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Fast precision rectifier/absolute-peak-detector

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thanks for the advice, i had compltely skipped your mention of that part in that vid when i watched it originally.
Very nice device, i ran a quick simulation and seems to work, now to try and get one of those(and they're dip as well, hooray!), i won't be needing the 20Mhz as the ripple will probably be below 1Mhz, but it can't hurt either.
What i do notice is that if i try to AC couple the input, it adds quite the offset to the input after the decoupling cap, does the AD8036 requires a low-impedance input to work?

--- Quote from: EEVblog on March 01, 2011, 06:19:34 am ---For a fast (20MHz) full wave precision rectifier, try the AD8036:
See the app note.
Mentioned in blog #38


--- End quote ---

scrat, actually it won't go down to 0Hz, i think the lowest freqs will be around 20 to 50Hz.
It isn't viable to build a reference value and a diference amplifier as the voltages are high and the values are nominal, thus there can be variations which would be problematic to differentiate.
Time response is not of the essence as long as it outputs the peak.

tecman, the large spikes(under/over) will better be handled by a dedicated window comparator, i rather not dwelve into logarithmic amps and dynamic range issues as i need the best of both world in as simple package as possible.

What's the best way to AC couple the input signal?, a simple CR highpass filter?, or a buffered highpass with an opamp(but that can distortion the original ripple if i'm not careful about the amp) to provide low impedance to the AD8036?

Since the opamp is powered by dual voltage, I see no problem in simply adding a series cap at the input, having 1/(2*pi*Rg*C) << lowest frequency.

to narrow down on the design i'll block it this way(and since i have no idea about the clipping on the input of the AD8036):
Active sallen-key highpass filter with 20Hz cutoff based on OP277 or LM310 follower -> AD8036 precision rectifier -> peak-hold/peak-detector -> [possibly x2/x3 opamp] -> ADC input

the problem i'm having is the response of the peak-hold/detector in a wide range of frequencies, i've built this circuit http://www.discovercircuits.com/DJ-Circuits/peakdet1.htm but behaves quite poorly below ~1Khz, and i can dispense of the 2nd LM393 in that circuit as well as i don't need large current capabilities.
the gain stage is because the resulting rectified peaks will be very small (around 100mV~) and that's 10~codes in the 10bit ADC, but maybe i won't need it...
so back to the peak detector, is there any other "fancier" solution(not clock sampled/holdfed, as i need it to be continuous) for wideband peak-holding?

I think the only solution is to lower down the peak detector output "filter" corner.
However, very low pass filtering the AD8036 output should make the peak detector not necessary.

the peak detector is to get a stable DC input for the ADC and to have a stable "worse-case" sampling which is what i'm after.
the simple rectified output is too "variable" and i could be missing small very transient spikes in software
of course i could do this in software with max/min, but the microcontroller is already busy with other stuff running and i can't be sampling fast enough to depend on software max/min(and i'd rather not).

i could also use a very-large time constant RC network after the AD8036, but that would skip fast spikes...., even an active one...


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