# Understanding a common mode signal conditioning module

As far as I understand, normally an ideal differential amplifier(or an instrumentation amp) can reject common mode(CM) voltages up to around its rail voltages.

Let's say the rails of the diff. amp are +/-10V DC. And let's say the inputs are such that the normal/differential mode voltage is 1V, and the common mode voltage is 20V. As far as I understand, this will saturate or maybe damage the amplifier because 20V>10V. But if the common mode voltage at the input would be 2V this would be rejected well and the amplifier would work fine because 2V<10V.

There is this signal conditioning module and below is some specs from the data-sheet:

I know that after this signal-cond. module, the signal are coupled to data acquisition inAmps whose rails are +-5V.

Normally if there were no conditioning a 20V common mode voltage would saturate the amplifier. But this module says "CMV, Input to Output 1500V max."

Does that mean that the module can get rid of a common mode voltage of 200V or more? I also came across the following:

:

Does that mean the 5V battery above can be read as 5V even though Vcm=1200V? Is that what this module doing and acting as a common mode filter or choke?

It means that the input amplifier is isolated galvanically from the output stage and therefore can have 1500 volts between circuits at the input and voltages at the output. In effect this does mean the common mode input range is around 1500 volts but this will have somewhat degraded performance as that voltage gets bigger and certainly the common mode noise rejection will be much poorer at higher frequencies. A link to the data sheet should help.

• Thanks for the feedback I also understood like that but wanted to be sure. I dont have enough time to research and bult CM filters or chokes so was planning o use such a ready module.
– GNZ
Commented Jun 2, 2018 at 16:05

That module is isolation amplifier - a differential amp which has galvanically isolated actual differential amplifier input section and the output section. The input section gets its supply power via internal DC/DC converter. The output section gets it directly. The signal (=measured voltage difference) is transferred to output module as pulses which can easily go through otical isolation or transformer. Input section has a pulse modulator and the output section has a demodulator.

Pulse modulation has a price: Signal bandwidth is low. Otherwise high speed parts are needed and the price jumps up.

CMRR is high, common mode voltage limit is determined by the insulation between the input section and the output section. Critical places are the internal DC/DC converter and the signal path transformer or optocoupler.

NOTE: If the common mode voltage is not a static DC, but AC or high frequency noise or impulses, the module behaviour surely degrades. At 60Hz it claims to have still 100dB CMRR. Otherwise the performance degradation isn't clearly specified.

If the common mode voltage is 1200VDC the module surely can measure and output 5VDC battery voltage. If the common mode voltage is sharp 1200V pulses, probably not. It's not mentioned in the datasheet. The common mode voltage spec ends to 60Hz.

The module is not a filter nor choke, it's an isolation amplifier, a circuit which has a magnitude more complex operation principle than a filter or choke.Internally it probably has several filters and chokes.

• My question was "Does that mean the 5V battery above can be read as 5V even though Vcm=1200V? Is that what this module doing and acting as a common mode filter or choke?"
– GNZ
Commented Jun 2, 2018 at 18:56
• @Genzo the answer is fixed.
– user136077
Commented Jun 2, 2018 at 19:06