Specifications

1820/1822 Operator’s Manual
at I MHz’ in the Test Record. This is the
Differential Mode Gain at 1 MHz.
h.
Remove the leveled sine wave generator from
the input of the 1820/1822.
i. Connect a BNC cable from the output of the
high amplitude sine wave generator to the
Channel 2 input of the oscilloscope. Do not
terminate the cable into 50 f2, and verify that
channel 2 coupling is set to DC and 1 Mr2.
j. Set the oscilloscope to display channel 2,
vertical scale to 1 V/div, horizontal scale to
500 ns/div, and trigger source to external +10.
If necessary, adjust the trigger level for a
stable display.
k. Set the sine wave generator frequency to
i MHz, and the output amplitude to exactly 5
Vp-p (5 divisions on the oscilloscope).
I. Remove the sine wave generator output cable
from the oscilloscope. On the free end of the
BNC cable, install the female to female BNC
adapter, BNC ’Y’ adapter and two 6" BNC
cables.
m. Set both the 1820/1822 +INPUT and-
INPUT to DC.
n. Connect the two free ends of the 6" cables to
the 1820/1822 +INPUT and -INPUT.
o. Set the oscilloscope to display channel 1
(1820/I 822 output signal). Increase the
vertical sensitivity to maximum. Verify that
the oscilloscope is still triggered on the
Frequency Reference Signal Output of the
sine wave generator.
p. The displayed signal is the Common Mode
Feedthrough. Measure the peak to peak
amplitude. (Use the oscilloscope ZOOM
function if needed to increase the size of the
displayed waveform.
NOTE
The amplitude of the Common Mode Feedthrough
should be very small lf the output waveform appears to
be a 10 V square wave, check that both of the
1820/1822 inputs are set to DC.
q. Record the ’Common Mode Feedthrough at 1
MHz’ amplitude to two digit resolution in the
Test Record.
r. Calculate the Common Mode Gain by
dividing the Common Mode Feedthrough (in
4-8
mV) by 5,000 mV. Record the result to two
significant places as ’Common Mode Gain at
1 MHz’ in the Test Record. (Keep all of the
leading zeros or use scientific notation.)
s.
Calculate the Common Mode Rejection Ratio
(CMRR) at 1 MHz by dividing the
Differential Mode Gain at 1MHz (recorded in
step 6-0 by the Common Mode Gain recorded
in step 6-u. Record the result as ’Common
Mode Rejection ratio at 1 MHz’ to two
significant places in the Test Record. (Keep
all of the trailing zeros.)
t. CHECK -- That the CMRR at 1 MHz is
greater than 1,000:1 (60 dB).
u. Leave the test setup for the next tests.
7. Check Low Frequency CMRR.
NOTE
The attenuation of the 1820/1822 at 70 Hz and 100 kHz
is so insignificant that the Differential Mode Gain can
be assumed to be unity (1.0). However, the high value
of the CMRR specification requires a preamplifier to
boost the level of the common mode feedthrough to an
amplitude where it can be measured.
a.
Disconnect the output cable of the 1820/1822
under test from channel 1 of the oscilloscope.
Install a 50 f2 inline BNC termination the free
end of the cable.
b.
Connect the terminated input to the + input of
the oscilloscope preamplifier. Using another
BNC cable connect the output of the
oscilloscope preamplifier to the channel 1
input of the oscilloscope.
c. Set the input coupling termination of the
oscilloscope channel 1 to what is required by
the oscilloscope preamplifier (50 f~ ifa 1822
is being used.)
d. Disconnect the output cable of the sine wave
generator from the female to female BNC
adapter. Reconnect the free end of the cable
to the channel 2 input of the oscilloscope.
e. Set the oscilloscope to display channel 2,
vertical scale to 5 V/div and horizontal scale
to 10 ms/div.
f.
Set the sine wave generator frequency to 70
Hz and the output amplitude to exactly 30 Vp-
p (6 divisions). Adjust the oscilloscope
trigger level for a stable display if necessary.