Teledyne LeCroy DA1855A - Manual (Page 81)

Operator’s Manual
922258-00 Rev A 75
o. Calculate the Common Mode Rejection ratio (CMRR) at70 Hz by dividing the
Differential Mode Gain at 70 Hz (1.0) by the Common Mode Gain (recorded in step
6-p). Record the result as ‘Common Mode Rejection Ratio at 70 Hz’ to two
significant places in the Test Record. (Keep all of the trailing zeros.)
p. Check that the CMRR at 70 Hz is greater than 50,000:1(94 dB).
q. Remove the sine wave generator output cable from theDA1855A’s + input.
Reconnect the cable to the channel 1input of the oscilloscope.
r. Set the oscilloscope to display channel 1, The coupling to DC and 1 MΩ, the vertical
scale to 5 V/div and the horizontal scale to 5 μs/div. If necessary, adjust the trigger
level for a stable display.
s. Set sine wave generator to 100 kHz.
t. Adjust the output amplitude of the sine wave generator to20 Vp-p (4 divisions).
Readjust the oscilloscope trigger level of necessary to maintain a stable display.
u. Remove the sine wave generator output cable and reconnect it to the female to
female BNC adapter and cables attached to the DA1855A inputs. Refer to Figure 21.
v. Connect the DA1855A output to channel 1 of the oscilloscope
w. Set the oscilloscope to display channel 1, input coupling to DC and 50Ω and the
vertical scale as necessary to measure the amplitude of the displayed signal. The
displayed signal is the Common Mode Feedthrough.(Use the oscilloscope ZOOM
function and averaging if needed to increase the size of the displayed waveform and
to reduce noise.)
NOTE: This measurement needs to be made very carefully. The signal is only several
hundred μV in amplitude and measuring the peak to peak amplitude of this signal,
using oscilloscope measurement functions, may cause erroneous reading. Measure
only the amplitude of the common mode feedthrough, not the total value of the
signal plus noise.
x. Record the displayed amplitude as ‘Common Mode Feedthrough at 100 kHz’ to two
digits resolution in the Test Record.
y. Calculate the Common Mode Gain by dividing the Common Mode Feedthrough (in
μV) by 20,000,000 μV. Record the result as ‘Common Mode Gain at 100 kHz’ to two
significant places in the Test Record. (Keep all of the leading zeros or use scientific
notation.)
z. Calculate the Common Mode Rejection Ratio (CMRR) at