Peak Hold or Peak Follower OpAmp Ckt

[CanYouC3D]

Hi All

Way Back in the OpAmp AGE (Late 70's)
I had Text Book with some Ckt's like them in it.
now I can't find the book. I Remember one Ckt it had 2 opamp 2 diodes a res and a cap but I can't Remember how to wire it up.

Help

Thanks
Chris N. :p

[Robert Reed]

Sounds like an Absolute value circuit or an "Ideal" rectifier circuit. Not sure from your post what you are trying to achieve.

[CanYouC3D]

I'm looking to catch 1µSec., 5V max, AC pulse peek at a rate of 60Hz.
The pulse is 8µSec. to 13µSec. after high going trigger pulse. (noise from 3.5µSec. to 4.5µSec.)
Then read ADC to get the value.
All run buy a Basic Stamp or PIC.

Thanks

Chris N.

[Michael J]

I had a relay circuit in N&V APR 2005 p:50.
The op amp circuit in there charges up a
capacitor which turns on a small MOSFET to
latch on a relay for a second or so (microphone
version).
This may work for what you want, just change
the cap value and or the bleeder resistor on the
MOSFET or omit the MOSFET all together.

[Engineer1138]

THe circuit you're thinking of probably is a buffer opamp with a diode in the feedback loop. The voltage drop of the diode will be divided by the opamp's open loop gain, so you effectively have a "perfect diode" (zero voltage drop) circuit. It will then feed a low-leakage capacitor to record the peak. You'll also need a way to discharge the cap. This is in the Analog Devices _Nonlinear Circuits Handbook_, but my copy is probably still packed away after moving.

The problem I see is that the peaks you're trying to detect are very short. You'll need a very low value capacitor with extremely low resistance to measure something like that accurately. It won't be easy!

Here's what I would try instead. Pick up a Sample & Hold chip (can't offer a part #, sorry), and figure out a circuit to trigger the S&H when the pulse comes through. All the hard work is done inside the S&H and your A/D converter can read it.

Better yet, find an ADC with built in *fast* S&H and an external trigger. They do exist, but you're on your own with that search

[MrAl]

Hi there,

I was going to say "get a sample and hold ic
chip" too. National has one but it's probably
not fast enough (or maybe with some mods).
Analog Devices probably has something that
will work down in the 1us region...check both
sites.

Take care,
Al

[Robert Reed]

Chris
If all you want to do is capture a short duration pulse to make it usable for other cicuitry, there is a simple circuit thats been around forever. Its called a pulse stretcher and is configured as such:
Basically an emitter follower (2N3904 or 6) .emitter resistor 10K ohm or so is parraleled by a suitable value capacitor, however the cap is fed by a diode from Re. Pulse charges up cap instantly--cap discharges slowly thru what ever resistance you choose to meet your desired RC time. There are some draw backs such as juntion drops but its quick to bread board to play around with to see if it meets your needs.If shaping is required, you can follow this with an and gate (for shape and time).

[Engineer1138]

Robert: he said he wants to read the pulse with an A/D converter. The stretcher will change the amplitude.

[rshayes]

The simplest peak detector is a diode and a capacitor, with a high impedance amplifier to read the voltage on the capacitor. This can be fast with a small capacitor, but it does have the diode offset voltage to contend with.

An emitter follower with a capacitor as a load can be fairly fast. When the input signal falls, the emitter-base diode stops conducting and leaves the capacitor charged to the peak voltage minus the base-emitter voltage.

A partial compensation for the base-emitter voltage error is to add a PNP emitter follower ahead of the peak rectifier. This will compensate most of the offset and temperature errors of the single transistor. This can also be used with the diode approach.

Op amps improve the accuracy at the expense of response timmme. The National LF398 is based on op amps. Its acquisition time is about 6 microseconds, which slightly slower than what you are looking for.

One trick is to cascade two peak rectifiers. The first uses a small capacitor, which can acquire the peak voltage quickly. If it can acquire the pulse in 1 microsecond and store the value for 10 microseconds, then a second peak rectifier can use a larger capacitor which may take 10 microseconds to acquire, but may hold for several milliseconds.

Many A/D converters have a sample and hold circuit built in, so a second stage might not be necessary. If the signal does not rise too rapidly, then a track and hold circuit may help. Here the capacitor is charged as the signal rises, but is not discharged when the signal falls.

[CanYouC3D]

Thanks To All

I Did come up with a Peak Follower Ckt with the diodes, I used a small sig FET to discharge the Cap
it works well, but I still need to work on it some more I was seeing the peak and the follower voltage tracking but there was some droop on the follower ckt (? Caps old mica)

Will post CKT Soon

Thanks

Chris N.