jgpeiroAnother situation with which I found doubts was in the design of the filter at the entrance. If I remember correctly, to get a high input impedance, I used a resistive divider, which also increases the input range. The problem is that with this relatively high impedance, at high frequencies, the opamp had a lot of losses.
To avoid this, I put the capacitor C28, which is driven at high frequencies and allows to drive the input of the opamp. Again I have my doubts about this decision and I wanted to know your opinions.
Is it bad architecture? Are there other solutions?
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That's ok, the same principles as oscilloscope probe compensation - you've got divider with some capacitance at bottom so you compensate that with capacitor parallel to top resistor so that response is flat.
If you want to use a typical 10x oscilloscope probes, consider adjusting the values. Input resistance should be 1M. You may need to increase also input capacitance (C28 and C29), you may be unable to compensate probe if it's too small (typical is more like 10-20pF, you've got only 1pF). Higher capacitance also helps to avoid effects of nonlinearity or temperature coefficients of protection diode and op-amp input, also small parasitic capacitances in PCB doesn't hurt that much if they are bloated with C28 and C29 large by design.
Of course, higher capacitance loads the measured circuit more which is bad if you use 1x probe, not 10x (then C28 and C29 should be small - so decide for which probe you optimize the circuit).
C28 or C29 may need to be tuned without probe to make sure there are no effects of under/over compensation. Also check if leakage current of protection diodes won't make any voltage at the resistive divider (if you didn't).
Are you sure? yes | no
About the 10x proves, I have a Keysign bench oscilloscope and I always prefer to use the probes in 10x option, but for this design it was out of my "scope".
With your comments I feel more confortable about the decision of C28.
Are you sure? yes | no
(The detail with the C18 and the opamp)
https://lh6.googleusercontent.com/RAOTFIXQyuUMxWdHtHo9OkiVyvgmQMkuR9YqIURg0bbmb-HxjP8XDgRH3BF4Ryj8Wl_ez4fy84lKbXHIS8YYUjkQGRuxlE8LjNS-bCAMBPnoFseZHFI53UIau2NG3bMpo3OMYPBTodos9OD8dPbZVwc5RrzKkhvMrJWQl3XEv2yp3byo4W-Lm7Lg
Are you sure? yes | no