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Strange behavior from peak detector's certain op-amps

Lefteris Kostoulas
Prodigy 170 points
Other Parts Discussed in Thread: OPA379, OPA350, LMC6492, LMC6482, OPA340

I built a multiple feedback 2kHz band-pass filter with gain Vo/Vi = 4 and an envelope detector for this filter as shown in this great video tutorial EEVblog #490 - Peak Detector Circuit

The peak detector i have built is the same with the attached picture

When i applied a 1v 2kHz square wave signal at the input of the BP-filter i got 4v peak to peak sine wave signal (2v positive and 2v negative).So far so good.

For the peak detector, in the beginning, i chose two OPA379 op-amps.

When i connected the output of the filter to the input of the peak detector and measured the filter's output and the demodulated voltage with the oscilloscope something strange happened.

The filter's output was at 2v (positive peak) as i expected but The demodulated detector's output voltage was 1v instead of 2v (exactly half of what i expected, since the detector holds the peak voltage). Then i changed the two OPA379 op-amps with two OPA340 op-amps and the detector's output voltage was at 2v (as expected in the first place). So i cannot understand why this thing happens. Both OPA379 and OPA340 (or OPA350 or LMC6482 or LMC6492) are single supply cmos op-amps. Why did the OPA340/50 (or OPA350 or LMC6482 or LMC6492) worked but the OPA379 didn't? Ι assume that i have disregarded something in the datasheet but i don't know what this can be.

Can somebody help me?


Thank you

  • 0
    TI__Guru 62035 points
    Hello,

    The OPA379 is really not comparable to the OPA340 or OPA350 devices which feature significantly higher output drive and bandwidth than the OPA379. The OPA379 is optimized for ultra-low power applications because of its 2.9uA quiescent current. Bandwidth, output drive strength, and capacitive load drive capabilities are typically directly related to the quiescent current of the amplifier which is why the OPA379 with 2.2uA of Iq current only features 90kHz of bandwidth, slew rates of 0.03V/us, and low capacitive load drive capabilities.

    My guess is that either the bandwidth, slew rate, or capacitive load drive of the OPA379 is insufficient to properly work in this circuit. Without seeing your full circuit it's hard to definitively say what the issue is, but it's likely one of the ones I listed.
  • 0 in reply to Collin Wells
    Prodigy 170 points
    Thank you for the info. I decided to change the OPA379 with OPA340. By the way, since you mentioned it, when i looked in the datasheet for the capacitive load the OPA379 can drive, the datasheet gives only a graph of the overshoot with increasing frequency (page 8 fig.17) which didn't say much to me. I used an isolation resistor between the two stages of the peak detector in case of phase margin decrease but this didn't work neither. The output of the peak detector was constantly attached to my oscilloscope which has a huge input impedance so even if the the OPA379 can give a small output current the oscilloscope needs a few pA current for its channel.