LeCroy DA1855A Operation page 18

DA1855A Differential Amplifier
The DXC100A Differential Probe Pair
Probe Grounding
3-18
(one probe always on the +INPUT and the other always on the –
INPUT).
The DXC100A is a high performance matched passive differential
probe pair designed for use with LeCroy DA1855A series
differential amplifiers. The probe pair consists of two well-
matched individual probes that share a common compensation
box to allow the attenuation factor on both probes to be
simultaneously switched between ÷10 and ÷100. When used with
the DA1855A Differential Amplifier, the probe's attenuation factor
is automatically incorporated into the effective gain display and
the decimal properly located in the Precision Voltage Generator
(PVG) display.
The DXC100A Probe Pair is supplied with accessories that allow
for three methods of connecting probe grounds.
In most cases, when the common mode portion of the signal
consists mainly of low frequencies (1 MHz and below), the probe
ground leads should not be connected to the ground of the circuit
under test. They should be connected to each other. This
minimizes the effects of ground loop currents. The signal
corruption caused by not having the probes connected to the
ground of the circuit under test will be common to both inputs and
will be rejected by the differential amplifier.
However, when working in an environment with high RF ambient
noise, it is best to connect the probe ground leads to a good RF
ground near the point where the signal is being measured.
The best way to determine which probe grounding technique
should be used is to try both methods and use the one that gives
the least corruption of the differential signal.
When adjusting the compensation and probe CMRR, the use of
probe tip to BNC adapters is required. They provide the best
performance of the three grounding method.
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ISSUED: July 2002 DA1855A-OM-E Rev A