I'm trying to measure a low frequency (0.1 - 100 Hz) signal with an ADC module. After powering the sensor up the signal can have common mode voltage from -15 V to +15 V and is tuned manually (the mechanical part of the sensor). The problem is that the ADC inputs can withstand only +/-10 V in unpowered state.

I want to add some buffer op-amp with +/-10 V supply to protect the ADC, besides this particular ADC module has no internal anti-aliasing filter, so op amp will work as 2-order LPF. The op amp itself will be protected with a pair of BAS85 diodes to rails and a current limiting resistor on the filter's circuit input.

The situation that I'm most concerned about is when the signal source is powered first and op amp is unpowered. As far as I understand the sensor will be charging op amp's supply's output capacitors via current limiting resistor and one of two diodes, depending on the common mode voltage polarity and can theoretically charge them to more then +/-10 V.

The solution that I came up with is to add a small reed relay between the signal source and the filter circuit. After powering the op amp up relay closes with a small delay provided by simple circuit.

So my question is: are those precautions really necessary and will the solution I described above work properly?


1 Answer 1


The reed relay should work, and the reed will provide excellent isolation. It's large, expensive and some have experienced reliability issues. I think I would use a telecom relay if this was deemed to be the approach.

As far as "necessary"- the frequency is very low so perhaps you could simply insert significant series resistance and clamp the signal with a Zener diode or diodes.

A slightly more sophisticated approach is to bias the Zener diodes and clamp to that with diodes. That gives you the guaranteed (low) leakage of diodes and the clamping of the Zeners (or even TVS diodes):


simulate this circuit – Schematic created using CircuitLab

In some applications the diodes might be ultra-low leakage or ultra-low capacitance types (or other devices repurposed for their diode-like characteristics).

  • \$\begingroup\$ Thanks. Do I need to use different diodes for lower supply voltages? For example +/-9V? \$\endgroup\$
    – Tolik4
    May 27 at 4:57
  • \$\begingroup\$ You can use different zener voltages. As always there has to be some margin between the maximum voltages you can accept and the operating voltage range. \$\endgroup\$ May 27 at 5:00

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