Fast(er) op-amp for a precision full-wave rectifier - Page 1
Forum section: electronics / projects

author: ltz2000
author_rank: Regular Contributor
timestamp: April 02, 2015, 09:51:12 am
content: I happened to have a large quantity surplus MC33079 op-amps which I thought would do the job. Very low offset voltage (but it is bipolar, so need to be careful with the impedance balancing). The prototype worked fine down to a few mV.Except the frequency response. Perfect up to a few kHz, but I measured -4dB at 20 kHz for low level signals.The resistance values are as low as they can be and the diodes (1N4148) can't be the problem. So I need a faster op-amp.Last time the solution was an LM301 (?) with a special compensation scheme making it faster. But that was more than 30 years ago.I know that there must be a number of suitable op-amp available and new products introduced every year.But what would be the "industry standard fast op-amp"? Low cost, available almost everywhere.

author: dannyf
author_rank: Super Contributor
timestamp: April 02, 2015, 10:49:41 am
content: QuoteNE5532, good for a few Khz. Not exactly your circuit but works just as well.

author: ltz2000
author_rank: Regular Contributor
timestamp: April 02, 2015, 11:23:55 am
content: Quote from: dannyf on April 02, 2015, 10:49:41 amThe 5532 is actually worse than the 33079 that I used. No improvement in speed, but higher offset and known to be unstable in non-linear circuits.

author: blackdog
author_rank: Frequent Contributor
timestamp: April 02, 2015, 12:21:06 pm
content: Hi LTZ2000,This....But what would be the "industry standard fast op-amp"? Low cost, available almost everywhere.Is looking for the holy grail...Faster opamp you can buy, are all relatively bad DC performers.You can use somthinge like the old LM318 and use some feedforeward, but, not so good DC performance.Why not use a AD736 or a LTC1968, but be shure to use the right level (dynamic range)To make your schematic better, use low power schottky diode's for a lower voltage drop and more speed.Keep the resistance value is relatively low, use bias compensation resistors on the + inputs of the opamp.The are there are plenty of fast opamps available from Analog, Linear and TI, but no yelly beans...Kind regarts,Blackdog

author: Zero999
author_rank: Super Contributor
timestamp: April 02, 2015, 12:31:13 pm
content: Try the circuit on page 17.http://www.ing.unlp.edu.ar/electrotecnia/circelec/CA3140.pdfIt has to drive a 10k load and has a gain of 0.5 but a buffer or gain stage can be added if needs be.

author: blackdog
author_rank: Frequent Contributor
timestamp: April 02, 2015, 12:40:46 pm
content: Hi Hero999,The problem whit this schematic is, try 10mv, is wil not be -3dB @ 290Khz :-)A fast schottky diode can help...Kind regarts,Blackdog

author: Zero999
author_rank: Super Contributor
timestamp: April 02, 2015, 12:47:56 pm
content: Quote from: blackdog on April 02, 2015, 12:40:46 pmWhat do you mean? I don't understand.

author: blackdog
author_rank: Frequent Contributor
timestamp: April 02, 2015, 01:05:35 pm
content: Hi Hero999,With low input voltages, the opamp must work verry hard to overcome the diode threshold.Thats why you need a fast opamp, and low threshold diodes(schottky) helps.Kind regarts,Blackodg

author: ltz2000
author_rank: Regular Contributor
timestamp: April 02, 2015, 01:10:08 pm
content: Quote from: blackdog on April 02, 2015, 12:21:06 pmMakes the "dead zone" smaller. I thought that the difference would be minimal, but definitely worth trying. Any recommendations for a holy grail small signal fast schottky diode?Quote from: blackdog on April 02, 2015, 12:21:06 pmAlready done, as mentioned in the first post.Quote from: blackdog on April 02, 2015, 12:21:06 pmAlready done, as mentioned in the first post.

author: dannyf
author_rank: Super Contributor
timestamp: April 02, 2015, 01:37:29 pm
content: Only slew rate matters here. So you zero bias diodes or germanium diodes would help.Also, a current sensor is a uni-direction rectifier. With two back to back current sensors, you have a rectifier, especially good for small signals.

author: dannyf
author_rank: Super Contributor
timestamp: April 02, 2015, 01:38:13 pm
content: Also think of current transmission vs. voltage transmission. The ne5532 rectifier I built went all the way to 120Khz.

author: Zero999
author_rank: Super Contributor
timestamp: April 02, 2015, 02:12:13 pm
content: Quote from: blackdog on April 02, 2015, 01:05:35 pmThat's true, but that circuit is still easier on the op--amp than other circuits. Try changing the diode for a Schottky.Another thing you could do is amplify the signal before the rectifier but that will limit the maximum input voltage.

author: Liv
author_rank: Regular Contributor
timestamp: April 02, 2015, 04:08:06 pm
content: I used the AD8512 opamp and Schottky diode BAS70-04 for detector in frequency response meter up to 500 kHz.

author: ltz2000
author_rank: Regular Contributor
timestamp: April 02, 2015, 08:06:02 pm
content: Quote from: blackdog on April 02, 2015, 12:21:06 pmQuote from: ltz2000 on April 02, 2015, 01:10:08 pmTested at 5mV signal level:1N4148: -4dB at 20 kHz1N5711 (Schottky): -3dB at 20 kHz1 dB improvement. Still need the faster op-amp...