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HP E1415 Noise Rejection
Normal Mode Noise
(Enm)
Common Mode
Noise (Ecm)
Keeping Common
Mode Noise out of
the Amplifier
364 Wiring and Noise Reduction Methods
See Figure 6-8 for the following discussion.
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 HP E1502,
E1503, E1508, and E1509 SCPs.
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 (HP E1501). However, high frequency Ecm is
rectified and generates an offset with the amplifier and filter SCPs (such as
HP E1502, HP E1503, HP E1508, and HP E1509). 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.
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 HP E1415 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, HP elected to handle this with the High Frequency Common Mode
Filter option to the HP E1586A Remote Rack Panel (HP E1586 Option 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 HP E1415 chassis ground through a very low impedance.
(Not via the guard terminal - The HP E1415 guard terminal connection
shown in the HP E1415 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. We recommend
that you follow this practice, and if you see high frequency common mode
noise (or suspect it), tie the shield to the HP E1415 ground through a 0.1 µF
capacitor. At high frequencies, this drives the shield voltage to 0 volts at the
HP E1415 input. Due to inductive coupling to the signal leads, the Ecm
voltage on the signal leads is also driven to zero.
Appendix E
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