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clipping a fast negative pulse

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capt bullshot:

--- Quote from: brumbarchris on May 09, 2019, 08:24:29 pm ---

--- Quote ---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).
--- End quote ---
Thank you for the suggested schematic, kapitän, but would that circuit be any more accurate than the clamping diodes? Their forward voltage varies quite a lot!


--- End quote ---
Yes, the "precision" clipping isn't done by the diodes but the OpAmp. The negative supply is e.g. -0.8V, if you use an RRIO amp, the output voltage follows the input as long as it is above -0.8V, but can't go below. The diodes clip to -(0.8V + diode drop voltage) to protect the amp input.

OwO:
I don't understand why you can't just clip the signal inaccurately first and then clip it again in software after it is digitized.

brumbarchris:

--- Quote ---What's there to ADC if your clipping circuit works?
--- End quote ---
The remaining slow decaying waveform between -0.8V and GND. That is where the actual information in this signal is.


--- Quote ---Quote from: brumbarchris on Today at 06:24:29 am


    Quote

        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).

    Thank you for the suggested schematic, kapitän, but would that circuit be any more accurate than the clamping diodes? Their forward voltage varies quite a lot!

Yes, the "precision" clipping isn't done by the diodes but the OpAmp. The negative supply is e.g. -0.8V, if you use an RRIO amp, the output voltage follows the input as long as it is above -0.8V, but can't go below. The diodes clip to -(0.8V + diode drop voltage) to protect the amp input.
--- End quote ---

I'll have to look at that in more detail then, could be a winner!


--- Quote ---I don't understand why you can't just clip the signal inaccurately first and then clip it again in software after it is digitized.
--- End quote ---
We could probably do so, but as the whole system is a bit unblurry now, and we do not know yet what we'll use down the line exactly, we want to make it as easy as possible for it and take the difficulties in the portions of circuits that we are designing now. It might be that later, when we will ourselves know more about these (what type of ADC we'll use, what analog signal processing we will employ before it, how the signal slope varies from product to product etc; we first need to learn some things and perform additional measurements), we could also do what you suggest here.

Best regards,
Cristian

OM222O:
well then let me tell you that you're building your circuit backwards. the saying would be "making a coat for a button that you found" ... when nothing about your circuit is determined (something as basic as the ADC!) why do you even bother creating analog section for a very specific type of signal? what kind of information are you even trying to look at that can't be gathered from simulations? LTSpice is very accurate in that regard.

T3sl4co1l:
This sounds very much like an eddy current sensor, for whatever purpose: material resistivity, thickness, metal detection or identification, etc.  You don't need to be cryptic about it, there's nothing special about this.

The impulse method never impressed me.  In particular, why discard whole microseconds of the strongest part of your precious signal, when you can get the long tail (low frequency) components directly, with a sine wave impedance bridge method instead?

A frequency sweep with a DDS and ADC, under MCU control, would be very easy, require no fiddly current source or beyond-the-rails voltages, and as it has no dead bands, and integrates the signal continuously, offers an arbitrarily high SNR.

Tim

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