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Fast precision rectifier/absolute-peak-detector

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Eliminateur:
Hello again,
this time i'm racking my brains on another module of my bigger project and i turn to you guys as i've hit a larger brick wall than before.
I need to measure ripple in a DC line and also monitor large spikes(overvoltage/undervoltage).
for the spike part, i was thinking of a windows comparator using 2 opamps/comparators and a flip/flop/scr output to stay latched(so that the uC can read it and i don't miss the pulse, i could also use an interrupt pin as well but i'm pin constrained), nothing fancy, it needs to catch large and or fast spikes of several volts outside norm.
Dc input would be AC coupled to rectifier, i'm expecting a freq from 0 to 150kHz in the ripple, and also asymmetric ripple.

now... for the ripple part, my initial idea was to use a precision rectifier followed by a peak-detector, then input that to my ADC and done with it..., as a side note, i think it would also catch the spikes i mentioned before... i think....

I've been trying to find example circuits of fast precision rectifiers but come up short, the examples i've seen(TI sboa68.pdf for example) show problems in the negative cycle, or totally fail in high frequencies.
I've also come across a peak detector circuit using LM393, but it appears it only works for the positive cycle(i would have to put this after the precision rect output) http://www.discovercircuits.com/PDF-FILES/peakdet1.pdf

so.. i'm kind of baffled in this part, looks like the "perfect" device would be an absolute peak-detector..
Also, how do i AC couple in a 0 to 150kH< range without compromising the signal?, a lowpass RC filter with 150kHz~ cutoff?

refs i've seen:
http://sound.westhost.com/appnotes/an001.htm
http://www.analog.com/static/imported-files/application_notes/AN-573.pdf
http://ecelab.com/circuit-window-comp.htm
http://www.analog.com/static/imported-files/application_notes/130445851AB109.pdf
http://www.electronics.dit.ie/staff/ypanarin/Lecture%20Notes/K235-1/7%20Precision%20Rectifiers.pdf
and TI sboa68.pdf can't find the url

Simon:
for the spike detection you could use just a comparator that detects anything over a certain voltage and then sets some sort of holding circuit.

What exactly are you trying to achive for the ripple ? if your just trying to check the ripple stays under a certain voltage you could AC decouple the voltage with a capacitor and rectify the ripple and smooth it and measure it with a comparator. Now sure how you would do both neg and pos, you may have to mirror everything and use a dual supply although someone else might be able to advise on how to keep it all on the positive side of the rail

EEVblog:
For a fast (20MHz) full wave precision rectifier, try the AD8036:
http://www.analog.com/static/imported-files/data_sheets/AD8036_8037.pdf
See the app note.
Mentioned in blog #38

Dave.

scrat:
Of course, since AC voltage at 0Hz is a contraddiction itself, perfect AC coupling in the 0-150kHz range is not possible.

If you need to monitor very low-frequenc noise and DC error too, and the DC has a fixed known value, you could reproduce it (with a good voltage reference) and subtract to center the measuring range around zero. Low-pass filtering couldbe then added, if needed.

If you don't need precise time response (i.e. you can accept some delay between the spike or noise amplitude change and its detection), you can use >1 order low-pass filters to effectively limit bandwidth at the desired value. If possible (enough high sampling rate), I'd consider digital filtering as an option, too.

tecman:
What you need is a precision rectifier, followed by a peak detector.  There are quite a few op-amps fast enough to make a fast precision rectifier.  Problem will be one of dynamic range.  You are trying to measure ripple, and large spikes.  The amplitude of the spikes will define the allowable gain of the rectifier.  The limit may be that the amplitude of the ripple may be too small to effectively measure.  You may need a log amp to cover the range, followed by the rectifier-peak detector.

Paul

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