Problem with settling time for high voltage in a peak detector

[lats]

Dear all
i am designing a voltmeter for AC mains line. i m using this peak detector circuit which i have taken from this site. The opamp i am using is TL074. the circuit works fine but the problem is the settling time for the voltage is very high. suppose i feed 1 volts Ac peak to input of peak detector i get 2 volts dc output and after that it starts dropping gradually to 1.9 volts ,1.8 volts ...and so on and reaches 1volts dc value after 40-50 seconds. when i make the capacitor value small say 0.1 uF then settling time becomes 10-20 seconds but the value becomes unstable and incorrect.how to solve the problem?

thanks

 

[ahmad_abdulghany]

Re: settling time

try to increase the frequency of input square wave signal..

 

[lats]

peak detector

hi thanks for replying but the frequency will not be high cause its AC line voltmeter and frequency cannot be more than 50 hz..

 

[Siniša]

Re: peak detector

Try bypassing 22k with cap. There is delay in feedback and first opamp overshoots. Experiment with values.

 

[on1aag]

Re: peak detector

Hi Lats,

The problem is the reverse leakage current of the diode.

Select a fet opamp with a higher output current. If you compare the
maximum output swing of a TL074 and a LF353 you will find that a LF353 can
supply a lot more output current than a TL074 under the same conditions.

Select a fet opamp with a rail-to-rail output swing. Meaning: you can lower
the supply voltage of the opamp and therefore also the reverse leakage
current of the diode.

Replace the diode with a base emittor junction of a transistor, a transistor
juntion will have a lot less reverse leakage current than a diode. That's
why some manufacturers prefer using transistor junctions to protect the
input of their DVM's.

Replace the diode by a special diode like the BAV45. If you can't get those,
construct one yourself. Select a N-channel fet with a maximum gate current
of 50 mA. Connect drain and source together. The gate of the fet will be the
anode, the drain and source connection will be the cathode.

You don't have to apply all these changes at once, see what parts you have
on hand and try until the result is satisfactory.

on1aag.