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MODEL
616
OPERATING INSTRUCTIONS
b.
Fast.
Mode
C
ur
rent Measurements,
(Useable for
currents from lO^^A
and smaller.)
in this
mode,
the
Model
616
measures very small currents with fast
response.
When
the
FAST/NOEMAL switch
is
set
to
FAST, the
Model
616 operates
as a
feedback-type
picaommeter in which
the
current flows through
the
feedback resistor
of the
voltage amplifier.
The
Model
616
indicates
the
voltage developed across
the range
resistor where Vp
x Ry.
This
method
of
me.asurement provides fast
response since
the
effect
of the
input capacitance from lengthy
cables
is
diminished.
To use
the
Model 616 as
a
feedback-type picoammeter,
set the front
panel
controls
as
follows:
RANGE
—
10"^^ AMPERE
SENSITIVITY
~
AUTO
FAST/NORMAL
—
FAST
Make connections
to
the
Model
616 using a
shielded
triaxlal cable.
The zero control may need occasion-
al
adjustment
to
reduce any voltage offset
which
may occur
due
to
temperature variations.
To rezero
the
Model
616, set the ZERO
CHECK switch
to
CHECK
position
and adjust
the
front panel ZERO control
for a
O-O-O-O display.
(The
recommended
warm-up
time for the
Model
616
is
two hours
although it
is
useable immediately after power
is
turned on.)
NOTE
The Model 616 may exhibit
a
large amount
of
zero offset or drift immediately after an
overload
or a
long period
of
storage.
In
addition,
the
offset current may exceed the
specification
for
a
short time until the
Model
616
is
well below offset found in
conventional voltmeters,
the
effects of the
offset current charging
the
input capaci-
tance
may be noticed when
the
input
is
open.
Use
the ZERO
CHECK switch
to
discharge the
offset charge.
S)
Considerations When Measuring Current.
When
using
the
Model
616 in
either
the
NORMAL or FAST
modes, 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.
First, small
value resis-
tors are of
higher quality compared
to
large values
with respect
to
accuracy, time stability, tempera-
ture, and
voltage coefficient.
Second,
ideal plco-
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
in faster response.
However, since noise and zero
drift become more significant when
the
voltage
amplifier sensitivity
is
set
to 10 or 100
millivolts.
the
appropriate range multiplier should be selected
with
a one
volt full scale sensitivity.
The use
of
the
Model
616 in the
FAST mode enables measurements
of
very small currents while reducing
the
source
leading and input
RC time
constant.
Since the range
resistor
is
connected in
the
feedback
of the
volt-
age
amplifier,
the
effective input resistance
is
determined
by the open-loop gain where
= Ry
t
A.
(The
value
of
A
is
typically 10,000.)
Another
ad-
vantage
of
the
feedback mode
is
Che
reduced effect
of
input capacitance since
the input RC time con-
stant
is
decreased by
the
gain A
"
RC
^
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 across
the
resistor (usually
a
few pF).
Thus,
when compar-
ing shunt versus
feedback modes,
it is
evident Chat
Che
feedback measurement offers superior performance
for
most applications.
TABLE
3-4.
Current Settings
Input
Range
Sensitivity
Digital
Typical
Current
Setting
Setting
Display
Accuracy*
10"^
A
lO'I^A
";S
10
10
mV
10
mV
.00010
.00100
+
+
2
d
6
d
10
:^A
io"::a
10
mV
.01000
+
51 d
100
mV
.1000
+
51 d
1
V
1.000
±
51
d
10"J^A
lo-f
1
V
1.000
±
51 d
10"^
1
V
1.000
±
51 d
10-®
A
10"®
1
V
1.000
±
21 d
10
'
A
10"^
1
V
1.000
±
6
d
loi
A
10-^5
1
V
1.000
±
6 d
10
,
A
1
V
1.000
±
6
d
lO-A
A
10"^
1
V
1.000
±
6
d
10"
^
A
1
V
1.000
±
6
d
10-2
^
10"^
1
V
1.000
±
6 d
10
A
10
1
V
1.000
±
6 d
*exclusive
of
noise, zero drift, and loading
error.
NOTE
The use
of the
FAST mode
for current measurements
is
subject
to
certain restrictions
as
follows:
1.
The range resistor value should not be
greater than the source resistance, otherwise
feedback division results In zero instability
at
the output.
2.
The largest value
of
current measured
should not exceed
10"^ ampere, otherwise
the
full feedback current cannot be developed.
The NORMAL
mode should be
used for currents
from
200
mA
to
10~^A.
0374
9
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