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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?
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
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.
