Op-Amp spec for full wave rectifier

it used 4 Op-Amps, and with when I used the LF411 Op-Amp, it worked fine, but I switched to the OPA820 and I couldn't get the full wave rectifier to work... the first stage op-amp worked ok, but once I hooked up the second o p-amp, my output disapeared and the thing started oscillating at like 14MHz (the input signal I'm rectifying was only 1kHz). I built this up on a sold erless breadboard.

o figure out what a good op-amp would be for my circuit? Is there something on the OPA820 datasheet that should've clued me in that it wasn't the righ t op-amp for me?

/8891

95

Hmm at a guess, stray capacitance (somewhere) on the solderless bread board . and an opamp that's too fast. Build it on copper clad and see if it works. Do you want this to work at hgh frequency?

George H.

The OPA820 is a pretty high-performance op amp. To do full wave rectification at a few kilohertz doesn't require gain-bandwidths in the hundreds of mHz and slew rates of hundreds of volts per microsecond.

My guess is that it's oscillating due to a layout issue. Try using an opamp that has a slew rate in uS approximately equal to your signal bandwidth in kHz and a GBW in mHz a couple times your signal bandwidth in kHz and I think that should be enough...

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Also, solderless breadboards are garbage, don't use them ever. Use a sheet of FR4 and "dead bug style" construction for high frequency, and a piece of strip board or blob board for audio.

Use sockets if you have parts you need to reuse.

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It's probably your peak-detector loop that's oscillating.

See "FullWaveRectifier.pdf" on the S.E.D/Schematics page of my website for a simpler way to go.

...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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I love to cook with wine.     Sometimes I even put it in the food.

Putting a 300MHz op-amp on a solderles breadboard (presumably with a breakout board since the op-amp is an SOT-23-5 type) sounds like a recipe for disaster.

--sp

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My guess is missing power supply bypass caps.

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-TV

I was wondering if anyone knew what Op-Amp spec I need to be looking at to figure out what a good op-amp would be for my circuit? Is there something on the OPA820 datasheet that should've clued me in that it wasn't the right op-amp for me?

This is my circuit

This Op-Amp Worked

This Op-Amp didn't

Thanks!

There is an interesting full-wave rectifier on this page:

Looks like something a PhD would concoct without thinking thru the error budget >:-} ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Assuming the power supply decoupling caps are in place and layout is as tight as possible then you could try a small capacitor across D2 - try

4.7pF or 10pF - my reasoning being to keep the output coupled back to the inverting input during the time between D1 conducting and D2 conducting.

piglet

Solderless breadboards (one of those semi-translucent nylon things, right?) add lots of parasitic capacitances between adjacent rows, and without much care lots of inductance between pins of the same node. This along with a very high bandwidth device like the OPA820 is a recipe for failure.

Why use such a high bandwidth device for a 1kHz signal? (There are other things to think about but you could start with these two...