amplifying 0-50mV to 0-5 v ?

[vicky29]

dear freinds

i want to amplify the voltage level from 0-50mV to 0-5 v to give it to adc . given below daigram is what i was using from book jan axelson .but it is giving say for input 45mV -output as 4.1v.its not working well. i have removed offset between pin 11 nad 6. please help me out

 

[master_slave]

My recommendation is for you to try it first on a variable gain amplifier. It seems to me that you are using a fixed gain amplifier, whose gain is derived from calculations. However, in practice, the ideal gain is not exactly achieved, because of the imperfections in the circuit that are usually neglected. Like for these case, I think the imperfections here are those in the op-amps and the unexactness of the resistor values (it has tolerance around a nominal value). If you use a variable gain amplifier, you can practically tune it in the actual circuit and, by trial and error, find the appropriate component values to meet your required gain to amplify 0-50mV to 0-5V. I suggest using an instrumentation amplifier, whose gain can be tuned by a variable resistor. BTW, in-amps are also good for purposes of isolation.

 

[sunking]

It not difficult to get a close loop gain of amplifier about 100. But pay attention to that the open loop gain should large enough such as 10000 or so

 

[echo47]

Somehow I don't see any link to your jpeg image. Nothing in you message is clickable.

Do you mean that the output signal has a DC offset, or the output/input slope is wrong?

If you are using an LF347, then beware it has up to 13mV of input offset.

Offset to pin 11?? That's the V- power supply of an LF347. That chip normally needs positive and negative power supplies?

 

[vicky29]

Thanks everyone for replying. I don't know why the image isn't opening....I'm sending the jpg file agai. Please suggest me accordingly what should i do...should i use any other op-amp??

 

[IanP]

I think better solution than a 3-stage amplifier is 4-opamps instrumentation amplifier configuration. For instrumentation amplifier 100 gain is not a problem.
Example inside attachement ..
Regards,
IanP

 

[sunking]

test the value of res and open loop gain of lf347

 

[echo47]

If you ground the input, the output will probably be hundreds of millivolts instead of zero. That's caused by the opamp's imperfect input offset voltage. I suggest you shop for a "precision" FET opamp that has an input offset spec that meets your requirements.

IanP - An instrumentation amplifier configuration is convenient for several reasons, but it won't solve an input offset problem, assuming that's what is wrong.

You can save some parts by converting the three-opamp arrangement to an ordinary single-opamp non-inverting configuration.

 

[vicky29]

Thanks everyone for replying. Will a single OP07 work like in figure i've shown below..please help me out....

 

[Borber]

It will work, offset error will be about 5mV without correction.