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

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.

attachments.zip
• Hi Raj,

My understanding is that you have problem with the your DC offset from your sensor changing from 2-1.5V, you wanted to extract it's AC component and have it amplified 5 times and then add it up to a 2V offset. Is that correct?

Why did you use a 1.8V offset as an output to your sensor in your simulation?

Regards,
Goz

• Hi Gauzz,

The sensor has a nominal DC offset of 2V and when pressure is applied, the output changes (AC signal with 0.1Hz) on top of 2V DC offset. Ideally, a differential amp would have worked perfect, if my DC offset doesn't change because of small variations on the sensor reference port (sensor is differential).

You are correct in the second statement. Just as an FYI, the AC signal is a slow changing signal (0.1Hz).

I used 1.8V because, i wanted to see what happens to the circuit if my offset changes (As i said, the offset if not fixed at 2.0V, this can change vary between 2V +/- 0.3V)

Thanks,

Raj

• Hi Raj,

May I know the datasheet of the sensor you're using, it can be a really great help if you can provide it. Does the AC component of the output should be the only one that varies when measuring the pressure? Is your simulation circuit your actual set-up or the solution you taught about?

Regards,
Goz

• Hello Goz,

I have tried to send you personal message because of confidential information, but couldn't find the option to send to send a PM.

Anyways, i have compiled all the information into the PDF file attached to this message. Please let me know if you need more information.

FYI, keep in mind that the signal frequency is very low 0.1Hz-0.2Hz.