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INSTRUCTION MANUAL
Automatic Ranging Digital Picoamneter
Model 445
COPYRIGHT 1974, KEITHLEY INSTRUMENTS, INC.
PRINTED JULY, 1978, CLEVELAND, OHIO U. S. A.
DOCUMENT NO. 29105
MODEL 445
CONTENTS
SBCCiOU PM,.2
SPEClFIMTI(JNS ---------------_-------------------------- ii
1. GENERAL DESCRIPTION ------------ ------------------- 1
2. 'PEmTION ---------------_-_--_______________I____--- 3
3. CIRCUIT DESCRIPTION __-_------------------------------- 11
4. ~p~`-~LE PARTS ________________----_____I________-_ 29
5. C&TBmTl,,N __-______---__-__---------------------------------- 51
SCHEMATICS __-_----_------__---______ll______l_____------ 57
PRINTrn JUL 74 0174
iLLUSTF.ATIONS
ILLUSTRATIONS
Fiu. No. Title Page
1 Front Panel. 1
2 Front Panel Conrrols. 2
3 Rear Panel. 2
4 Triaxial Receptacle. 3
5 Reading Time. 4
6 Digital Display. 5
7 Prfnter/Control ConneccLr. 9
8 Timing Diagram. 10
9 Feedback titer. 11
10 Range Calibration. 11
11 Overall Block Diagram. 12
12 Autoranging Block Diagram. 13
13 A-Lo-D Converter Diagram. 14
14 Delay Hold Diagram. 15
15 Integrator Block Diagram. 15
16 zero cro**ing DetacLor. 15
17 Model 4401 Buffer stage. 17
1B Chassis, Top View. 19
19 PC Board Locationa. 20
20 Componanr Layout, PC-207. 21
21 cMnponenc Layout, PC-208. 21
22 COmponent Leyout, PC-209. 22
23 Component Layout, PC-212. 22
lb Component Layour. PC-214. 23
25 cmponent Layout, PC-217. 23
26 Colnponent LayoUr, PC-218. 24
27 component Layout, PC-219. 24
25 component Layout, PC-221. 25
corQpon.nt Layout. PC-229. 27
:i Componenr Layout, PC-230. 27
31 Mechanical Assembly. 31
32 Test Equipment Set-up for A/D Calibration. 54
33 AID Converter Zero Adjuetmenrs 55
:: Test Cover Template, Top Panel. 57
Teat Cover Template, Bottom Panel. 58
0714
SPECIFICATIONS HODEL445
SPECIFICATIONS
RANQE: lo-' amper. full scale (lo-" ampere. least PRINTER OUTPUTS AN0 REMOTE CONTROLS:
significant digit) to 104 ampsra in eight decade Printer Output.: SCD positive output represents
ranges with 100% ov.rr.nging on all ranges. each of the four digits, rang.. polarity. overrange.
DISPLAI: Four digits from 000 to 1999: range ~4. r.nge changing and zero check. Standard cod. is
pon.nt digit from 2 lo 9: paerity. overload and 1.2.4.8. "0"<+0.4 volt: "l">+ 10 volts at up to
Range Changing indication. one milliampw.: O=OOOO.
POLARITY SELECTION: Automatic. Print bmm.nd A: Positive pulse of 14 volts from.
2200.ohm source with 1 volt per mlcros.cond rise
RANQE SELECTION: time. 100 mkroseconds minimum pulse width. Print
Automatk: Range change possible after each A to commrnd given after each A to 0 conversion.
D conversion. An undrrrangscondition ( causer one range change to next more sewdive cept pres.nt only when displaying an onrsnge
rsng.. An overr.nge condition (21999 digit.) on reading.
any range ca"ses rang. change to IO-' A range.
Remote Control.:
Mmu~l: Front panel swi1ch.s permit manual r.nga
control. Hold # 1: Closure* to ground inhibits A to D
conversion .t that instant.
ACCURACY AND RESWNSE TIME:
Hold # 2: Closure* to ground inhibits A to D
conversion .fter r..ding h.s been compl.t.d.
Trigger: Cbsure* to ground initiates one conv.rsn)n
when in Hold #2. Integration period starts 8.3
millisecond. (10 milliseconds on 50.H~ models)
.fter "Trigger" or r.le.se of Hold # 2.
R.nSr HoM: Closure* to ground prevents range
change.
I~ C,llbr.l~.l1.c1011 IllT .n*.nlb. bl.m,l Id,"Itmmnt.On 10.. to ,w 2ero Check: Closure* to ground places instrument
&lw,,. ran,,, to, loc.1,~1.llO".
1. I1 dW1 "4" `1l1.1WI ."d 100 Wol.l.dS Y"Ml", m. mow in zero check mode and prewnts range change.
1. WI" "0 10 $00 P'cOlll.as %h"nlm, tn. l"D"l l * rllt" wt.
101 Range: Cbsure' to ground places instrument
FILTER: Improves .c rsiection by Ikngthening rise tim. in 10-a ampere r.nge and holds until lo-* Range
to approximately 3/4 son lO-' to IO-* A ranger. rolnsed.
LEN0 DRIFT: Lass than 0.5% of full scale per week; ISOLATION: Circuit ground to chassis ground: Greatsr
le.1 than O.OS%/`C. after %.hour warm-up with th.n l(r ohm. shunted by .02 microfarad. Circuit
source volt.ge greater than 2 volts. ground mey bi floated up to 100 volts with respect
DISPLAY RATE: 24 r..dings per second maximum (20 to ch.s.is ground.
per second with 50+lz units), adiusbble toapproxi. COMMON MODE REJECTION: 100 V de or 200 V
mstely 1 reading per two seconds. pe.k.to.p..k rt line frequency will not affect resdmg.
With Filter in. m.ximum display rate is abeut l/s. CONNLCTORS: Input: Teflon.insul.ted triaxi.1. Analog
INPUT VOLTAQE DROP: Less than 1 millivolt for full. output: Amphenol SO.PCZF. Printer outputs and
scale display on all r.ng.s when properly zeroed. rrmote controls: 50.pin Amphenol Micro.Ribbon..
Low md C.S. ground: Binding posts.
LINE FREQUENCV REJECTION: 60 dS (ratio of peak.
to.pe.k current of power line frequency or multiple FOWEL 105.125 or 210.240 volts (switch selectsd).
which will cause less than 1 digit of error, to that 60 Hz. 50.H~ models .v.il.bl.. 30 watts.
error). 100 dEon LO-rto 10-v A rmges with Filter DIMENSIONS. WEIQHT: 5'1," high x 19" wd. by lo"
in. P..k input current should not .xc.ed 20 mA. deep; nrt weight, I5 pounds.
MAXIMUM INPUT OVERLOAD: 1000 volt. using . ACCESSORIES SUPPLIED:
Keithlay or other current lim#t.d (up to 20 mifli. Model 4451 Input Cable: 48" triaxial cable wth
amperes) High Voltage Supply with Model 445 in triaxi.1 connector and 3 alligator clips.
autorange mod.. Inst.ntaneous input current must M.ting digit.1 output connector.
never exceed 125 m~lliamp.res.
ANALOG OUTPUT: tl volt from . 500.ohm sourc.
for full.scale display. Maximum output. 1 millian.
per*. Output polarity is opposit. input polarity.
IV 0874
MODEL
445 GENERAL
DESCRIPTION
SECTION 1. GENERAL DESCRIPTION
1-1. GENERAL. The Model 445 is B sensitive Automatic d. Digital Display. The digital display provides
Ran ing Digital Picoammeter with eight ranges from 3 digits with lo-= ampere resolution.
lo- 8 to 10-2 ampere plus 100% cwerranging.
e. Variable Dieplay Kate. A front panel DISPLAY
RATE control adjusts from 24 readingslsec. to 1 read-
l-2. FEATURES. ing/Z eec, to accommodate the mode of data retrieval.
a. Stability. Zero drift with time is less than 1 f. Autorsnning. Autorange circuit sense8 the in-
digit per day; drift with temperature is less than one put current and automatically selects the proper
digit per O/C, making frequent adjustment unnecessary. range far each meaeurement.
b. Overload Protection. A transient overload of g. Complete BCD Output and Control Lines. Binary-
1000 volts at up to 20 r,A will not damage the instru- Coded-Decimal Outputs are provided on a rear panel
nlent. connector for significant digits, range, polarity,
overrange, and zero check. Cantrr,l lines are also
C. Damping. A front panel FILTER Switch selects provided for remote control of ranging and A/D conver-
additional damping of noisy signals on the 10-7. lo:*. , *ions.
and 1O-g amp& r&es. - -
TABLE l-l.
Front Panel Contr018.
Control Functional Description Paragraph
POWER
witch (S102) Controls power to instrument. 2-3, B
ZEROCHgCK(S1204) Selects Zero Check Made. 2-3, b
ZEROADJUST (R1207) Adjusts the zero offset. 2-3, c
FILTER Switch (S1201) Selects additional filtering. 2-3, d
RANGE-HOLD/AUTOMATIC
(51203) Selects HOLDor AUTOMATICmodes. 2-3, a
RANGE- lo-Z/DOWN (51202) Permits manual range selection. 2-3, f
DISPLAY FATE (R1237) Adjusts the AID conversion rate. 2-3, 8
0971 1
GENERAL
DESCRIPTION MODEL445
ZEROADJUST ZEROCHECK
Control R1207 Switch S1204
I I
PO&R DISPLA; RATE PI Switch Switch 51205 Switch CHANGING HOLD/AUTO 10-2/DowN~
s102 51201 Lamp DS1201 Switch 51203 Switch S1202
FIGURE 2. Front Panel Controls.
TABLE 1-2.
Rear Panel Terminals.
Terminal Functional Description Paragraph
INPUT Receptacle (51213) Input connector, triaxial type. 2-1, a
LO Binding Post (51215) Input LO connector. 2-1, a
GNDBinding Post (51217) Chassis Ground connector 2-1, a
PRINTER/CONTROL
Connector (51214) Provides BCD outputs and control lines; 50 pins. 2-8, a
ANALOGOUTPU'I (51218) Analog Output connector, Mates with Keithley 2-7
Part NO. a-33. I
2 0971
MODEL
445 OPERATION
SECTION 2. OPERATION
1NPUT
LOW
HIGH
CASE
OPERATION MODEL445
!L Accurecv. The accuracy is specified in terme g. Reading Time. The time interval for a new
of a percent of reading on esch range. A" addition- reading to take place depends on the Model 445 front
al 2 1 digit is specified since the A/o conversion panel Settings as well as the chareceeristics of the
has-e" inherent 2 1 digit uncertainty. Noise and input signal. The various time intervals which co"-
saurce~resistance conditions should be evaluated es etitute the total reading time are described es
additianal meeeurement considerations. follows. The intervals are graphically shown in
Figure 5.
a. Source Resistance. The value of eowce resist-
ance can affect the measurement if the loading effect 1. T1 = A/D Conversion Time until first range change.
of the picoammeter is significant. To avoid a de-
gradation of zero drift, the picoammetee range should 2. T2 = Total Time for all successive range changes.
be selected so that the range feedback resisrar is
much lesa than the e~urce resistance. The zero drift 3. Tj = A/D Conversion Time to first an-scale reading.
epecificseio" is valid only for e~urce voltagee greaf-
er the" 2 volts. 4. Tq = Total Settling Time until reading is final.
f. Overloads. A unique input circuit provides h. Off-Ground Measurements. The Model 445 can be
complete overload protection with feet recovery. The used far off-ground meeeurements with the circuit low
maximum overload is 1000 volts using e current limit- floated up to 100 volts with respect to chassis ground.
ed supply (up to 2OmA) such as Keithley Models 240.4, Circuit low to chassis ground isolation is greater
245, or 246. Instantaneous input current mwt "ever then 106 ahms shunted by .02 microfarad. When making
exceed 125 milliamperes. With a" input voltage up off-ground meeeuremente discannect the LO to GND
to 1000 volts use e current limited supply or a ser- shorting link on the rear panel. A triaxial cable
ies limiting resistor (8000 ohms minimum). should be used to maintain noise shieldine and to
provide ueer safety., The BCD autputs are"co""ected
to circuit low and therefore muet be used with de-
vices which ce" be Operated off-ground.
TABLE 2-1.
Typical Reading Time With Filter "OUT".
Input Initial Sampling Range Ranging Final Range Settling Total
Signal Range Time Tl Sec. Changes Time T2 Sec. Time Tg Sec. Time Tq Sec. Time Sec.
10-g A
10-9 10-2 A .042
.042 (1) 6
7 .252
.294 (7) .042
,042 (1) ,060
.120 (3) .498
.396 (12)
(1) (6) (1) (3) (11)
10-7 A 10-2 A .042 (1) 5 .210 (5) ,042 (1) .OlS (3) .312 (10)
10-6 A 10-2 A .042 (`1) 4 .168 (4) .042 (1) ,015 - .267 (6)
10-5 A 10-2 A .042 (1) 3 .126 (3) ,042 (1) ,012 - .222 (5)
10-4 A 10-2 A .042 (1) 2 .084 (2) .042 (1) .003 - ,171 (4)
:;I; ; .042 (1) 1
0 ,042
0 (1) ,042 (1) .003
,003 - ,087
,129 (3)
(1) (0) (1) (2)
PIGoRE 5. Reading Time.
4 0971
MODEL445 OPERATION
2-3. FRONTPANEL CONTROLS. 2-4. OPERATINGPROCEDURE.
a. POWER Switch (S1021. This switch controls the a. Preliminary Pracedure.
line power to the instrument. A rear panel fuse
PlOl protects the instrument in case of malfunction 1. Check the 117-234" Switch (SlOl) on the rear
of the power supply or power switch. panel for proper line voltage.
b. ZEROCHECKSwitch (S1204L This switch selects 2. Check for proper rated fuse.
either "NORMAL"or "ZERO CHECK"modes of operation.
I" "ZERO CHECK" "asitio", B relay is activated which 3. Connect the power cord, place the POWER Switch
places a short across the feedback of the picoammeter. ON, and allow a 30-minute warmup for critical meas-
Zero Controls R1238 and Rl207 ca" be used to adjust urements.
the zero offset as necessary.
4. Adjust the ZEROContra1 (R1207) as necessary.
c. ZEROADJUST (R12071. This control provides Zero is indicated by alternately flashing + polarity
"FINE" adjustment of the zero offset. The internal lights.
zero control Rl23S provides "COARSE"adjustment.
5. Connect the source as described in paragraph
d. FILTER Switch (51201). This switch selects 2-l.
either mininu,, damping ("OUT" position) or additional
damping ("IN" position). A more complete diacussio"
of the "FILTER" mode is give" L" paragraphs 2-2, c b. Measurements. The Model 445 measures current
and 2-2, g. over a full-ran e from 10 milliamperes (10m2A) to 1
"anoampere (lo- 6A) with resolution to 1 picoampere
e. RANGE- HOLD/AUTOMATIC Switch (51203). This (lo-12A). The display will indicate either a posi-
switch selects automatic re."e.i".z weratio"
_ _ in "ADTO- tive or negative input current autamatically. (A
MATIC" position or manual ranging in "HOLD" position. p"sitive current is defined as a positive "conven-
I,, "HOLD", the range is held at the existing range tional current" applied at Input High with respect
displayed until the switch is changed or the range to Input LOW).
is manually changed by either S1202 or the external
control lines.
2-5. DIGITAL DISPLAY. The display has three digits
f. RANGE- 10-2/DOWNSwitch (51202). This switch plus one for overrange indication. The range is
is a nomentary-contact, "armally open switch for ma"- displayed in scientific notation where the exponent
ual range changes. This switch should be used with is automatically displayed for ranges 10-2 through
Switch 51203 in the HOLDposition. When Switch S1202 10-9 amperes, To interpret the display refer LO
is nmentarily set to "lo-2", the Picaawneter resets Figure 6. A" overload condition will cause the dis-
the range display t" 10e2. However, each time the play cubes to blank o"t so that "o ambiguous display
switch is set to "DOWN"Lmsition. the Dicoammeter down is possible. This situation is described under 2-4,
w to a more sensitive range: The'sequence af- d Autoranging.
dawn ranging would be typically 10-2, lo- , 10 4, 10-5,
;riO$ith a reset after 1O-g range is reached, that
) 10-9, 10-q.
g. DISPLAY RATE (R12371. This control adjusts the
A/D Converter conversion rate from 24 readingslsec.
(MAX) to 1 reading/Z sec. (approx).
How to Interpret the Digital Display
The reading shown in Figure 6 above should be
interpreted as follows. The polarity is dis-
played by a lighted "f" or "-" indicator. The
decimal point remains fixed with respect to the
digits. I,, the example, the reading is 1.676
(three digits plus a "one" for overrange). The
range multiplier is 10-9 (the exponent is a"to-
matically selected by the autorangi" circuit).
The complete reading is +1.676 x lo- 8 amperes.
FIGURE 6. Digital Display.
0971 5
OPERATION MODEL445
TABLE 2-2.
Output Information and Controls.
lode1 4401 Printer Output Cards: Provide SCD outp"t
and external control of Model 445; may be purchased
installed or separately for field installation (no
wiring required).
Printer Outputs: SC0 pwitive o"tp"t represents
each of the four digits, exponent, range, zero,
polarity and averrange. Standard code is l-2-4-8.
"O"< +0.4 volt; "1" 5 +lO volts at up to one milli-
ampere; O=OOOO.
Print Command: Positive pulse of 14 volts from a
2200-ohm source with 1 volt per microsecond rise
time, 100 microseconds minimum pulse width.
Remote Controls:
Hold #l: Closure to ground inhibits A to D conver-
sion at that iridtant.
Hold 82: Closure to ground inhibits A to D canver-
sion after reading has been completed.
Trigger: Closure to ground initiates one conversior
when in Hold 112. Integration period starts 8.3 ms
(10 ms on SO-Hz models) after "Trigger" or release
of Hold i/Z.
Connector: SO-pin Amphenal Micro-Ribbon mounted on
Model 445. Output mating connector supplied with
4401.
NOTE
The term "Closure to Ground" or "Grounded control
line" means a short to commn directly or through
a saturated NPN transistor (VC, < +0.4V). Only
the "COMMON" supplied at the PRINTER/CONTROL
as
connector should be used for closures to ground.
TABLE 2-3.
Full Scale Magnitude.
Connector
Pi" No. Output Decimal Digits
1 .l x 10" 1
2 2 x 10" 2
26 4 x 10" 4
27 8 x 10" 8
3 1 x 101 1
4 2 x 101 2
28 4 x 101 4
29 8 x 101 8
5 1x 102 1
6 2 x 102 2
30 4 x 102 4
31 8 x 102 8
0971
MODEL445 OPERATION
2-6. A!JTORANGING. The sutomatic ranging feature of b. Output Codes and Levels. The PRINTER/CONTROL
the Model 445 permits rapid measurements of B wide 0ut"uts are Sinarv Coded Decimal sianels with l-2-4-
range of currents. 8 S&da-d Code. . The Standard signal levels are as
follows:
a. AUTOMATIC Mode. In this made the Model 445 will
automatically select the full-range that permits a Logic "0" < +0.4 volt
display from 0.100 to 1.999. The Model 445 changes Logic "1" > +lO volts at up to 1 milliampere.
range in decade steps from .a less sensitive range to
a mare sensitive range. This process is defined as c. Output Information. The various Output Informa-
"down-ranging". If the input current causes the dis- tion and Controls sre surmnarized in Table 2-2.
play to exceed 1.999, the" the Model 445 will a"to-
matically change to "10-2" range and "down-range" 1. Full Scale Magnitude. The Magnitude of the
until the display Is between 0.100 and 1.999. reading is indicated by SCD o"tp"ts which corres-
pond to the three front panel display lights (three
1. DOWN Range Control. The "se of Switch S1202 significant digits) as shown in Table 2-3.
permits manual range changing. This switch is a
momentary-contact type switch. Each time the switch 2. Overrange Indication. Overrange is indicated
is depressed to "DOWN"position, the range will by the fourth (from the right) display light and
"down-range" from a less sensitive range to a more corresponding SCD output as shown in Table 2-4.
sensitive range. When the 10-V range is reached, Overload is indicated by a blanked display and
actuation of the switch will cause the range to go corresponding BCD output. The output at pin 33
to 10-2 or "reset" position. will be a logic "1" while the magnitude will be
represented by logic "0" as shown in Table 2-4.
2. 10-2 Range Control. When switch 51202 is set
to "10-Z" position, the range will be "reset" to TABLE 2-4.
the lo-2 range regardless of the current input. Overrange and Overload Outputs
Once the switch is released the Model 445 will re-
vert to normal operation. Connector Decimal
Pi" NO. output Digits
b. HOLDMode. When switch 51203 is set to "HOLD"
position, the full range of the instrument will hold 7 1 x 103 1
regardless of the input current. This switch posi- 8 Gammon 0
tion inhibits the operation of the DOWN/lo-2 switch. 32 COSDIIO" 0
33 Overload (8 x 103) 8
NOTE
3. Polarity and Zero Check Indication. The
If the input current exceeds 1.999 for the polarity is indicated automatically by the Polarity
range in "HOLD", the display tubes will indicator and corresponding BCD output as shown in
blank indicating a" overload on that range. Table 2-5. Additional BCD levels are available at
If Switch 51203 is set CO "AUTOMATIC" the pins 14 and 39 for use with some printers. The
Model 445 will change to 10m2. Then if the zero check mode is indicated by a BCD o"tput from
input current does not exceed 1.999 on the pin 38.
10-2 range, the display will indicate
normally when the final range is selected. TABLE 2-5.
Polarity output
2-7. ANALOG OUTPUT. The analog OUtput on the rear bnnector Decimal
panel (51218) provides a voltage of 1.05 volts corres- `in No. output Digits
ponding to 1.000 o" any range. Since this output has
a source resistance of 499 ohms, the recording device 13 Polaritv
must have a load resistance greater than 10 kilohms 14 +15v .
to obtain 1.000 volt for full range. If a recorder 38 Zero Check
such as the Keithley Model 370 is available, an Isola- 39 + 15v
tion Amplifier (such as Keithley Model 399) must be
used to provide up to 1 milliampere current for full 4. Range Indication (Exponent). The range or
range. exponent has a corresponding BCD output as shown
in Table 2-6.
2-8. DIGITAL OUTPUT. TABLE 2-6.
Lange or Exponent Indication
8. General. The Model 445 has o"tp"t buffer print-
ed circuit boards which provide Binary Coded Decimal onnector Decimal
(BCD) outputs. A factory wired SO-pin PRINTER/CONTROL Pi" No. output Digits
connector is provided on the rear panel (51214). A
spare cutout is provided on the rear panel for instal- 9 1 x 10 Range 1
lation of an additional PRINTER/CONTROL connector. 10 2 x 10 Range 2
34 4 x 10 Range 4
35 8 x 10 Range 8
7
OPERATION MODEL445
5. Ranging Signal. A single binary signal is "PRINT COMMAND8" This control is similar
available at pin 49 which corresponds to 8. ranging toe)`Print Command except that na signal is
A"
signal. The logic level is "1" whenever range present if the instrument is changing range.
changes are occurring so as to indicate invalid
readings. f). Zero Check. This control places the Model
445 in Zero Check and prevents automatic range
d. External Control. change when a closure to ground is made.
1. General. To obtain aptim~ system perform- 9). Range Hold Control. This control prevents
ance, Ft is often desirable t" "perate the Model automatic range change when a closure to grand is
445 synchronously with other digital equipment such made.
as printers, paper tape punches, computers, and
other data handling devices. h). 10e2 Range Hold Contral. This contra1
causes the range t" change to low2 when a closure
2. Applications. Several approaches may be used to ground is made. The range is held until the
in designing the overall syetem contra1 scheme. line is released.
*). The Model 445 can be used to provide master i). High and Law References. Two reference
control of external devices so that maximum poss- voltages are provided t" define the "HIGH" and
ible conversion rates ca" be obtained. "LOW" digital output states far external printers
or other devices; "HIGH"** +Bv, "LOW" = +2v.
b). An external device can be used far master
control; such as a high speed printer.
e. Summary of Digital 0"tp"ts and Controls.
Cl. A completely independent "master clack" 1. Standard Output Codes and Levels. The sta"-
can be used for system caner01 for maximum flex-
ibility. dard output code for Model 4401 Printer Output Cards
is l-2-4-S Binary Coded Decimal (BCD). A binary
3. Description, coded decimal digit is represented by a four-bit
binary code as shown in Table 2-7.
=). "HOLD 1". This control inhibits A to D
conversion at the instant a closure to ground is TABLE 2-7
made. The conversia" cycle will resume immediate-
ly when the "HOLD 1" line is opened. Decimal
Number 4 bit 3 bit 2 bit 1 bi,
b). "HOLD 2". This control inhibits A to D
conversion after a complete reading cycle. Further 0 0 0 0 0
conversions are inhibited as long as a closure to 1 0 0 0 1
ground is made. The conversion cycle will resume 2 0 0 1 0
immediately when the "HOLD 2" line is opened. 3 0 0 1 1
4 0 1 0 0
Cl. "TRIGGER". This control initiates one com- 5 0 1 0 1
plete conversion when "HOLD 2" line is grounded. 6 0 1 1 0
Closure t" ground may be momentary or any longer 7 0 1 1 1
duration to initiate a conversion. 8 1 0 0 0
9 1 0 0 1
d). "PRINT COMMAND A". This control provides
* positive going pulse of 14 volts after * corn-
plete A to D conversion is made and all data out-
puts are final readings. No change can take place
(except Zero Check) for 25 milliseconds. This
"Print Command"signal is present regardless of Refer to Figure 17 for a circuit diagram of the
the mode (ADTOMATICor HOLD). Model.4401 Standard Printer 0"tput buffer stage.
TABLE 2-8.
Typical Digital Outputs.
Fr""t Panel Range Sign. Digit Range Ranging
Digital Display Exponent Polarity or overload gag. EXP. Signal Interpretation
+ 0.275 10-5 + 0 275 5 0 + ,275 x 10-5 A
+ 1.347 10-9 * 1 347 9 0 + 1.347 x 10-V A
- (blank) 10`2 8 000 2 0 negative overload
+ (blank) 10-7 + 8 000 7 0 positive overload
- 0.023 10-3 0 023 3 0 - ,023 x 10-3 A
+ 1.962 10-6 + 1 962 6 1 ranging
- 0.586 10-4 0 586 4 1 ranging
I
0971
8
MODEL 445 OPERATION
TABLE 2-9.
PRINTER/CONTROL
Connector Pi" Identification.
in No. Output F"llCtiOll Pin No. Output Function
1 1 x 100 oata 26 4 x 100 mea
2 2 x 100 Data 27 8 x 100 oaea
2 2 x 101
1 x 101 Data
Data 29
28 8
4 x 101 Data
5 1 x 102 oata 30 4 x 102 oata
6 2 x 102 oata 31 8 x 102 oata
7 1 x 103 oara 32 Comon ___
8 Caman 33 8 x 103 Overload
9 1 x 100 Range 34 4 x 100 Range
10 2 x 100 Range 35 8 x 100 Range
11 Blank --- 36 Common ___
12 Common --_ 37 cormnon ___
13 1 x 100 P&WiQ 38 1 x 100 Zero Check
14 + 15" ___ 39 + 15v ___
15 Blank ___ 40 Blank ___
16 Blank ___ 41 Blank -_-
17 + 15v ___ 42 - 15" ___
18 + 3.6V ___ 43 COllllTlO" _-_
19 Grounded Remote Zero Check 44 Grounded Hold ii 1
20 Grounded Remote w2 45 Grounded Hold ii 2
21 Grounded Remote Range Hold 46 Grounded Trigger
22 "lank --_ 47 Blank ___
23 + 14" Pulse Print Command
A 48 + 14" Pulse Print Command
B
24 + BV Hi Reference 49 1 x 100 Ranging Signal
25 + 2" Law Reference 50 Blank
FIGoRE 7. Printer/Control connector.
0971
OPEBATION MODEL44s
2. PRINTER/CONTROL Connector. The PRINTER/CON- b). Conversion Cycle Timing. The Conversion
TROL Connector used on the Model 445 provides for Cycle is composed of three timing periods, namely
connectiom to 50 ~,ins as shown in Table 2-9. The Integrator Zero, Integrator Sampling, and AD
mating connector supplied is an Amphenol Part No. Counting period. Refer to Timing Diagram Figure
57-30500 or Keithley Part Number CS-220.
1. Integrator Zero Period (ZERO). When a
3. Analog-to-Digital Conversion Cycle, trigger pulse initiates a new conversion cycle,
the Integrator circuit is zeroed for a period
a). The analog-to-digital conversion cycle not to exceed 8.33 milliseconds for 60 Hz oper-
can be initiated in any one of three ways. ation. (The Integrator Zero Period is 10.00
milliseconds for 50 Hz operation).
1. DISPLAY RATE Control Set at MAX. With
the DISPLAY RATE Control set at MAX, the end 2. neegratOr sampling Period (INTEGRATE).
of one complete conversion triggers a second The Integrator Sampling Period follows automat-
conversion to obtain the maximum conversion ically the Integrator Zero Period and lasts for
rate of 24 readings per second. the duration of 16.67 milliseconds for 60 Hz
operation. The Integrator Sampling Period lasts
2. DISPLAY SATE Control Set at Other Than for B duration of 20.00 milliseconds for 50 Hz
MAX. With the DISPLAY RATE Control set at operation.
some position other than MAX, (uncalibrated
control setting) the end of one complete con- 3. AD Counting Period (COUNT). The AD Count-
version triggers a second conversion which is ing Period is initiated immediately following
delayed by a specific time interval (DELAY). the Ineegratar Sampling Period. The actua$
The time delay is a function of the position counting time duration will depend on the actual
of a continuously variable control to provide integrator voltage up to a maxilll"m Of 2000 clock
a conversion rate from 24 readings per second pulses of 16.67 milliseconds. Following the
to 2 readings per minute. counting period a Buffer/Storage command is
automatically generated in order to store the
3. `HOLD 2" With TRIGGERControl. With the new reading in the output registers.
`TlOLD 2" camand grounded, a closure to ground
of the "TRIGGER" command initiates one complete
conversion cycle. A second convereion will 2-9. 117-234V LINE POWER OPERATION. The Model 445
fallow only if the TRIGGERcommand is removed is shipped far use with 117V ac line power unless
and reapplied a second time. The maximum con- ordered for 234V operation. TO cO""ert any inserunlent
version rate using an external trigger is 24 for either 117V or 234V operation, use a screwdriver
readings per second. to set the 117-234" Switch on the rear panel. The
10 0971
MODEL445 CIRCUIT DESCRIPTION
SECTION 3. CIRCUIT DESCRIPTION
3-l. GENERAL. The Model 445 consists of three sec-
eions packaged together in one chassis: a sensitive
picoameter, an automatic ranging circuit, and an an-
alog-to-digital converter.
a. Picoammeeer. The picoammeter is a linear dc
amplifier connected as a feedback anmxeter with eight
current ranges.
b. Autarangina Circuit. The autoranging circuit
senses the magnitude of the display so 88 to trigger
the range changing circuits and cycle from 10-2~ through
lob9 amperes full scale.
FIGURE 9. Feedback Ammeter.
C, Analog-to-Digital Converter. The A/D converter
is a dual slow, integratina t"Pe converter with med-
ium conversion &ace, cold cathi& readout tubes, BCD
outputs end optional external controls.
3-2. PICOAMMETER.
a. Operation. The picoammeter consists of a linear
dc amplifier with a 1 volt full scale mensitivity. The
amplifier provides an analog output up to Z~volts for
100% overrange display. The RANGE resistors are con-
nected across the feedback of the amplifier.. A sim-
plified diagram of a feedback ammeter is shown in
Figure 9.
FIGURE 10. Range Calibration.
b. Circuitry. The amplifier input stage is a pair
of insulated-gate, field-effect transistors (IGFET)
designated 91201 and Q1202 connected in a differential 3-3. AUTORANGING.
configuration. The circuit designated 234308 is a
spedlal overload protection circuit on the Input PET a. operation. The autoranging circuit operates
board. The gate,of Q1201 is connected to the input from the BCD outputs of the A/D converter. If the
through 10 Megohms while the gate of Q1202 is refer- reading is less than O-l-O-0, then an "Under-range"
enced to ground. Potentiometer R1238 is an internal commandis generated and the range relays are operated
COARSE ZEROadjustment. Potentiometer R1202 is an to "Down-range" to a more sensitive range. If the
internal BALANCE control. Transistors Q1203-Q1204 reading is greater than l-9-9-9, then an "Over-range"
form a second differential amplifier stage. Potentio- commandis generated and the range relays are oper-
meeer RI207 is a front panel ZEROadjustment. TrZl"S- ated so as to "reset" and then "Down-range". If the
istors Q1205 and Q1206 (emitter-follower) provide HOLD/AUTOMATIC Switch is set to "HOLD", the autorang-
sufficient gain for the analog output and A/D can"eT- ing circuit is inhibited and the range will hold re-
ter. The analog output is connected through R614 gardless of the current being measured.
(499CL) to the dc amplifier output (the junction of
R1215 and 01212). The full scale current sensitivity b. Circuitrv. Integrated circuits QA703, QA704A,
is determined by the RANGEresistor connected across and QA704B code the BCD outputs to generate "Under-
the feedback. The RANGgresistance RP is composed of range" and "Overrange'! c,ommsnds. The "Buffer-stare"
a fixed resistance 81 plus a calibration adjustment command is delayed for approximately 5 microseconds
resistance K2 as shown in Figure 11. The RANGE re- by Mono stable gates QA705A and QA705B, to allow
sistors are arranged in "parallel" such that, an the signals to settle. The range counter gates (QA801,
"1O-2'1 range, all eight RANGEresistors are connected. QA802) is a three-bit binary counter ("000" corres-
Range selection and zero check are performed using ponds to 1O-2, "111" corresponds to 10-9 ampere). The
reed relays which are electrostatically shielded from output of the range counter is decoded by gates QA803,
the high impedance circuitry to preve?f.coupling of QA804 and QASOSinto an "eight-line" code. The eight-
switching transients. The FILTER Switch 51201) pro- lines are encoded by gates QA806 and QA807 to yield
"ides additional damping on the 10-7, lo- $ , and 10-9 BCD range information. Gates QA809 and QAEIO store
ampere ranges with an additional 40 dB rejection of the range information. The range display tube is
60 Hz frequency. With the FILTER Switch set to "IN", driven by transistors Q901 through Q908. Gates QA901,
the A/D conversion rate is set to 1 reading per second QA902 and QA903 drive transistors 9909 through Q915
maxinlum. to operate the read relays.
0971 11
CIRCUIT DESCRIPTION MODEL445
3-4. ANALOG-TO-DIGITALCONVERTER
OPERATION. d. BCD counter. The BCD Ccuneer counts the Clock
pulses with a Wea.1 range of 5000 c~unte. The C~unec
a. General, A detailed block diagram of the A/D is composed of 4 individual cwnters designated 1, 10,
cO""erter is shown in Figure 13. The analog-to- 100, and 1000,
digital converter aperates using a dual slope inte-
gration technique which has inherent line frequency
noise rejection. The analog signal is applied to the
integrator far one complete line frequency cycle.
The analog signal is ehen removed from the integrator e. Delay Hold. The Delay Hold circ"it controls the
input. The voltage on the integrator is then driven DISPLAY BATE function and external Hold and Trigger
to zerc, to complete the voltage-to-time conversion. commandsas shown in Figure 14. It determines ehe
The time interval to reach a "Zero Grassing" is count- length of time between A/D conversions when the front
ed and displayed on the "Readout" in proportion ta panel DISPLAY RATE Control is set t0 any position
the original analog signal. The sequence is then re- other than MAX. The clock is stopped at the beginning
peated for a second reading. A Timing Diagram is of the ZERO (2) period for a time determined by the
shown in Figure 8. rotation of the DISPLAY RATE Central. If eneures thee
when the Hold 2 Fe grounded the conversion in process
b. Circuits. The A/D Converter is composed of nine will be completed and new data will be stored in the
major circuits which perform the analog-to-digital output storage register. Then the clock will be in-
conversions and provide various control commands. hibited at ebe beginning of the ZERO period (2). The
instrument will remain in this condition indefinitely
1. Oscillator or Clock until Hold 2 %s released or until Trigger is shorted
2. BCD counter to ground. After cowersian, the instrument will
3. Delay Hold again be inhibited at the beginning of the period (2).
4. Program/Decoder If both Switches Sl and S2 are closed, the canversion
5. Integrator cycle works in the following manner. After the pre-
6. zero crossing Detector vious conversion has been completed, the leading
7. Buffer/Storage Register edge of the program co,+,nd (2) reeete the flip-flop.
8. DecaderfDriver In this new condition Q is high and, therefore, the
9. Numerical Readout clack gives no output. At that time, the unijunction
timer begins its cycle and, after the appropriate
time, produces e pulse that sets the flip-flop. This
C. Oscillet~r or Clock. The Oscillaear praduces changes Q to a low state and a new converslan cycle
pulses at a rate of 120 kilohertz for Electrometer begins. After the reading has bee,, completed, the
using 60 Hz line power. (The 50 Hz units have e pulse (2) command again reeets the flip-flop and the timer
rate of 100 kilohertz). again issues a new pulse to set the flip-flop.
DIGITAL
DISPLAY
r
T '
FEEDBACK
INPUT+ b A-TO-D CONVERTER OUTPUT
PICOAMMETER
rc
a---
AUTO RANGE
RANGE
RANGE
DISPLAY