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I am using the MCP601 for rectification of small AC signals. I got it working.

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

however at high frequencies (20kHz) weird behavior starts to surface such as this. enter image description here

I understand is has something to do with the chip spending too long in saturation and can't catch up. I also heard that you can get a faster OP AMP in terms of slew rate and it would fix the problem as well.

But shouldn't this OP AMP Already be suitable as the Slew rate equation is \$2*\pi*F\$

if we do the math

\$SR = 2*\pi * 20kHz\$

\$SR = 0.1256V/uS\$ the op amp is stated for 2.3V/uS ?

Can anyone explain to me why is this is happening with this op amp? and why isnt the current slew rate satisfactory enough for this current application.

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The op-amp output has to go from saturation at 0V to +0.6 instantly to keep the rectified output from swinging below zero. That's because of the diode forward drop.

The op-amp recovery time from saturation is not specified by the slew rate, and in fact it's seldom specified at all. It can be much, much worse than you would think from the slew rate.

Eg. LMV2011 with slew rate of 4V/us but \$t_{rec}\$ recovery time from saturation of 50ms typical.

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  • \$\begingroup\$ I see, I am assuming even the LMV2011 wouldn't be fast enough as T = 1/f right? T = 1/20kHz = 50us \$\endgroup\$ – Pllsz Jul 3 '18 at 21:55
  • \$\begingroup\$ No point in even thinking about it with 50 millisecond recovery \$\endgroup\$ – Spehro Pefhany Jul 3 '18 at 23:04
  • \$\begingroup\$ Yeah kk, just use it as a standard? How would a 45v/uS be ish? \$\endgroup\$ – Pllsz Jul 4 '18 at 0:17
  • \$\begingroup\$ and I would one fix the recovery time of an op amp? do we have to delay a signal for the op amp to catch up? \$\endgroup\$ – Pllsz Jul 4 '18 at 0:38
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    \$\begingroup\$ How do I choose an op amp that can meet my spec? As you mentioned not a lot of op amp state it's recovery time. How would I ago about this? \$\endgroup\$ – Pllsz Jul 4 '18 at 1:29

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