I am developing an embedded solution to measure and record the analog signal from a PCB Piezotronics pressure sensor. Specifically the Model 124A21.
The sensor has some integrated circuitry to boost the signal. My understanding is you need to drive the sensor with a 2mA constant current source, and then the sensor will drive the voltage as the signal.
The common mode voltage of the signal is ~10V, and the swing can be +-5V (the datasheet mentions up to 7V swing, but it's heavily non-linear outside of 5).
I care about the frequency content between 0.5 Hz and ~15 kHz (sensor natural frequency).
My plan so far is to use a 16 bit ADC that has 30-50+ KSPS, which leaves SAR adcs as the best bet. I was also thinking of 100kSPS to leave additional room for additional digital and analog filtering to keep the 15 kHz signal intact.
My main question is now, how do I get this weird 10+-5V signal into either a differential signal or a 0-5V signal? Considering the datasheet seems rather uncertain about the Vcm.
The diagram below shows the sensor on the left with its cable in the middle and the constant current source on the right. The signal conditioning guide mentions the decoupling capacitor generally has values around 30uF, which means a 1 Ohm impedance at 5kHz.
Ideally, I would like a DC-coupled system, which has other benefits, like detecting if the sensor fails.
EDIT: Okay so I think what I want to achieve here is take this 5-15V signal, shift its bias to 2.5V and scale it from +-5 to +-2.5. So that the final signal is a 0-5V signal.
To do this I need to
- Get a reference of the Vcm using a heavy low pass filter.
- Us an op amp between the Vcm ref and the true signal. On this opamp I can do my scaling. I reference the output of this opamp to a 2.5V reference.
Maybe do some active low pass filtering with the op amps while I'm at it :)