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Instruction Manual
Model 225
Current Source
Publication Date: August 1976
Keithley Instruments, Inc.
Cleveland, Ohio, U.S.A.
Document Nufnber: 29039
WARRANTY
Keithley Instruments, Inc. warrants this product to be free from defects in material and workmanship for a period of 1 year from date of
shipment.
Keithley Instruments, Inc. warrants the following items for 90 days from the date of shipment: probes, cables, rechargeable batteries,
diskettes. and documentation.
During the Wmanty period, we will, at OUT option, either repair or replace any product that proves to be defective.
To exercise this warranty, write or call your local Keithley representative, or contact Keithley headquarters in Cleveland, Ohio, You will
be given prompt assistance and return instructions. Send the product, transportation prepaid, to the indicated service facility. Repairs
will be made and the product returned, transportation prepaid. Repaired or replaced products are warranted for the balance of the origi-
nal warranty period, or at least 90 days.
LIMITATION OF WARRANTY
This warranty does not apply to defects resulting from product modification without Keithley's express written consent, or misuse of
any product or part. This warranty also does not apply to fuses, software, non-rechargeable batteries, damage from battery leakage, or
problems arising from normal wear or failure to follow instructions.
THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING ANY IMPLIED
WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. THE REMEDIES PROVIDED HEREIN ARE
BUYER'S SOLE AND EXCLUSIVE REMEDIES.
NEITHER KEITHLEY INSTRUMENTS, INC. NOR ANY OF ITS EMPLOYEES SHALL BE LIABLE FOR ANY DIRECT, INDI-
RECT, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OF ITS INSTRUMENTS AND
SOFTWARE EVEN IF KEITHLEY INSTRUMENTS, INC., HAS BEEN ADVISED IN ADVANCE OF THE POSSIBILITY OF
SUCH DAMAGES. SUCH EXCLUDED DAMAGES SHALL INCLUDE, BUT ARE NOT LIMITED TO: COSTS OF REMOVAL
AND INSTALLATION, LOSSES SUSTAINED AS THE RESULT OF INJURY TO ANY PERSON, OR DAMAGE TO PROPERTY.
CONTENTS MODEL 225
CONTENTS
SP*CIFICATIONS--------------------------------------------------------------------- 1
1. GENERAL DESCRIPTION------------------------------------------------------------ 2
2. OPERATION---------------------------------------------------------------------- 4
3. CIRCUIT DESCRIPTION------------------------------------------------------------ 8
4. ACCESSORIES-------------------------------------------------------------------- 10
5. SER"ICING---------------------------------------------------------------------- 11
lj. CALIBRATION-------------------------------------------------------------------- 12
7. REPLACEABLE PARTS-------------------------------------------------------------- 19
SCH~rIC------------------------------------------------------------------------
31
ii 3
ILLUSTRATIONS
ILLUSTRATIONS
I
Figure NO. Title Page
1 Prone Panel. ............................ 1
2 Front Panel Controls ........................ 3
3 Rear Panel Controls. ........................ 3
4 Connections. ............................ 4
5 Resistive Load ............................ 6
6 Currenf Sink ............................ 6
7 Capacitive Load. .......................... 6
8 Inductive Load ........................... 6
9 Floating Supply. .......................... 6
10 Volt Limit Characteristic. ..................... 7
11 Modulation ............................. 7
12 Block.Diagram. ........................... 8
13 Component Layout, PC-227 (Switch Board). .............. 13
14 Component Layauc, PC-228 (Mother Board). .............. 13
15 Component Layout, Switch 5101. ................... 14
16 Chassis - Top View ......................... 14
17 Accuracy Check ........................... 18
18 Regulation Check .......................... 18
19 Modulation Check .......................... 18
20 Inductive Load Check ........................ 18
21 current Mode "S Voltage Made Waveforms ............... 18
22 Top Cover Assembly ......................... 21
23 Bortom Cover Assembly. ....................... 21
I
0676 iii
4
CAUTION MODEL225
CAUTION
IF THE DECADE CURRENT DIALS ARE SET TO O-O-O and if the Output Selector switch is not at STANDBY,a
COMPLIANCE VOLTAGE to 100 volts may be present at the oucpuc terminals.
up For r&x"rn operator
safety the Model 225 Current Source should always be set to STA!IDBYmode when no current output is
needed. The Output Selector switch (S105) should also be set to STANDBY prior to changing current
dial seteings and/or Voltage Compliance. This will ensure that no transient currente will be
generated.
WARNING
This insrrument is no? approved for use in "hazardous locations" as defined in the National
Electrical Code, Classes I, II, and III.
Class I: Those locations in which flameble gases or vapors are or may be present in the air in
quantifies sufficient to produce explosive or ignitable mixcures.
Class II: Those locations that are hazardous because of the presence of combustible dust.
Class III: Those locations that are hazardous because of the presence of easily ignitable fibers
or flyings.
0676
5
MODEL 225 SPECIFICATIONS
SPECIFICATIONS
1
OUTPUT: LOADREGULATION: t0.005% offullrangefrom
D.C. Current: lo-' ampere full range to lo-' no load to full load on the lo-' to 10-6 am.
ampere in seven decade ranges, 3.digit in. pere range, 50.05% on the lo-' ampere
line readout. range (with FILTER "OFF").
Voltage: 100 voltsmaximum. Compliancelimit LINE REGULATION: k 0.005% Of full range for
continuously variable from 10 to 100 volts. 10% change in line voltage.
Polarity: Positive or negative. OVERLOAD PROTECTION: Voltage limited tocorn.
Floating: c 500 volts maximum off chassis pliance voltage setting of polarity selected.
ground, less than 5 ppm of full range change Automatic recovery from overload.
in output current per volt. MODULATION: Transformer input permitsmodw
lation of current from 50 Hz to 500 Hr.
RESOLUTION: Three significant figuresfrom lo-' Input 2 approximately 500 ohms.
to 10-l ampere. "Trim" potentiometer permits FILTER: For operation with inductive loads up to
0.02% of full range or better resolution. 100 millihenries having greater than 10.6L/R
ACCURACY: z 0.5% of reading. 2 0.05% of full ratio. Limits shunt output impedance to 1
ra"ge. microfarad shunted bygreaterthan 1010ohms.
STABILITY: -0.02% of reading, _cO.O05% of ENVIRONMENT: 50% relative humidity limit at
full range on the 10-l to 1O-6 ampere range, 25'C.
(e-0.1% of reading 20.02% of range on the CONNECTORS: Output (front and rear panels):
10-T ampere range) the first hour or in sub. BNC
sequent E-hour periods after a l.hour warm-
up and at reasonably constant ambient tern. POWER: 105.125, 210.250 volts (switch se.
perature. lected), 50.60 Hz; 25 watts.
NOISE: Less than 0.01% rms of full rangeabove DIMENSIONS, WEIGHT: 5%" high x 8%" wide x
5 HZ. 10" deep; net weight, 8 pounds.
225 CURRENT SOURCE
.IWE
0571 1
6
GENERAL
DESCRIPTION MODEL225
SECTION 1. GENERAL DESCRIPTION
1-I. GENERAL. The Model 225 is a current source with c. Accuracy. The three decade dials adjust the out-
full scale ranges frarn 10-7 to 10-l ampere. put current to +.05% of full range and +.5% of reading.
1-2. FEATURES. d. Polarity. The OUTPUTSELECTOR Switch provides
polarity selection without changing connections c" the
a. Resolution. The thpe decade dials plus a "Trim" load.
potentiometer provides a resolution of .02% of full
range. e. Floating Capability. The instrument LOWcan be
floated up t" 2500 volts above chassis.
b. Compliance Voltage. A front panel Compliance
Cantral provides a" adjustable maximum compliance volt- f. Modulation. The ""tput current can be modulated
see from 10 to 100 volts. over a frequency range from 50 t" 500 Hz.
TABLE l-l.
Front Panel Controls
CQ"trOl Punctianal Description Paragraph
Power Switch (S202) Controls line power to instrument. Decimal point serves 2-4, a
as a pilot lamp.
OUTPUTSELECTOR
Switch (SlO5) Selects positive or negative polarity or standby pasitio". 2-4, b
VOLTAGE COMPLIANCE
Control Sets the maximum compliance voltage which can be developed 2-4, c
(R117> at the OUT terminal.
LIMIT Indicator (DSlOl) When lighted, the instrument is in Voltage mode. 2-2, e
Range Switch (SlOl) Selects the full scale range. The decimal point is auto- 2-4, d
matically positioned by the Range switch.
Selectar Switches (S102, Selects the magnitude af current in 3 decade positions. 2-4, e
s103, SlO4)
FINE Control (R13S) Adjusts current. 2-4, f
FILTER Switch (5106) When set to "ON", the switch connects a 1 p capacitor 2-4, s
at the OUT terminal.
TABLE 1-2.
Rear Panel Controls end Termirals
Control or Terminal Functional Description Paragraph
Power Cord (P201) Pravides line power to instrument.
LINE VOLTAGE Switch (S201) Sets instrument for 117 or 234 V line operatia".
FUSE (F201) Type 3AG. slow slow. 117 v: 1,4A; 234 Y: l/S`4 2-1, a
OUT Receptacle (3104) Output high cannectio". 2-1, a
LOWTerminal (5201) Output law connection. 2-1, a
GROOND
Terminal (520) Ground (chassis) connection. 2.1, a
!4ODUlATEReceptacle (5101) Connection to modulation transformer. 2-1, b
Shorting Link Provides cbnnection between LOWand GROUND.
7 0571
7
MODEL225 GENERAL
DESCRIPTION
0571 3
8
OPERATION MODEL225
SECTION 2. OPERATION
2-l. CONNECTIONS. 2-2. MEASUREMENT
CONSIDERATIONS.
8. OUtPUt conneerors. a. Accuracy. The accuracy is specified in terms
of the front panel dial setting or reading as well as
1. Front Panel. The connectors on the front the full range selected. (The FINE current control
panel are BNC type receptacles (Keithley part no. must be set to "0" position to obtain the stated
cs-15). The OVP receptacle (5103) provides conr,ec- accuracies.) Since the total accuracy of the Model
tion to the Model 225 Output High. (The center 225 is the s of the readina accuracy and the full
terminal is Output High while the outer shield is a accuracy, the user should select the lowest
connected to chassis ground.) Connections to the full range for best possible accuracy. Table 2-l.
output load can be made using the OUT receptacle shows the total accuracy for typical dial settings.
when the Model 225 is not floating. (For a more (The accuracy on the lOOkA range is +.05% of 1000 or
complete discussion of floating operation, refer to o.s"A plus 2.5% of 1000 or %A for a coca1 of 5.5nA.)
paragraph 2-2 f.) The LOWreceptacle (JlO5) provides
a connection to the Model 225 Output Low. (The
center terminal is Output Low while the outer shield TABLE 2-l.
is connected to chassis ground.) Accuracy For Typical Dial Settings
2. asac Panel. The OUT receptacle on the rear Output Decade Dial RANGE Total Accuracy
panel is also a BNC type. Connections can be made lhrL-elLt setting Setting (FINE c0ntr01
to either the front panel receptacle s the rear set to "0")
panel receptacle with satisfactory results. The
rear panel LOWterminal (3104) is a binding post 1 x lo-6A o-o-1 1000 &k4 + .505 VA
(Keithley part no. BP-1lG) which is connected to the 1 x lo-6A O-1-0 100 $ + .055 NA
center terminal of 5105 (on the front panel). The 1 x 104A 1-o-o 10 @ + .OlO *
rear panel GROUND terminal (3201) is a binding past 1 x lo-6A 10-O-O 1000 UA + .0055 p.4
(Keithley part'no. BP-118) which is connected to
chassis ground. The shorting link (supplied) permits
LOWto chassis ground connections. CAUTION
When the OUTPUT SELECTOR set to + or - and VOLTAGE
is
b. Modulate Connector. The NOD"UTE receptacle COMPLIANCE set to 100, up to 100 volts will be
is
0101) is a BNC type connector (Keithley part no. CS- present at the output terminals.
15). This connector can be used for modulating the
output current using an ac modulation signal. The b. Stability. The stability is specified in terms
center terminal is connected to a transformer-coupled of the front panel dial setting or reading as well as
modulation circuit while the outer shield is connected the full range selected. Since the m stability
to chassis ground. specification is the e of the readinR stability and
the full range stability, the user should select the
c. Cables. Connections should be made using co- lowest full range for best possible stability.
axial cables which are low-noise types with graphite
coating between dielectric and shield braid. For c. Line Regulation. The line regulation is stated
custom length cables, Keithley Part No. SC-9 Low Noise as +..005% of full range for a corresponding 10% change
Coaxial Cable should be used. A mating BNC connector in line voltage.
is available by ordering Keithley part no. CS-44.
d. Load Rezulation. The load regulation is spec-
ified a8 2.005% of full range far a change from no
load to full load. The no load to full load varia-
tion corresponds to an output compliance voltage change
from 0 volts to 100 volts.
e. Output Impedance. The effective output imped-
ance of the Model 225 is a function of the load reg-
ulation specification and the high to low insulation
resistance. The output impedance for each Range set-
ting is given in Table 2-2. The insulation resist-
ance is 2 x 1012 ohms with the FILTER Switch OFF, but
is 1 x lOlo ohms with the'FILTER Switch ON. There-
fore the maximum output impedance is 2 x lOl2 on 100
nA and 1OOOti ranges with the FILTER Switch OFF. The
effective output impedance R. ~a" be decermtned by the
following equations.
FIGURE 4. Co"*ectm"s.
0676
MODEL225
TABLE 2-2. c. contra1 setti"*s. The output current is selected
Output Impedance by three decade switches and a seven position Range
Switch. Refer to the discussion of the fronr panel
Range setting O"tput Impedance* concrok in paragraph 2-4 before power is turned on.
100 m.4 2 x 10; n
10 mA 2 x log n 2-4. FRONTPANELCONTROLS.
1000 liA 2 x lOlOG
100 ,,A 2 x 1OlI R a. Power Switch (S2022. This switch controls the
10 UA line-power to the instrument.
1000 nA 2 x l0l2 ",
100 nA ", : :;12 n b. OUTPUTSELECTOR Switch (SlO5). This switch has
four positions designated "-", "STANDBY", "STANDBY",
*With Filter switch set to off. I and "+". The + and - positions set the output current
polarity. The two "STANDBY" positions divert ehe out-
put current as shown in Figure 5. The output load is
For the 100 mA range, not disconnected at any position of the switch. (The
shunting effect should be considered, since a small
R,, = 103n and % regulation = 0.005% voltage can be developed across the lo-milliahm shunt
RL and therefore across the output terminal.)
since I regulation = 100 x
qq
I 1 c. VOLTAGE COMPLIANCEControl (R1171. This control
is a continuously variable potentiometer which can be
Then % - see for a compliance voltage from lo~volts to 100
volts.
d. Range Switch (SlOl). This switch selects the
full scale ranpe from 10-7 BmDePeto 10-l amoere.
The Model 225 will deliver the dialed current for The dial is deiignated in re&s of engineering Airs;
a load resistance from 0 ohms to WK. The val'ue that is, nA, IJA, and mA. The full range is determined
of w is determined by the dialed current I and by the position of the lighted decimal point end the
the compliance voltage setting VC where Rw.K = designated engineering unit.
WI. If the load resistance exceeds RMAx then
the Mcdel 225 will automatically switch into Volt- e. Selector Switches (SIOZ, SlO3. S104L. These
age Limit mode indicated by the LIMIT lamp (DSlOl).. switches select the magnitude of `the o"tput current
in three decades. The position of the dials determines
f. Floatiw Operation. The instrument can be floated the significant digits from O-O-O to V-9-9.
"I) to +500 volts off chassis nround with less than 5
ppm of-full range change in &put current per volt f. FINB Control (Rl38)-. This control is the inner
off ground. For floating operation, the shorting link dial of a dual concentric control. The fully clock-
on the rear panel m,,st be m. In this case, two wise position "10" represents 1 digit added t., the
coaxial cables should be used to connect to output right-hand dial (S104). The fully counter clockwise
high and low. The outer shell of the front panel O!Jf position is the "0" or off position. When the FINE
and LOWreceptacles are always at chassis ground for Control is set to "0". the output current is determined
safety when flokting the instrument. A filter capaci- only by the calibrated dials within rated specifica-
tor C217 (,05 ~0 is connected between the LOWand tion. This control cm be used to "trim" the output
GROUND binding posts to minimize line frequency pickup. current to within +.02% of full range, although the
The LOWto GROUND isolation is approximately lo7 ohms. trim dial is adjustable t., within 2.005% of full range.
A typical example of floating operation is shown in
Fig"re 9. In this example, the Model 225 can be used NOTE
with a voltage supply such 8s Keithley Model 240A to
extend the maximum compliance voltage to +600 volts. For example, if the three decade dials were set
The Model 225 can be adjusted for a compliance voltage for 9-9-9 on the 1000 !,A range, then the FINE
of +500 2100 volts de. The maximum current is 10 milli- control could be set to 1500 clockwise to obtain
amperes (for the Model 24OA). a current .Y"tp"t of approximately 999.5 +4.
Since the stabilitv specification is +.02% (or better)
2-3. PRELIMINARY PROCEDURES. of reading, the useebie resolution w&d be ~i.2 ,,A.
If the FINE control was set to "O", then the output
a. Power. Check the 117-234V Switch for proper line- would be 999.0 ti 2.5 ,JA.
voltage setting. Check the fuse for proper rating.
Set the Power Switch to off before connecting the power g. FILTER Switch (S106). This switch should be
card. used whenever the Model 225 must deliver current to
an inductive load.
b. Connections. The output load should be cwnected
as described in paragraph 2-l.
0972B 10 5
OPERATION MODEL225
2-5. OPERATING CHARACTERISTICS. The Model 225 Current
Source has a bipolar output, a constant cnreent up to
the maximum compliance setting, and automatic cross-
over to constant voltage mode.
a. Bipolar Output. The Model 225 can deliver 0'
-`Iff!?++iJ RI
accept current and as such it has a bipolar output.
1. Current Source. The instrument can be used as
a current source with a resistive load as shown in
___---------- I
Figure 5. In this case, the Model 225 delivers B
current set by the front panel controls. Tbe volt-
age developed across the load resistance RL is V, = FIGURE 5. Resistive Load.
IO x RL.
2. Current Sink. The instrument can be used as
a current sink as shown in Figure 6. In this case.
the polarity of the current is such that cw'eent
is flowing into the Model 225. The output voltage
"0 = E - IORL, where IC is selected by the front
panel dials, E is the external voltage source, and
RL is the load resistance.
b. Constant Current Mode. Tbe Model 225 will de-
liver the dialed current for resistive, capacitive,
and inductive loads unless the voltage required at the
output terminals exceeds the compliance voltage.
I
FIGURE 6. Current Sink.
1. Resistive Load. The instrument can be used to
deliver current to a resistive load from 0 ohms to
w where k = VC/IO and VC is the compliance
voltage setting.
2. Capacitfve Load. The instrument can also be
used to charge a capacitive load as shown in Figure
7. The capacitor will be charged to the compliance
voltage set on the front panel.
3. Inductive Load. The instrument can be used to
deliver current to an inductive load. When the ratio
of load inductance L to load resistance R is greeter
than 10-6, the FILTER Switch m"st be set to "ON" for
stability. The FILTER Switch connects a 1 pF cap*-
citor across the output as shown in Figure 8.
FIGURC 7. Capacitive Load.
:-
I
I
I
I
I
FIGURE 8. Inductive Load. FIGURE 9. Floating Supply.
6 0972
11
MODEL225 OPERATION
C. constant "Oltsge Mode. The Model 225 can be ed-
jwted for a compliance voltaza from 10 to 100 volts.
The voltage limiiing will o&r if the load resistance
is such that IORL - "C. The instrument "ill auto-
marically switch into a constant voltage mode (the
LIMIT lamp will be lighted) such that the output volt-
age will not exceed the compliance setting. If the
Mcdel 225 is used as a current sink, a,, output voltage t
of 130 volts could be developed regardless of the con- 1.3
pliance setting. The voltage limiting characteristic
can be shown as in Figure 10.
, I
2-6. M!JDULATION. For convenience in making dynamic RL-+
measurements, the Model 225 has a MODULATE input (JlOl) I I
so that the current can be modulated in the 50 to 500
HZ range. F1mP.E 10. Volt Limit Characteristic
a. Circuirry. The Model 225 uses a transformer
coupled input with 500 ohms impedance.
b. Characteristics. The peak-to-peak modulation
can be adjusted up to a maximum of 40% of full range
from 50 CD 100 Hz decreasing to about 8% of full range
at 500 Hz (with a 10 volt rms sine wave input). The
modulation circuit is not calibrated so that the user
m"st check with a knwn load resistance. Within a
selected w the modulation is independent of decade
dial setting. The capacitance of the output cables
can affect the modulated current signal so that the
modulation calibration should take into account this
CapC.CitaWX.
I I
C. "Olcage Complia*ce. The serring of the voltage Modulation.
compliance can cause clipping of the modulated cur- FIGURE 11.
rent as shovn in Figure 11.
NOTE
When the 225 output is modulated the average
value or dc level is determined by the dial set-
rings. The outpllt is essentially a sine-wave at
the input drive frequency. The peak-to-peak
modulation amplitude depends on both the magni-
tude and frequency of the drive signal so that
the exact modulation waveform will have to be
checked tith an actual output load connected.
Clipping of the output signal may occur de-
pending on the compliance voltage setting. The
modulation feature is especially useful when
making dynamic impedance measurements on semi-
conductor devices, etc.
0972 7
12
CIRCUIT DESCRIPTION MODEL 225
SECTION 3. CIRCUIT DESCRIPTION
3-1. GENERAL. The Model 225 is an all solid-state 3-3. CIRCUITRY.
current source which is composed of varicus circuits
as shown in Figure 12. These circuits include the a. High Gain Operational Amplifier `A3". This
following types. amplifier is composed of matched emitter followers
(4111 and Q114). e differential rpn gain stage (trans-
a. High Gain Operational Amplifier "A3". istors Qlli and Q113), a differential npn gain stage
b. Differential Voltage Sensing Amplifier "Al". (transistors QlOS and QllO), an emitter follower
c. Kelvin Divider with Precision l-volt Reference. transistor Q115 and a second npn gain stage (Q116 and
d. Voltage Compliance Amplifier `A2". Ql17).
e. Compliance Voltage Control.
f. Compliance Voltage Suppies 2130 V. b. Differential Voltage Sensing Amplifier "Al".
g. Series Regulators. This amplifier is composed of a marched pair of FET'S
h. Range Resistor Controls. (transistors Q118A and Q118B). These devices have a
i. Power Supplies. low temperature coefficient (5 ppm/W) and low typical
j. Modulation Circuit. offset-current (less than 10 pA). The offset-current
however, doubles every 10