Author Topic: Peak Detector Circuit Self Resetting  (Read 1211 times)

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Offline N3xus

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Peak Detector Circuit Self Resetting
« on: March 28, 2016, 06:02:14 pm »
Dear all,

I am currently facing a problem with one of my circuits and hope that someone can help.

Attached you can find the schematic. It is a normal peak detector circuit which resets after about 25ms. This is also shown in the simulation which is also attached. The incoming signal is a 40 kHz Signal coming from an ultrasound receiver plus gain.

In the attached pictures TP2_3_1..3 you can find the measurement on the testpoints TP2 and TP3 with my Rigol oscilloscope. There is this strange app. 2V voltage offset. I then measured the voltage directly after the capacitor C10 and saw (Figure TP_2_C10_1) that after the capacitor the voltage offset is introduced.

Can somebody explain me why this happens?
What can I do do improve the situation?

btw: This is a already manufactured print so I hope there is a modification that does not lead to a redesign.

Best Regards
Armin
 

Offline amspire

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Re: Peak Detector Circuit Self Resetting
« Reply #1 on: March 28, 2016, 06:28:58 pm »
I am not familiar with the LMH6654 myself, but if it is a normal opamp, it has to have a resistor from pin 3 to somewhere - probably ground to provide the input bias for the opamp. Currently, the DC voltage on pin3 looks undefined. The input bias current can be just over 10uA, so assuming you want less then 10mV DC error, a 1K resistor from pin 3 to ground would be good.

A resistor in series with the output may improve stability as well.
 

Offline danadak

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Re: Peak Detector Circuit Self Resetting
« Reply #2 on: March 29, 2016, 12:08:53 am »
From datasheet (also there is a graph in datasheet) discussing Riso.

Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 

Offline N3xus

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Re: Peak Detector Circuit Self Resetting
« Reply #3 on: March 29, 2016, 06:01:04 am »
Thanks for your replies so far! I tried to add the resistor which is connected to ground and pin 3 and it worked as expected. Unfortunately I did miss to discover the whole datasheet and so I missed to add Riso but I don't really have the chance to add one in this design. Seems to work without it so I will skip to add one. I also tried to short the DC blocking cap C10 but this did not work as good as when I add the resistor to gnd.

Thanks for your fast help!
 


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