Hello everyone!

I've been working on a circuit that takes in a 13MHz AC signal and attempts to get a DC voltage out corresponding to it's peak. (or as close to that as possible)

The active half wave rectifier works to perfection, the filter to smooth out the output however, does not work at all.
It's supposed to be an active low pass filter, but even after messing around a bunch with resistor and capacitor values, i never get what i want out.

Here's a pic of the circuit at the moment.


I've also attached the simulation file in case anyone want's to play around with it a bit.

Does anyone spot what I've done wrong here? Am I even in the right path?
Any help is much appreciated

 

hi,
Check C1 on the 2nd OPA input.
Is this what you require.?
E

 

Hi eric, thanks for taking the time to reply!

I wand DC out yeah! But there's an issue, what is happening there is the voltage is just going to the 2.5V rail of the op amp, so if i change the input voltage, the result will be the same.

What I want to see is the output voltage accompanying the peak of the output of Amp_Vout2 ideally. But I can't seem to find a resistor capacitor combination to make that happen

I think there might be something fundamentally wrong with the way I implemented the filter...

 

But I tried out C1 you added and it definitely does help!

I'm gonna experiment with it a bit

 

After experimenting with c1 at 10nF it out for a bit I arrived at a reasonably good output, It's not following the peaks of the signal but I suppose it's still fine.
Thanks so much for the suggestion eric!

 

hi S,
Could you configure a Voltage source in the sim that creates post #3 input.?
I can then try it out.
E

 

Sure thing!

So i guess I should speak about limits, the maximum input I expect is 1.5V to the first jalf wave rectifier, meaning I would want 1.5V input to produce a final DC output of 2.5V that is fine.

But for 1V input i would need something lower so I can distinguish a 1.5V input from a 1V input, the output will go into an ADC so I'll need a way of distinguishing between the two. Oh and the lowest input I'd expect is 0.1V.

Guess a simple way to correct that is changing the gain resistor R3?

I'll include the simulation with 1.5v input here.
Thanks again eric, i really appreciate the time!

 

Hi,
Have you considered a test source like this model.? Use V4 to sets params
E

Added lts file.

 

Oh that is awesome!
I'm gonna have a play around with it and see how my circuit reacts to different inputs.
Thanks once again! I think i should be ok now

 

hi,
OK, will be interested to hear how it goes.
E

 

Well here it is!



The finished circuit.
I'll be using it to measure a voltage caused by a magnetic field.
I'm pretty happy with how it turned out.
Thank you so much for the help Eric!

 

hi S,
Pleased its working to your design spec, many thanks for the positive feed back.
E