OPA615: High Frequency Peak Detect circuit

Hi Vishal,

The OPA615 has a current output (operational trans-conductance amplifier), which is difficult to find beyond this device, but any operational amplifier can be used to create a peak detector circuit. 

The only requirement is that the selected amplifier can charge a capacitor through a diode fast enough to detect the peak, and that the capacitor size is sufficient to hold that voltage for the required amount of time before it is sampled and reset. Typically this means that the amplifier must have a larger output current and a relatively stable capacitor drive frequency response, which is sometimes harder to find in a normal op-amp.

Can you tell me:

1. How long you need to hold the peak voltage

2. What peak voltage range is expected

3. The minimum peak duration that must be detected

4. Do you need to know the exact voltages of the peaks, or is a simple comparator that detects an "out of range" condition a better fit than an amplifier solution?

With this above information, I can help recommend a peak detector circuit and a sufficient high speed amplifier.

Best regards,

Sean

Hi Sean,

Thank you for response.

1. We have 1Msps ADC and we will need to sample at least 5 samples. So we need to hold it for at least 12us. 

2. Negative peak will vary between -48V to -10/-15V. Negative peak may be enough and we can ignore positive peak. Positive peak shows less variation compared to negative peak.

3. There is no specific requirement. Operating frequency is 1.65MHz.

4. Exact voltage is important. We are scaling down 48V signal by 10x. Just comparator will not work. We wanted to build data acquisition system to sample voltage waveform across transducer. But to keep cost low, we are now sampling peak voltage and frequency to estimate voltage waveform. So peak voltage accuracy is important.

Please let me know if you have nay other question.

Regards,  

 Vishal

Hi Kai,

Thank you. I am going to add MOSFET with series resistor do that we can discharge capacitor to check peak periodically. 

With regards,

   Vishal

Hi Kai,

I am confused with this thread.(Not your message thread in general) Do I have to put parallel resistor to C1 ? Because when I put paralel resistor voltage drops faster over time.

Hi Bethan,

whether you can omit R1 depends on the ratio of input bias current of U1 to leakage current of SD2: If the reverse current of SD2 is much higher than the input bias current of U1, SD2 can fully absorb the input bias current and R1 can be omitted. But if the input bias current of U1 is much higher than the reverse current of SD2, the input bias current can make the input saturate. Then R1 can help to provide an alternative path of input bias current to signal ground.

With the chosen components here, R1 can be made higher or even omitted.

Kai 

Hi again Kai,

Could you tell me what kind of capacitor should I use in peak detector circuit? Also,I really appreciate product suggestion.

Thank you in advance.

Regards,

Behtan Umut Mercan

Hi Bethan,

a simple NP0 or C0G will do.

Kai