Recently, I have started designing a USB oscilloscope frontend
https://www.eevblog.com/forum/projects/few-mhz-scope-frontend/msg4957573/ . To make up for the lowish 10 MHz bandwidth, I threw in a few extras to make life more interesting. One is a wide input offset shift - a few volts (+2 -5) on top of the 20 mVpp max input range (x1/10/100/1000). To achieve this, I added a bias voltage at the first FET input amplifier (x5) stage.
I had my doubts about this LM358 bias buffer (bias1 in the schematic), and indeed it shows lots of ripple and crossover distortion.
Thinking about it, the bias1 point should be stable to 0.1% (8 bit ADC and x5 gain), which means its impedance should be 1000 times lower than the feedback divider R12, R13, R49, which are 850 Ohms according to AD8065 datasheet spec for x5 gain.
So I need the bias to have an impedance of about 0.8 ohms from DC to 10 MHz. I found two ways how others did that:
First would be scopefun
https://gitlab.com/scopefun/scopefun-hardware/-/blob/master/KiCadSource/Scopefun_v2.pdf?ref_type=heads using a OPA692 video buffer. This is kind of brute force - but why not? Unfortunately it would add lots of extra noise due to its large bandwidth and filtering the output of a fast unity gain buffer is out of the question, these things are allergic to single digit pFs. Also these things are expensive and I have four bias points (one for each amplifier stage - I figured connecting those would create cross-coupling, making a nice oscillator).
Second solution I found in a reverse engineered scope schematic (can't remember where I found it), where the bias was buffered with a 220 uF capacitor, driven by an opamp followed by a BJT class B follower.
So I could either find a low enough noise/bandwidth (video?) buffer or try to drive a large enough capacitor. A 220 uF capacitor would have an impedance of 0.8 ohms at 904 Hz, so I'd have to design an opamp circuit driving those 220 uF and providing less than 0.8 ohms closed loop impedance below 900 Hz. (some literature on that:
https://www.analog.com/media/en/training-seminars/design-handbooks/Practical-Analog-Design-Techniques/Section2.pdf).
Not sure what the best way to proceed is here. As this will most likely stay and educational exercise, part cost actually does not matter but still it would feel inapproriate throwing several $5 parts into a design of such pedestrian bandwith (the original USB mixed signal scope sells for $20 on Ali).
Any ideas?