Teledyne LeCroy DA1855A - Manual (Page 78)

DA1855A Differential Amplifier
72 922258-00 Rev A
s. Verify that the oscilloscope is set to display channel 1 and the\ input coupling to DC
and 50Ω.
t. Increase the channel 1 vertical sensitivity to maximum.
u. Verify that the oscilloscope is triggered on the Frequency Reference Signal Output of
the sine wave generator.
v. Measure the peak to peak amplitude. 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: The amplitude of the Common Mode Feedthrough should be very small. If
the output waveform appears to be 1 Volt square wave, check that both of the
DA1855A inputs are set to DC.
w. Record the Common Mode Feedthrough amplitude to two digit resolution in the
Test Record as ‘Common Mode Feedthrough at 10 MHz’.
x. Calculate the Common Mode Gain by dividing the Common Mode Feedthrough (in
mV) by 5,000 mV.
y. Record the result to two significant places as ‘Common Mode Gain at 10 MHz’ in the
Test Record. (Keep all of the leading zero’s or use scientific notation.)
z. Calculate the Common Mode Rejection Ratio (CMRR) at 10 MHz by dividing the
Differential Mode Gain at 10 MHz as recorded in step 5-f by the Common Mode
Gain recorded in step 5-y.
aa. Record the result to two significant places as ‘Common Mode Rejection Ratio at 10
MHz’ in the Test Record. (Keep all of the trailing zero’s).
bb. Check that the CMRR at 10 MHz is greater than 316:1 (50 dB)
cc. Disassemble setup by removing all cable, adapters, etc. from the instruments.
6. Check Low Frequency CMRR.
NOTE: The attenuation of the DA1855A 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 the zoom and averaging function to boost the level of the
common mode feedthrough to an amplitude where it can be measured.
a. Connect a BNC cable from the output of the high amplitude sine wave generator to
the channel 1 input of the oscilloscope. Do not terminate the cable into 50 Ω, and
verify that channel 1 coupling is set to DC and 1MΩ.