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clipping a fast negative pulse

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OM222O:

--- Quote from: SiliconWizard on May 08, 2019, 06:42:37 pm ---
--- Quote from: OM222O on May 08, 2019, 05:01:45 pm ---the main question still remains: WHY WOULD YOU WANT TO CLIP A PULSE TO A VERY SPECIFIC VOLTAGE WITH LESS THAN 20mv ACCURACY!

--- End quote ---

I'm not sure why either. 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? Because this is the only way I see that those two requirements would match?



--- End quote ---

He mentioned it's NOT ADJUSTABLE and the value doesn't matter. it just has to be accurate for some reason  ??? :wtf:

David Hess:

--- Quote from: brumbarchris on May 08, 2019, 11:06:18 am ---A simple diode will not do, as it is by far not accurate enough. To put a figure on it, let's say the clamping voltage needs to have a tolerance of +/-20mV, and of course, it has to be within the already stated limits.
--- End quote ---


--- Quote from: brumbarchris on May 08, 2019, 02:59:54 pm ---The variable forward drop on the schottky is far to great for this application. Definitely larger than 20mV, even for the same device, across temperature.
--- End quote ---

When diodes are used as precision clamps, then usually two are used with one compensating for the voltage drop and change in voltage drop with temperature of the other.  This leaves the logarithmic change in voltage with linear change in current (60 millivolts per decade) but you gave no specification for current.

If more precision is required and the speed is not too great, then a shunt regulator can be used as a clamp.

brumbarchris:
Hello everybody, thank you for your replies and patience.
Here are some clarifications:


--- Quote ---WHY WOULD YOU WANT TO CLIP A PULSE TO A VERY SPECIFIC VOLTAGE WITH LESS THAN 20mv ACCURACY!
--- End quote ---
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.


--- Quote ---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?
--- End quote ---
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.


--- Quote ---If more precision is required and the speed is not too great, then a shunt regulator can be used as a clamp.
--- End quote ---
Unfortunately speed is quite high, and I do not think a basic shunt regulator would provide this accuracy, particularly if it is BJT based.


--- Quote ---that solution is also using a diode  ??? how can that be any more accurate?
--- End quote ---
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.


--- Quote ---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.
--- End quote ---
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

capt bullshot:
See that quick and dirty schematic, hope you get the point. Component choice is up to your requirements, choose R2 to limit the current into the OpAmp input to a level typically lower than 5mA, better 1mA (check the datasheet and appnotes).



Anyway, your approach appears rather unusual, for what you describe (accurate and fast measure the input signal above the clamping level) I would consider coarse clamping by diodes and wouldn't rely on a stable clamping level for the rest of the circuitry. Maybe you should re-think the rest of the design once more.

Edit: AFAIR, some special OpAmps exist that allow for signal clamping to a defined level (set by a voltage applied to a pin), independent of the supply voltage. I believe, they were mainly used in video applications.

OM222O:
A basic simulation here:
http://tinyurl.com/y4eeqnjd
it shows that the negative edge is clipped well, but so is the positive edge due to the supply rails of the op amp (+-15v) so that can be a limitation.
Also it has so sink / source considerable current in order to do that (about 30mA which is on the high side for op amps, they're not meant for driving loads)
I'm not sure if this will actually work in real life when you build it.

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