Tektronix 414S Instruction Manual

INSTRUCTION MANUAL

COPYRIGHT

1977,

Model
Picoammeter

414s

KEITHLEY INSTRUMENTS, INC.

PRINTED MAY 1978, CLEVELAND,

OHIO,

U.

S.

A.

CONTENTS MODEL

CONTENTS

1.

2.

3.

4.

5.

6.

414s

7.

ii

0675

MODEL

414s

ILLUSTRATIONS

ILLUSTRATIONS

Fig.

No.

Title

Page

3

3

5

6

11

14

15

25

0675

25

iii

SPECIFICATIONS

SPECIFICATIONS

MODEL

414s

RANGE:

ACCURACY:

ZERO DRIFT:

10-lo ampere full scale to ampere in seventeen

lx

and

3x

ranges, positive or negative currents.

&2%

j=4%

of full scale on

0.05%

per

of full scale on to ampere ranges:

3

x

10-9

to

10-10

ampere ranges.

Less than

“C

on any range after a 10-minute warm-up and

0.5%

of full scale per week plus

with source voltages greater than 1 volt.

INPUT VOLTAGE DROP:

meter deflection

EFFECTIVE INPUT RESISTANCE:

ampere range, increasing to less than

Less than 1 millivolt for full-scale

on

any range when properly zeroed.

Less than

0.1

lo7

ohms on

ohm on

ampere range,

RISE TIME, MAXIMUM:

Range,

atnoere

10.10

10.9

10.6

10-7

10

-6

and

Seconds, from

above

With

Across

less

10%

up

0.75

0.15

0.13

0.09

0.05

than

to

to

Innut

90%.

500

pF

0.001

MAXIMUM INPUT OVERLOAD:

Transient:

(or other 10

Continuous:

1000 volts for up to 3 seconds using a Keithley

rnA

limited) High Voltage Supply.

600

volts on

10-lo

to 10-6 ampere ranges, de-

creasing to 12 volts on the ampere range.

CURRENT SUPPRESSION: (Model

414s

only):

Up to

times full scale.

Suppression Range:

ampere to ampere

decade steps, precision 10-turn potentiometer for inter-

polation between steps. Selectable polarity.

CONNECTORS:

Input: Teflon-insulated UHF type. Output:

Amphenol8OPC2F.

RECORDER OUTPUT:

Output:

fl

volt or 1 milliampere for full-scale meter

deflection. Output polarity is opposite input polarity.

Noise:

414s:

414A: Less than

Less than

POWER:

5

watts.

105-125

or

DIMENSIONS, WEIGHT:

0.3%

1%

of

of full scale peak-to-peak.

full scale peak-to-peak.

210-250 volts (switch selected),

5%“

high x 8%” wide

x

50-60

10”

deep;

net weight, 8 pounds.

ACCESSORIES SUPPLIED:

Mating input and output con-

nectors.

lom2

10’lo

100

in

7

Hz,

iv

0675

1

c

MODEL

414s

GENERAL

DESCRIPTION

SECTION

1-1.

GENERAL.

a.

The Keithley Model

state

pcioammeter which measures currents over

ranges from to

is

52%

of full

24%

and
ranges.

tors
stage,
stage. Negative feedback
accuracy.

1-

Picoammeter

of full

b. The Picoammeter employs matched MOS FET transis-

in

the input followed by a differential amplifier

a

2.

FEATURES.

a. The

ammeter.

requiring rezeroing. Zero drift with

0.5%

of full scale per week. Temperature drift

equally

less

small

than

scale

scale

transistor driver and a transistor output

time

and temperature stability of the

is

unmatched by any other inexpensive pico-

The Model

-

a

0.5%

change of 10°C affects the reading

of full scale

414s

is

a

completely solid-

10-lo

ampere full scale. Accuracy

on

the to

on the

is

414s

will

3

used for stability and

on

10-8

x

10-9

operate for days without

the most sensitive

1.

17

ampere ranges

to

10-10

ampere

time

is

less

4145

is

GENERAL DESCRIPTION

than

b. A unique circuit provides complete overload pro­tection for Model
standing features

meter

will

volts without damage, and overload recovery
instantaneous

c.

the Picoammeter.

its

most sensitive range. For maximum stability, how-

ever, about 10 minutes warm-up

d. One volt or

tion

ers.

Keithley
recorders are used, the output voltage can be conven­iently divided by a shunt resistor and adjusted for

full scale with the rear panel Calibration potentio-

meter.

withstand transient overloads up to

Fast warm-up

on

all

The 1 milliampere recorder output

370

414s

of

the

.

is

an inherent characteristic of

It

can

1

ranges

Recorder directly.

milliampere

is

provided to drive chart record-

without compromising the out-

MOS

FET input.

be used

time

almost

at

When potentiometric

The

Picoam-

1000

is

almost

immediately on

should be allowed.

full-scale deflec-

will

drive the

P

P

?-

0272

1

OPERATION

Con

t

r

ol

TABLE

Front Panel Controls

2.

Functional Description

MODEL

Par.

414s

RANGE

Switch

METER

Switch

ZERO

ADJ.

Control

Pilot Light

SUPPRESSION,

SUPPRESSION, FINE and Polarity
Switch

t

r

o

1

Con
INPUT Receptacle

OUTPUT Receptacle

MAX

AMPERES Switch

Connects input to source. Receptacle
UHF Connector.

Connects output to monitoring device.

Selects full-scale current range instrument

measure.

Turns instrument
instrument zero.

Zeroes
Glows to indicate instrument

Selects suppression current magnitude and turns sup­pression current off.

Concentric switches: one determines polarity of sup­pression current, the other (FINE) varies the suppression
from zero to maximum

meter

Rear

Functional Description

on;

selects

on

any range.

set

TABLE

3.

Panel Controls

~~

meter

is

by the

is

polarity; checks

on.

MAX

AMPERES

a

Teflon insulated

is

to

Switch.

I

2-2

2-2

2-2

---

2-2

2-4

1

Par.

2-1

2-5

1

MA

-

1

MA

CAL

117-234

Fuse

I

V

Switch

Control

V

Switch

1

Selects output of instrument:

Adjusts output from 0.95 to 1.05

Sets instrument for 117 or 234 volt ac power line.

3AG

Slow-Blow.

117

volt - .125

milliampere or 1 volt.

mA.

A;

234 volt - .062

A.

2-5

2-5

2-2

2-6

2

0272

6

MODEL

414s

GENERAL DESCRIPTION

RANGE

-WITCH

S

103

3

r

INPUT
5102

I-

POWER ON METER ZERO

INDICATOR

DS201

I

SWITCH CONTROL POLARITY
Slol R131 CONTROL CONTROL SWITCH S102

FIGURE

1.

I

ADJ

S

Front

I

SUPPRESSION SUPPRESSION

104

Panel Controls.

IMA

CA1

FINE

R108

IMA-IY

I

MAX

/

AMPERES

I-

OUTPUT

1MA-lV

TWITCH

S

105

AUTPUT

5103

1

17

-23

f

-ELECTOR

4V

s201

w

I

FIGURE

0272

2.

Rear

Panel

Controls.

3

OPERATION

MODEL

414s

PICOAMPETER

SECTION

2-1.

INPUT

when using the Picoammeter on

a. The INPUT Receptacle of the Model
lon-insulated
the high impedance terminal, and the outer shield

case ground.

b. Carefully shield the input connection and the cur-

rent source being measured, since power line frequencies

well

are

ranges. Unless shielding

definite meter disturbances.

c. Use high resistance,
as polyethylene, polystyrene or Teflon

tion. The insulation resistance of test leads and fix­tures should be several orders of magnitude higher than

the source resistance. Excessive leakage
accuracy. Any coaxial cable used should be a low­noise type which employs a graphite coating between the
dielectric and the surrounding shield braid.

d. Any change in the capacitance of the measuring
circuit to ground
ing, especially

measuring setup as rigid as possible, and tie down con-

necting cables to prevent their movement. If a contin­uous vibration
as a sinusoidal signal and other precautions may be
necessary to isolate the instrument and the connecting
cable from the vibration.

CONNECTIONS. Use

UHF

connector. The center terminal

within the pass band of the Picoammeter on all

will

on

is

cause disturbances in the read-

the more sensitive ranges. Make the

present,

the following precautions
the more sensitive ranges. etc. positions, use the upper meter scale.

414s

is

is

thorough, pickup may cause

low-loss

it

NOTE

materials - such

-

for insula-

will

may appear at the output

2.

a Tef-

is

is

reduce

OPERATION

1.

When the

scale current range
ting.

2.

When the
etc. positions, use the lower meter scale.
scale current range is equal to the
setting

2-3. MEASUREMENT CONSIDERATIONS,

a. The Picoammeter employs the fast method of cur-

rent measurement
the amplifier input and output in the feedback loop.
This method largely neutralizes the effect of input
capacity and greatly increases the response speed.

Also,

of one millivolt on any range.

b. Rise time varies with the current range and the

input capacity (see specifications, Table

time,

across the input; however,
Picoammeter nearer the current source than to the data
reading instrument. Transmitting the input signal
through long cables
of capacitance
noise.

c. The internal resistance of the unknown source
should not be less than the reciprocal of the current
range being used, otherwise the zero stability
affected. The instrument
ever, but the stability
amount given by equation

Stability = 0.5%/week x (Rs + Rf)/Rs

.

the input voltage drop

though,

RANGE

Switch

is

RANGE

Switch is set to 3,

-

the measuring resistor

is

not affected with up to

-

-

with greater than

will

increase response time and meter

will

is

set to

equal to the

is

reduced to a maximum

it

is

better to place the

will

still

be degraded by the

1.

10,

1,

0.1,

RAEiGE

RANGE

be operable, how-

Full

SiJitch set-

0.3,

Full

Switch

is

between

1).

The rise

500

picofarads

500

picofarads

will

Equation

0.03,

be

1.

.

where

Rf

is

Keep the shield cap on the INPUT Receptacle
when the Picoammeter

2-2.

OPERATING PROCEDURES.

a. Check the fuse and the 117-234

proper line voltage.

b. Connect the power cord to the power source.

is

not in a circuit.

V

Switch for the

For example, if the source to be measured has a resis­tance of
back resistor
gain of the Picoammeter
stability of .5%/week
the offset due to temperature

.5%/'C.

RANGE

Set the

c.
SAtch
read zero. Zero the meter with the

After a few moments increase the current sensitivity b

advancing the

ampere range. Continue zeroing with the

trol. The instrument

the instrument to warm up for at least

and turn the METER Switch to the polarity of the input
signal,

Switch until the greatest on scale deflection is achieved,

to

(+).

d. If long term measurements are to be made, allow

e. Attach the current source to the INPUT Receptacle volts without damage.

+

or

-.

Switch to

Within seconds the meter needle should

RANGE

Switch in decade steps to the

is

Increase sensitivity with the

lom2

ampere, the

ZERO

now ready to use.

10

METER

ADJ.

Control.

10-

ZERO ADJ

minutes. meter can withstand overloads of up to

Con-

RANGE

have the source resistance at least equal to the feed­back resistor.

15

d. Overload Protection.
complete overload protection for the Model 414s with­out compromising the features of the MOS
Recovery

1.

for 3 seconds and continuous overloads of up to

2.

is

a function of rated power dissapation in the resistor.

the feedback resistance in ohms;

R,

is

the source resistance in ohms.

105

ohms and the current

will

This

is

is

instantaneous for most overloads.

At

the ampere range and below the Picoam-

Above amperes, the max. continuous overload

4

be

106

ohms. his means that the

is

106/10T

will

be

.5%

the reason that

will

A

unique circuit provides

is

lom6

then the feed-

=

10.

X

be

it

Then the zero

10 = 5%/week, and

.05%

X

is

lo*=

advantageous to

E'ET

input.

1000

volts

600

02

-

..4

72

MODEL

414s

OPERATION

f-

Allowable Overloads

Range Voltage Overload Current Overload

10-5

A

10-4

A

10-3

A

A

3. For maximum protection, use
240A Voltage Supply, or some other 10 milliampere FIGURE 3. Divider Circuits Across Picoammeter Output
current limited supply, in combination with the for Driving

Picoammeter.

2-4.

RECORDER

a.

For recording with the Model 414S, use the Keith­ley Model 370 Recorder for
and performance. The Model 370

10 chart speeds and
put cable has
the OUTPUT Connector

interface problems often encountered between a measur-

ing instrument and
when

set

preamplifier

r

b. Other recorders, oscilloscopes and

struments can be used with the Model 414s. The Pico­ammeter has two outputs,

to amplify signals for recorders, oscilloscopes and

similar

recorder amplifiers to the OUTPUT Receptacle. Pin

1

the

is

range. Internal resistance

Noise
METER Switch does not reverse the output polarity.

Output polarity

instruments to the OUTPUT Receptacle. Pin

the high terminal. Set the

The output

scale
apply
adjust the
full scale, Check the recorder and
repeat adjustment if necessary. The METER Switch does
not

rsite

across the Picoammeter OUTPUT Receptacle. See Figure

3. Set the OUTPUT Switch to

sc"

Control to
flections. Operation

instruments. These can be used

c. 1-Volt Output. Connect oscilloscopes and pen

is

the high terminal and pin

1

mA

now

21

is

d. 1-Milliampere Output. Connect 1-milliampere

meter

a

known full scale signal to the Picoammeter and

reverse

input polarity.

e.

For servo rebalance recorders, use a divider

OUTPUTS.

a

connector which

to the 1 mA

is

needed. No special wiring

-

1

V

Switch to

volt for full scale meter deflection

less

than

is

is

approximately 1 milliampere for full-

deflection

1

mA

CAL

the output polarity which

trim

the output for full-scale recorder de-

TABLE

4.

on

Ranges Above Ampere.

Max. Continuous Max. Continuous

300

V

120

v

30

V

12

v

a

Keithley Model

ease,

economy, versatility

is

1%

linearity. The Model 370's in-

on

the Picoammeter; this avoids

a

recorder. The Picoammeter output,
position,

a pen recorder with

mates

directly with

will

drive the 370;

similar

51

volt and

1

V.

is

1%

peak-to-peak of full scale. The

always opposite input polarity.

1

mA

on

any range. For

Control until the recorder reads

1

is

the

same

21

milliampere,

on

no. 2 is

The Picoammeter output

approximately 1 kilohm.

-

ground. Set

1

V

Switch to

exact

meter

zero and

is

always oppo-

mA.

Use

the

as

for current outputs.

1mA

4mA

10

mA

40

mA

is

required.

all

ranges.

no.

1

mA

no

in-

no.

on

any

1

is

1

mA.

output,

CAL

1

OUTPUT

5103

RECORDER
OUTPUT

L

50

and 100-Millivolt Recorders.

2-5.,

234-VOLT OPERATION. The instrument
for use with
ordered. To convert the Picoammeter for 234-volt
sources, use

on

the back panel to the 234-volt position. Change'

the fuse from 0.125 ampere to ,062 ampere.

adjustment
volt operation, reverse the procedures.

2-6.

SUPPRESSION

provides up to 100 times current suppression over a
range of
ground currents, variations as
signal can be displayed full scale. Currents of either
polarity may be suppressed.

a. Suppression
opposite polarity to the input current at the input of
the Picoammeter. This suppression current
from

well

with resistors.

b.

Use

1.

-,

as

2. Set the SUPPRESSION,

same

the 10-turn FINE Control for

meter.

one decade. Adjust the FINE Control for

ing. This

tain

of the

meter.

fluctuations

current.

c.

When using

resistor used in the suppression circuit

tenth the value
this mode the zero drift and temperature drift
approximately ten

discussion

a

117-volt power source unless otherwise

a

screwdriver to change the slide switch

is

necessary. To switch from 234 to

loe3

CURRENT

to

lom9

OPERATION. The Model 414s

ampere. By suppressing back-

small

is

achieved by applying a current of

regulated 212 volt supplies in conjunction

the suppression circuit

Set the SUPPRESSION, Polarity Control to

necessary.

sensitivity

Increase the sensitivity of the

100

times

RANGE

In this mode the Model 414s

in

as

the

RANGE

is

ten

times

suppression, increase the sensitivity

Switch one more decade and zero the

of

1%

100

of

the current measuring resistor. In

times

paragraph 2-3c for explanation).

current suppression. To ob-

of full scale of the Suppression

times

full scale suppression the

greater than normal. (See the

as

MAX

AMPERES Switch to the

Switch setting. Adjust

a

zero reading

is

shipped

No

as

1%

in a larger

is

follows:

RANGE

a

zero read-

is

able to detec*

will

be

obtained

on

other

117-

+

or

the

Switch

one-

will

be

I

I

0272

5

CIRCUIT

DESCRIPTION

MODEL

414s

SECTION

3-1.

GENERAL.

amplifier with
3 volts. By using the front panel controls, shunt
sistors
of
223943 for circuit designations.)

3-2.

effect input transistors followed by

transistor stage,

sistor output stage. Figure 4 shows the simplified

circuit for the Picoammeter.

are

17

current ranges. (Refer to schematic diagram

AMMETER

a.

The amplifier has matched insulated-gate field-

The

Keithley Model 414s

a

full scale sensitivity of either 1 or

selected to make measurements over a total

OPERATION.

a

a

transistor amplifier and a tran-

Rs

3.

is

a

linear dc

differential

CIRCUIT DESCRIPTION

3-3.

re-

AMMETER

a.

Two

sistors,
put. Resistors R125 and R126 and circuit designation
21850B (see schematic) protect the gate of transistor

QlOl,

the active field-effect transistor, from over-

loads. The gate of Q102

ground.

b. Turning the

places

the instrument.

and Q104, drives an amplifier stage, transistor
which in turn drives the output
sistor 4106.

Control, R128 varies the source voltage of transistor

QlOl

Control, R131, varies the drain voltage of transistor

QlOl

current through transistors
the source bias,

a

c.

A

Two

d.

with respect to transistor 4102. The
with respect to transistor 4102.

e.

The

CIRCUIT.

balanced insulated-gate field-effect tran-

QlOl

and 4102,

short from the input to the output and zeroes

differential amplifier stage, transistors Q103

zero controls

DC

METER

Bal

Potentiometer,

are

used for the amplifier in-

is

returned to amplifier

Switch to

are

ZERO

CK

position

emitter

used. The Coarse Zero

R127,

QlOl

and 4102 by varying

follower, tran-

ZERO

sets

the drain

.

QlOS,

ADJ

FIGURE

b.

ein,

flows through the measuring resistor,

But the output voltage,

voltage

Therefore, from equations 2 and

From whence,

where

Thus, the input voltage drop

tion,

put resistance

Simplified Diagram for

4.

If

it

is

is

negligible, then

eo

=

times

eo

A

ein

ein/iin

1/A,

assumed that the input voltage drop,

-iin

R,.

the amplifier gain.

-A

ein. Equation 3.

=

iin

R,.

is

the effective input resistance.

of

the output voltage, and the effective in-

is

1/A

all

eo,

of the measuring resistor.

Ammeter

the input current,

is

also equal to the input

3

we

is

kept

Circuit.

R,

,

and

Equation 2.

get

Equation 4.

Equation

at

a

small

iin,

5.

frac-

f. The voltage drop across R123 plus R124, or R124
alone, determines the full scale sensitivity of the
amplifier
and 3 x

10-10

put causes
R123 and R124. The
volt output. Resistors R138 and R139

current.

g. The full scale current sensitivity
by the range resistors R109 through R122 in combination
with resistors R123 and R124. The current measuring

resistor
figuration increases the response speed by minimizing
the effects of input capacity.
input voltage drop to

h. The
milliampere flowing through resistor R143.
milliampere output mode an external load
for R143. Potentiometer R141 varies the current to the
external load.

3-4. POWER SUPPLIES. The Model 414s has positive and
negative supplies, which provide power to the ampli­fier and output.

a.

secondary of transformer T201
D201 and D203 and filtered by capacitor C201 to provide

+25 volts for the output. Zener diode, D205,

to provide

b. -25 and -12 volt supply. This supply uses diodes
D202 and D204, capacitor C202 and Zener D206 to per­form the

-

either 3 volts from throu h 3

lom9

ranges.

a

and 3 x

Applying

1

milliampere current to flow through

1O-lo

meter

or 1 volt on

a

full

scale

signal to the in-

is

connected across the 3-

lO-Q,

set

is

is

connected in the feedback loop. This con-

It

less

than 1 millivolt.

1

volt recorder output

also reduces the

is

derived from

is

+25 and +12 volt supply. The power supplied from

a

regulated

same

functions

+11

is

rectified by diodes

volts for the amplifier.

as

the positive supply.

x

the

meter

determined

1

In the

substituted

is

used

and

1

c

?

6

0272