VXI VT1422A - Manual (Page 488)

486 Wiring and Noise Reduction Methods Appendix E
VT1422A Noise Rejection
Normal Mode Noise
(Enm)
This noise is actually present at the signal source and is a differential noise
(Hi to Lo). It is what is filtered out by the buffered filters on the VT1502A,
VT1503A, VT1508A, and VT1509A SCPs.
Common Mode
Noise (Ecm)
This noise is common to both the Hi and Lo differential signal inputs.
Low frequency Ecm is very effectively rejected by a good differential
instrumentation amplifier and it can be averaged out when measured
through the Direct Input SCP (VT1501A). However, high-frequency Ecm is
rectified and generates an offset with the amplifier and filter SCPs (such as
VT1502A, VT1503A, VT1508A and VT1509A). This is since these SCPs
have buffer-amplifiers on board and is a characteristic of amplifiers. The
best way to deal with this is to prevent the noise from getting into the
amplifier.
Keeping Common
Mode Noise out of
the Amplifier
Most common mode noise is about 60 Hz, so the differential amplifier
rejection is very good. The amplifier Common Mode Noise characteristics are:
120 dB flat to 300 Hz, then 20 dB/octave rolloff
The VT1422A amplifiers are selected for low gain error, offset, temperature
drift and low power. These characteristics are generally incompatible with
good high frequency CMR performance. More expensive, high performance
amplifiers can solve this problem, but they aren't required for many systems.
Shielded, twisted pair lead wire generally does a good job of keeping high
frequency common mode noise out of the amplifier, provided the shield is
connected to the VT1422A chassis ground through a very low impedance
(not via the guard terminal). The VT1422A guard terminal connection
shown in the VT1422A User's Manual does not consider the high-frequency
Ecm problem and is there to limit the shield current and to allow the DUT to
float up to some dc common mode voltage subject to the maximum
±16 volt
input specification limit.
This conflicts with the often recommended good practice of grounding the
shield at the signal source and only at that point to eliminate line frequency
ground loops, which can be high enough to burn up a shield. It is
recommended that this practice be followed and if high frequency common
mode noise is seen (or suspected), tie the shield to the VT1422A ground
through a 0.1 µF capacitor. At high frequencies, this drives the shield
voltage to 0 volts at the VT1422A input. Due to inductive coupling to the
signal leads, the Ecm voltage on the signal leads is also driven to zero.