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Amplifing AC Signal & Shifting DC Level

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Ayne

New Member
I am trying trying to make such a circuit which amplify AC signal comming from sensor(Sensor may be any one, no talk on sensor). Sensors output is varing from 0~20mVolts feeding this output to OPAMP (TL072) amplify this output, now ouput is 0~1.25 Volts coming from opAMP output, Now the problem is how to super impose this wave to a DC level of 1.25 Volts using OPAMP.

I mean i want to amplify the AC signal and want to change the DC level to 1.25 Volts using opAMP.

In simple First picture is Input & Second picture is output.

How we do this using opAMP, guide me.

Thanks.
 

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Sceadwian

Banned
It sounds like what you want is an RMS to DC converter. There are many chips out there which do this. I don't think it can be done with a discrete opamp circuit, at least not easily.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
You can easily bias an opamp to give a 1.25V DC output, and AC couple the input, this will do what you want. The only question really is why do you want to do it?.
 

Roff

Well-Known Member
Sceadwian said:
It sounds like what you want is an RMS to DC converter. There are many chips out there which do this. I don't think it can be done with a discrete opamp circuit, at least not easily.
Sceadwian, sometimes your thought processes baffle me. :confused:
 

hentai

New Member
Hi, u dont need an opamp to put a dc level in a signal. Just use some resistors (high value) to divide ur supply to the level u want and couple it with the signal through a capacitor...
 

Roff

Well-Known Member
hentai said:
Hi, u dont need an opamp to put a dc level in a signal. Just use some resistors (high value) to divide ur supply to the level u want and couple it with the signal through a capacitor...
You apparently didn't notice that he also needs gain.
 

audioguru

Well-Known Member
Most Helpful Member
Here is a circuit with some magic resistors to do what you want:
 

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Ayne

New Member
Thanks all who reply.....
Audioguru ur circuit is working and giving required output, but the problem is, it is AC coupled with sensor, Is there any other technique, a circuit that give same result but not AC couple.

What happened when Sensor's output is varing is very slowly!!!


And i want to attached this circiut to PIC Microcontroller ADC.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Ayne said:
Thanks all who reply.....
Audioguru ur circuit is working and giving required output, but the problem is, it is AC coupled with sensor, Is there any other technique, a circuit that give same result but not AC couple.

What happened when Sensor's output is varing is very slowly!!!


And i want to attached this circiut to PIC Microcontroller ADC.

You keep changing your requirements! - how are we supposed to offer advise when you change your requirements all the time?.

I asked before, and you didn't reply - WHY DO YOU WANT THIS SILLY DC OFFSET - it makes even less sense now you've mentioned feeding a PIC ADC.
 

hentai

New Member
hmm well if ur sensor's output impedance is low enough u could use 2 diodes instead of a capacitor like in the first circuit... u must tweak the bias resistor to get the desired bias level. Or u can use 2 opamp like in the second pic...
Mb others can verify this...
P.s. R5 from second pic should be higher (depending on ur req) i forgot to delete the value :p
 

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Ayne

New Member
You keep changing your requirements! - how are we supposed to offer advise when you change your requirements all the time?.

I asked before, and you didn't reply - WHY DO YOU WANT THIS SILLY DC OFFSET - it makes even less sense now you've mentioned feeding a PIC ADC.

Why I want to make This circuit?
I have made different boards to use with PIC(like re232 to ttl converter, Ir Board etc etc.) .Now i want to make a such a universal Amplifier that can be used with PIC. This universal amplifier will take signal from any sensor (Mic, Thermocouple or any voltage source) and the amplify this signal and shift it's DC level because PIC only accept variations above zero volts, not below zero volts. And then PIC will do my desired action.

I want to make modules like Nigel Mades in his PIC tutorial program.
 

ericgibbs

Well-Known Member
Most Helpful Member
hi ayne,

There are many ways to meet your requirement.

This is a basic opa which I use for level shifting.

It has an optional voltage ref, in case you dont want to use the PIC's 5v ref.

If you don't want the Vref opa part , reconfigure it say, as a dc amp , driving your level shifter.
Remember, you will require a -Vsupply to the opa, if you want to amplify -Vdc signals

Eric
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
Ayne said:
Why I want to make This circuit?
I have made different boards to use with PIC(like re232 to ttl converter, Ir Board etc etc.) .Now i want to make a such a universal Amplifier that can be used with PIC. This universal amplifier will take signal from any sensor (Mic, Thermocouple or any voltage source) and the amplify this signal and shift it's DC level because PIC only accept variations above zero volts, not below zero volts. And then PIC will do my desired action.

You are trying to do too much at one time, simply bias the opamp with two resistors (like all the single supply opamp circuits do), this will give half the supply voltage at the output. Then simply capacitor couple the AC input, requiring a DC input is a different matter!.

There's no problem making a general purpose opamp board, but it will be far more complicated than it needs to be.
 

audioguru

Well-Known Member
Most Helpful Member
It is easy to make a DC-coupled opamp preamp if the sensor passes the input DC reference voltage like this:
 

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Sceadwian

Banned
There's not much unusualy about my thought process Ron, I missed viewing his second image though, he wanted a DC offset, I thought he wanted a 0-20mv AC to 0-1.25 DC. The wording wasn't overly clear and I read it too fast.
 

Roff

Well-Known Member
Sceadwian said:
There's not much unusualy about my thought process Ron, I missed viewing his second image though, he wanted a DC offset, I thought he wanted a 0-20mv AC to 0-1.25 DC. The wording wasn't overly clear and I read it too fast.
No disparagement was intended, I assure you. I was truly wondering how you had come to your conclusion. You've explained it. :)
 
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