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MODEL 616
b.
Fast Mode C
rrent Measurements.
�
currents from 10 - A and smaller.)
Model 616 measures very small currents with fast
response.
When the FAST/NORMAL switch is set to
FAST, the Model 616 operates
picoammeter 1n which the current flows through the
feedback resistor of the voltage amplifier.
Model 616 indicates the voltage developed across
the range resistor where V
method of measurement provides fast response since
the effect of the input capacitance from lengthy
cables is diminished.
feedback - type picoammeter, set the front panel
controls as follows:
RANGE
SENSITIVITY
FAST/NORMAL
Connect a Model
Triax cable (or other shielded
6011
triaxial cable) to the INPUT receptacle.
control may need occasional adjustment to reduce any
voltage offset which may occur due to temperature
variations.
To rezero the Model
CHECK switch to CHECK position and adjust the front
panel ZERO control for a 0-0-0-0 display.
The Model 616 may exhibit a large amount of
zero offset or drift immediately after an
overload or a long period of storage.
addition, the offset current may exceed the
specification for a short time until the
Model
is sufficiently stabilized.
616
though the
offset current of the Model 616
is well below offset found in conventional
voltmeters, the effects of the offset current
charging the input capacitance may be hot iced
when the input is open.
switch to discharge the offset charge.
�)
Considerations When Measuring Current.
using the Model 616 in either the NORMAL or FAST
nodes, there are certain considerations which ensure
optimum performance.
In general, it is advantageous
to use the smallest possible value of shunt resis
tance for a picoammeter.
tors are of higher quality compared to large values
with respect to accuracy, time stability, tempera
ture, and voltage coefficient.
ammeters require very small input resistance so that
source loading does not affect the overall accuracy
of the measurement.
Since the input resistance
in a shunt picoammeter is determined by the range
resistor, the RANGE multiplier should be set at the
lowest value.
Third, the use of a low value resis
tor reduces the input RC time constant and results
tn faster response.
However, since noise and zero
trift become more significant when the voltage
lmplifier sensitivity is set to
J 4 76
(Useable for
In this mode, the
8S
a feedback - type
The
x R
,
This
=
F
F
To use the Model 616 as a
11
10 -
AMPERE
AUTO
FAST
The zero
set the ZERO
616,
In
Al
Use the ZERO CHECK
First, small value resis
Second, ideal pico
or
millivolts,
10
100
the appropriate range multiplier should be selected
with a one volt full scale sensitivity.
the Nodel 616 in the FAST mode enables measurements
of very small currents while reducing the source
loading and input RC time constant.
reSistor is connected in the feedback of the volt
age amplifier, the effective input resistance is
determined by the open - loop gain where R
(The value of A is typically
vantage of the feedback mode is the reduced effect
of input capacitance since the input RC time co�
stant is decreased by the gain A (RC E FF ECTIVE
A).
As a result, the measurement speed is governed
by the feedback time constant which is a function
of the feedback resistor and stray capacitance ac ross
the resistor (usually a few pF).
ing shunt versus feedback modes, it is evident that
the feedback measurement offers superior performance
for most applications.
HOW TO SELECT APPROPRIATE AMPERES RANGE.
Current To
Be Measured
10- 15 A
1O - 1 4 A
10 - 1 3A
-12
A
10
lO - 11 A
1O
1O -
A
10 - 9 A
10- 8 A
7
A
10 -
10 - 6 A
10- 5
A
10 - 4 A
10- 3 A
10- 2 A
A
10- 1
When
The use of the FAST mode for current measurements
is subject to certain restrictions as followst
1.
greater than the source resistance, otherwise
feedback division results in zero instability at
the output.
2.
should not exceed 10 - 5 ampere, otherwise the
full feedback current cannot be developed.
The NORMAL mode should be used for currents
from
OPERATING INSTRUCTIONS
10,000.)
TABLE 3 - 4 .
Appropriate
Appropriate
Range
Sensitivity
10- 11
1
mV
0
-
11
1
mV
0
10
11
mV
10 -
10
- 1
1
1
mV
0
100
10 - 11
1
V
10
1
V
10 -
-9
1
V
10
10 -8
1
V
7
V
10 -
1
10- 6
1
V
1 0
1
-
5
V
10- 4
V
1
10- 3
V
1
10- 2
V
1
10 - 1
1
V
NOTE
The range resistor value should not be
The largest value of current measured
mA
4
to
200
10-
A.
The use of
Since the range
= Rr f
A.
IN
Another ad
RC f
=
Thus, when compar
x lO - 11 A
. 00010
x lO - 11 A
.00100
x lO -11 A
.01000
11
x 10 -
A
. 1000
x 10 - 11 A
1.
000
10
x 10 -
A
1. 000
x 10 -9
1.
A
000
1.000 x 10 - 8
A
7
A
x
10 -
1. 000
x 10 - 6
A
1.000
1. 000 x
10- 5
A
x 10 -4
A
1.000
- 3
A
x
1. 000
10
x 10- 2
A
1.000
x 10 - 1
A
1. 000
9
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