Precision Rectifier Gain Resistors

Thread Starter

leogomide

Joined Dec 19, 2019
8
Greetings,

First i want to thank you for the Precision Rectifier Explained video, it was a really amazing explanation.

I have a question about the value of the resistors to set the gain for this circuit:

sine.png

I need to set the gain of 50, so should i use 20k and 1M, or 2k and 100k?

Here some parameters:
- My signal will go from -100mV to 100mV at 4kHz max
- The opamp will have a single power supply of 0-24V (no way to have a dual power supply)
- The resistors are 1% rated
- I need to measure only the peak voltage of the signal

Best Regards :)
 

ronsimpson

Joined Oct 7, 2019
400
Information on the TL 072:
The inputs do not work when near the (-) supply. Should stay 3V off the negative supply. Can go to the (+) supply.
The outputs do not go from gnd to +24V, in your case. They run out of go near either supply. (designed to work in the center of the supplies)
I think you chose a good high frequency amp which is needed.

I think ground and 24V are the only supplies.
The schematic show the input going below ground.
Is the output going to a ADC? Is that why it must be sitting on ground?

I think you should choose a different type of amplifier. Look for R-R output and R-R input. Your amp needs to work at the (-) Rail and I don't care about the (+) rail. So you could look for "common mode input voltage range" that includes the (-) supply.

Also you have the feed back connected to the output of the amp and it should be connected to the "Vout".
----EDITED-------
There are examples of using R-R op-amps to make rectifiers. I think your input/output requirements match these examples. I can't find an example now but, If you made a amp with a gain of 50, it will only have the plus half of the wave on the output. No negative half will appear because the amp has not negative supply. I think you need two amps to get full wave. +50 gain and -50 gain in the other amp. Then add the two output together. No diodes. Sorry I can't find the application note right now.
 
Last edited:

Thread Starter

leogomide

Joined Dec 19, 2019
8
Information on the TL 072:
The inputs do not work when near the (-) supply. Should stay 3V off the negative supply. Can go to the (+) supply.
The outputs do not go from gnd to +24V, in your case. They run out of go near either supply. (designed to work in the center of the supplies)
I think you chose a good high frequency amp which is needed.

I think ground and 24V are the only supplies.
The schematic show the input going below ground.
Is the output going to a ADC? Is that why it must be sitting on ground?

I think you should choose a different type of amplifier. Look for R-R output and R-R input. Your amp needs to work at the (-) Rail and I don't care about the (+) rail. So you could look for "common mode input voltage range" that includes the (-) supply.

Also you have the feed back connected to the output of the amp and it should be connected to the "Vout".
Thanks for you reply :)

Yeah, sorry, the circuit was wrong xD

This was the intended circuit:

sine.png

Reading another post i see that the Silicon diodes are better suitable for this cases, so i changed to 1n4148.

Yes, the output voltage will go to an LPF and then to a MCP3202 ADC.

You have some recomendations of amplifiers to that case?

My goal is a good stability over temperature and achieve 0V at the output if the input signal is 0V.

I have found one that i think that will be suitable for this case, the TP2604 from 3PEAK, the datasheet is attached

The datasheet says that it can go to (-), but the test condition is using +-15V dual supply, it applies for the single supply too?

And about the resistor values, should i go for the higher or lower ones?

Sorry for the lot of questions, i still a newbie with amplifiers :p
 

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ericgibbs

Joined Jan 29, 2010
9,320
hi,
The TPI1252 d/s states:
Clip:
Ground Sensing and Rail to Rail Output
The TP1251/1252/1254 family has excellent output drive capability. It drives 2k load directly with good THD
performance. The output stage is a rail-to-rail topology that is capable of swinging to within 50mV of either rail.
The maximum output current is a function of total supply voltage. As the supply voltage to the amplifier increases, the
output current capability also increases. Attention must be paid to keep the junction temperature of the IC below 150°C
when the output is in continuous short-circuit. The output of the amplifier has reverse-biased ESD diodes connected to
each supply. The output should not be forced more than 0.3V beyond either supply, otherwise current will flow through
these diodes.

The go to you quote is:
Below-Ground (V-) Input Capability to -0.3V

E
 
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