Specifications

Reducing Common
Mode Rejection
Using Tri-Filar
Transformers
One VT1413 customer determined that greater than 100 dB CMR to 10 MHz was
required to get good thermocouple (TC) measurements in his test environment. To
accomplish this requires the use of tri-filar transformers which are an option to the
VT1586A Remote Rack Terminal Panel. (This also provides superior isothermal
reference block performance for thermocouple measurements.) This works by
virtue of the inductance in the shield connected winding presenting a significant
impedance to high frequency common mode noise and forcing all the noise voltage
to be dropped across the winding. The common mode noise at the input amplifier
side of the winding is forced to 0 volts by virtue of the low impedance connection
to the VT1419A ground via the selectable short or parallel combination of 1 kW and
0.1 µF. The short can’t be used in situations where there is a very high common
mode voltage, (dc and/or ac) that could generate very large shield currents.
Wiring and Noise Reduction Methods
Appendix D 375
Figure D-2: HF Common Mode Filters
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