel 0.
ment,bestresultswillbeobtainedifthemoduleisrehuned
for factory calibration. 7. Adjust R28 for a DMM Indication of 0 volts, within 50
microvolts.
8. Using the BASICA immediate mode, execute the fol-
Offset Adjustment lowing statement:


Equipment Required: POKE 4,224


Keithley Model 196 DMM or equivalent ThisstatementselectsXlOOgainandsingle-endedchan-
Keithley Model 500,5OOA, or 575 Data Acquisition System nel0.

9. Adjust R27 for a DMM indication of 0 volts, within 500
Procedure: microvolts.
10. Repeat steps 6, 7, 8, and 9 in order until no further
improvement is obtained.
1. Install the AIM3A module to be calibrated in slot 3 of
the Data Acquisition System. Install either an AMMIA
or an AMMZ into slot 1 of the Data Acquisition System,
Gain Adjustment
then tom on the system power. Provide 15 minutes for
the equipment to warm up.