# Remove unknown DC offset and Add Know DC offset to AC signal

Question asked by rajkanika on Jul 31, 2017
Latest reply on Aug 14, 2017 by rajkanika

Hello,

I have a pressure sensor, which has a nominal voltage output voltage of 2.0VDC from the sensor and when pressure changes, the signal changes from 2.0VDC +/0.2V DC. So there is a 2.0V DC offset and +/-0.2V if the AC signal on top of the 2.0VDC offset, the signal frequency is 0.1Hz (I have attached a picuture (originalsignal.png)).

I have to amplify just the AC signal on top of the 2.0VDC offset by 5 times but keep the DC offset to 2.0VDC, which i can do it by a differential amplifer by connecting the inverting pin to the signal and non inverting pin to 2.0V reference signal and add a reference voltage of 2.0VDC on the inverting pin to get  (3.0V positive peak and 1.0V negative peak with 2.0VDC offset).

The real issue i have is, my 2V DC offset  on top of AC signal from the sensor may change anywhere between 2V - 1.5V, because of things happening at the reference port of the pressure sensor.

I need to have a fixed 2.0VDC offset on the output signal and just amplify the AC signal (+/0.2V AC signal), no matter where my DC offset on the input signal  (DC offset may change from 2V DC - 1.5V DC). So i has an idea of if i can build a Differential amplifier with reference of 2.0V on the non-inverting pin, so i get the error signal (DC offset error of 0.2VDC when the offset of the sensor signal is 0.8V) and if i know what the error is, i can add it back to the signal source using Opamp Adder. But the problem is that, i will have +/-0.2V AC signal on top of 0.2V DC offset.

I need a constant 0.2VDC error voltage to add it back to the 1.8VDC offset +/- 0.2V AC signal, so that i can have a constant DC offset of 2.0VDC. (I have attached a LTspice simulation file for reference)

I have tried a high pass filter, but the problem is my roll of frequency of 0.1Hz. At this low frequency, there is a huge signal phase shift on the output signal (nature of a filter) and my design cannot afford any signal distortion or phase shift because of the application.

I would really appreciate your help and ideas.

Thanks,

Raj kenika.