Electronic – How to bias an AC signal for sampling

acbias

I want to sample a 10Vp-p 50Hz sinusoidal signal.

My ADC can only sample between 0V and 5V, so I need to bias the voltage around 2.5V.

I only have a positive 5V power supply.

To do this, I was going to feed the AC signal to a resistor divider so that it will be 5Vp-p, then add a 2.5V DC offset with another resistor divider pair (AC coupling each stage).

However, I'm having trouble with the DC offset, and I'm not sure this is the best approach to do this.

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As you can see, my voltage is still negative. How can I get a 5Vp-p signal offset around 2.5V?

Best Answer

Your sine wave isn't 5V peak-to-peak, it's 5V zero-to-peak.

In your simulation, try adding a 50-ohm resistor to ground between R3 and C1.

In the real world you would need to design the circuit so as to mitigate interactions between components, typically by using op-amps as buffers, but for your first pass in a simulator (where you don't care about the amount of virtual current that is going to be consumed by your virtual 50-ohm voltage divider) just adding the resistor is fine.

Of course, if you actually want to work with a 5Vp-p sine wave, then just change the simulated source's amplitude to 2.5V and everything should start working.

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