LeCroy DA1855A Operation page 17

Probes and Differential Amplifiers
DA1855A-OM-E Rev A
When using a differential amplifier it is very important to
understand the role probes play in the overall measurement
system performance. Probes not only make attachment to the
circuit under test more convenient, ÷10 and ÷100 attenuating
probes also extend the common mode range of the differential
amplifier. For example, the DA1855A amplifiers have a common
mode range of ±15.5 volts when their internal attenuators are set
to ÷1 and 155 volts when set to ÷10. The addition of a probe with
an attenuation factor of ten will extend the common mode range
to 1550 volts or the rating of the probe, whichever is less.
There is a trade-off, however. The Common Mode Rejection
Ratio (CMRR) capability of even highly matched differential probe
pairs is seldom as good that of the amplifier. In order to preserve
as much of the amplifier's performance as possible at the probe
tips, it is important to use probes that are designed for differential
performance. Attempting to use normal ÷10 or ÷100 attenuating
oscilloscope probes, even high quality probes, will result in very
poor CMRR performance. Nominally matching ÷1 probes
however, will provide excellent common mode rejection and are
recommended.
For applications which do not require additional attenuation, ÷1
probes present relative high capacitive loading to the circuit
under test, limiting their usefulness to low frequency
measurements.
When making differential measurements, accurate probe
compensation is much more important than in single-ended
measurements. Most probes depend on the accuracy of the
oscilloscope's 1 M input resistor to determine the accuracy of
the probe's attenuation factor. Two probes with a 1% accuracy
specification can yield a CMRR as low as 50 to 1 at DC while the
amplifier CMRR may be higher than 100,000 to 1. At high
frequencies, the CMRR will be worse.
A differential probe pair must allow for matching at DC as well as
over their useful frequency range. Changing the compensation of
a differentially matched probe set without following the proper
compensation procedure can result in a significant decrease in
the CMRR capability of any differential probe pair.
It is a good practice to compensate a probe pair for a given
amplifier and then leave the probe pair and amplifier together as
a system. Similarly, it is important that, once compensated for a
given amplifier, each probe always be used on the same input
ISSUED: July 2002
Operation
3-17