I want to use this Fully Differential Amplifier to design an OPAMP stage that is powered by +3.5 and -3.5 V DC power supplies, which will get a single-ended input signal that has AC and DC components.

The stage should amplify the signal and feed it to an ADC with a 85 Ω differential load.

The design will also get a DC input (External VDC), that I want to use to remove the input signal DC component.

What is the value of external VDC?

  • \$\begingroup\$ I have done something similar driving an ADC. At power up the device measures its DC level and then applies it to a 12 bit DAC to zero the level. Then it powers up the source device. The advantage here is that you can still measure DC levels and you can cancel out any offset in the amp. AC coupling would work too, but you lose DC and still have the amp offset. \$\endgroup\$ Feb 24 at 19:54
  • \$\begingroup\$ It makes for a poor design to pass spectrum that contains noise considering CMRR from DC to GHz without a great effort and broad spectrum experience . \$\endgroup\$
    – Hoagie
    Feb 24 at 20:12

2 Answers 2


Is there a reason to not just use a capacitor to block the DC? Trying to cancel it will involve getting VDC_EX rather precise and having it track any variations in Vin_DC.

Using a blocking capacitor should allow for a wider input signal range, the amplifier input range only has to accommodate the peak signal instead of the peak + DC.

If you need to cancel the DC without using a cap, one thing you might be able to do is derive VDC_EX from the input signal by using an integrator with a long time constant to extract the DC component which is then used as VDC_EX. This can also be done using the output signal instead of the input. This is sometimes done in audio amplifiers to remove any DC offset in the output.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thanks @GodJihyo, appreciated. I will try look deeper at your suggested solution, but for now, I was interested in certain direction for how to solve it, can you please check my question update? \$\endgroup\$ Feb 24 at 19:43

If you are going to go up to 8GHz, capacitive/inductive effects of all you components are going to have a major impact on any circuit operation.

As suggested by others, using a DC blocking cap will allow you to focus on these other factors. With a 10MHz lower cutoff, the components can also be selected for minor impact on accuracy.


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