Why does Auto-Zero have no effect for IEPE sensors?

The Prosig P8000 Auto-Zero function is used to remove the DC content of a signal.

For example, if you have a sine wave that is ±5 volts about 0 volts it has no DC offset. But if you have a sine wave with ±5 volt amplitude about 3 volts, then it has a 3 volt DC offset. Below is an example of these two sinewaves.

Two sine waves with and without DC offset

The autozero function would electronically remove the 3 volt offset for you automatically.

The Prosig P8000 would record the signal as a ±5 volt signal, but will not see the 3 volt offset as it would have been removed.

IEPE sensors however are AC coupled. This is because they are powered by a DC source.
If the IEPE sensor was not AC coupled then your signal would have a large offset (DC component) which is actually the power for the sensor. The IEPE sensor has only 2 wires, yet is powered and returns a signal through those same two wires.

The AC couple is basically a high pass filter, set to 1Hz. This means everything below 1Hz is filtered out.  Any offset (or DC component) will be at 0Hz and therefore will be filtered out by the AC coupling filter. And so, the auto zero function which only affects DC components of a signal will therefore have no affect on an IEPE sensors signal as the DC component has been removed.

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James Wren

Former Application Engineer & Sales Manager at Prosig
James Wren was Sales & Marketing Manager for Prosig Ltd until 2019. James graduated from Portsmouth University in 2001, with a Masters degree in Electronic Engineering. He is a Chartered Engineer and a registered Eur Ing. He has been involved with motorsport from a very early age with a special interest in data acquisition. James is a founder member of the Dalmeny Racing team.

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