The dynamic range
of
the display is
-+400,000
counts. This
means that
if
a
REL
level of -2.OOOOOV is established, then
applying +1.99999V to the input displays +3.99999V.
Once the REL level is established, that level is algebraically
subtracted from the input signal.
For
example:
if
a
+
1
.OOOOOV
REL level is established, then applying a
f2.OOOOOV level displays a
+1.OOOOOV
level.
The instrument accepts a +220,000 count input before going
into the overrange condition
(OL).
For example:
-1.OOOOOV
REL
level is established and 1.50000V is applied. The instru-
ment displays +2.500000V. If the input is 2.20000V the in-
strument overranges.
7
CAUTION.
1OOOV
MAXIMUM INPUT
INPUT RESISTANCE
=
10M. EXCEPT
>
lOOOM
on
200mV
and
2V
RANGES
o/
MODEL
197
I
Figure
2-2.
DC
Voltage Measurements
2.7.3
DC
Voltage Measurement
The Model 197 can make DC voltage measurements between
1pV and
IOOOV.
The basic procedure is as follows:
1.
Connect the test leads to the
HI
and
LO
INPUT
terminals
2. Select the DCV function.
3. Select a range consistent with the expected voltage. For
automatic range selection, press in the
AUTO
button.
of the Model 197.
NOTE
To prolong instrument life, manual ranging is
recommended for routine measurements above
200v.
4.
Connect the test leads to the source as shown in Figure 2-2.
If
the positive source terminal is connected to the
LO
ter-
minal of the instrument, the display shows a negative
value.
If
the negative source terminal is connected to the
LO
terminal, the display shows a positive value.
5. Observe the display;
If
the
"OL"
message is shown, select a
higher range until a normal reading is displayed. Always
use the lowest possible range for the best resolution.
6.
Take the reading from the display.
The term "when properly zeroed" means that the user must
establish a proper baseline for subsequent measurements on
that range. To zero the Model 197 use the following pro-
cedure:
1.
Short the INPUT
HI
and
LO
test leads together.
2. Wait until the displayed reading settles. Noise and thermal
emfs may require
a
few moments
to
settle out.
3. Press the
REL
button.
The 200mV DC range and the 2000 range require zero to be
set in order to achieve rated accuracy.
2.7.4
TRMS
AC
Voltage Measurements
The Model
197
can make TRMS AC voltage measurements
between
1pV
and 750V. To measure AC proceed as follows:
1.
Connect the test leads
to
the
HI
and
LO
terminals
of
the
2. Select the ACV function.
3.
Select a range consistent with the expected voltage.
For
automatic range selection, press in the AUTO button.
4.
Connect the test leads to the source as shown in Figure 2-3.
Model 197.
NOTE
There is a small amount of offset (typically 50
counts
or
less) present when using the
AC
func-
tion.
Do
not REL this level out. The reason for
this is that the offset is generally negligible as
compared to the input signal.
For
example:
offset
=
22pV
applied signal
=
20mV
displayed reading =\(20mV)2
+
(22pV)Z
=
.0200000121
=m-+484
x
10-iZ
This offset is seen as the last digit which is not
displayed on the Model 197. Therefore, the off-
set is negligible.
5.
Observe the display,
if
the
"OL"
message is shown, select a
higher range until a normal reading is displayed. Always
use the lowest possible range to obtain the best resolution.
2-6
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