Normal Mode Noise (Enm); Common Mode Noise (Ecm); Keeping Common Mode Noise Out Of The Amplifier - HP E1313A Manual

Normal Mode Noise
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,
(Enm)
HP E1503, HP E1508, and HP E1509 SCPs.
Common Mode
This noise is common to both the Hi and Lo differential signal inputs. Low
frequency Ecm is very effectively rejected by a good differential
Noise (Ecm)
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.
Figure F-1. HF Common Mode Filters
Keeping Common
Most common mode noise is about 60 Hz, so the differential amplifier rejection
is very good. The amplifier Common Mode Noise characteristics are:
Mode Noise Out of
the Amplifier
120 dB flat to 300 Hz, then 20 dB/octave rolloff
The HP E1313/E1413 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 are not required for many systems, HP elected to handle this with the
High Frequency Common Mode Filter option to the HP E1586A Rack
Mount Terminal Panel (HP E1586 Option 001, HF Filter).
Appendix F Wiring and Noise Reduction Methods 403
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