Specifications

VT1419A Noise Rejection
See Figure D-2 for the following discussion.
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 VT1419A 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 since they aren’t required for many systems, elected to
handle this with the High Frequency Common Mode Filter option to the VT1586A
Remote Rack Panel (VT1586A-001, RF Filter) discussed below.
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 VT1419A chassis ground through a very low impedance. (Not via
the guard terminal - The VT1419A guard terminal connection shown in the
VT1419A 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 VT1419A ground through a 0.1 µF capacitor. At high frequencies,
this drives the shield voltage to 0 volts at the VT1419A input. Due to inductive
coupling to the signal leads, the Ecm voltage on the signal leads is also driven to
zero.
Wiring and Noise Reduction Methods
374 Appendix D
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