Hello everybody, thank you for your replies and patience.
Here are some clarifications:
WHY WOULD YOU WANT TO CLIP A PULSE TO A VERY SPECIFIC VOLTAGE WITH LESS THAN 20mv ACCURACY!
The signal is going to be then "signal processed" and eventually fed to an ADC input. As there will be a large number of products that will be manufactured, we will need to account for component-to-component variation between various samples. I guess one of the concerns, something I should have probably mentioned in the first post, is that once the input signal goes above -0.8V it must be not altered or distorted in any way. That is where the actual information that we are interested in is.
Especially since the OP mentioned at first that it should clamp between -0.8V and -1.4V. So if 20mV accuracy is needed, does that imply that this clamping voltage is going to be adjustable?
As mentioned in the OP, the level does not need to be adjustable, but it has to be precise. So for instance if you suggest a solution that always clamps at -1.2V+/-20mV that is good. Equally good to a solution that clamps at -0.9V+/-20mV. Of course, the nominal clamping level needs to be known (if it is 1.2V or if it is 0.9V) and reproduce able across samples.
If more precision is required and the speed is not too great, then a shunt regulator can be used as a clamp.
Unfortunately speed is quite high, and I do not think a basic shunt regulator would provide this accuracy, particularly if it is BJT based.
that solution is also using a diode
how can that be any more accurate?
It is definitely more accurate as the opamp will drive its output so as to compensate any variations in the diode voltage drop until the VOUT node (which in this case is connected directly to the inverting input of the opamp) reaches the same value. AFAIK, the opamp drives its output so as to always keep its inputs at an equal level.
So one might consider a fast enough Rail-to-Rail I/O OpAmp to buffer the signal. Apply some coarse input protection using diodes, set its negative supply voltage to the desired clamping level and have your output clamped to the negative voltage.
Could you elaborate a bit more on that please? It's "sounds" relatively similar with the opamp based variant that I was considering, and the schematic of which I indicated in one of the above posts.
Best regards,
Cristian