Ultra low voltage amplifier

[Tommy1984]

Hi,
I tried to amplifier an ultra low voltage (0.05 mV) with help of AD8628, but, without any luck!
As it's shown in the picture, I have gain of 1000 or (1001), so the output must be something close to 45 mV. But all I got was 150mV.
Isn't so strange? Can anybody tell me where's my fault?

Tommy

[David Hess]

It should have worked.  You might need to add a feedback capacitor across R3 and another capacitor from the non-inverting input to ground to limit noise.

[Audioguru]

I hope the input signal is DC because it is not biased properly to be an AC amplifier.

[Zero999]

Yes, it should work.

What did you measure the output? Did you check it reads zero when the probes are shorted together?

[Tommy1984]

It should have worked.  You might need to add a feedback capacitor across R3 and another capacitor from the non-inverting input to ground to limit noise.

Thanks for your reply!
Any suggestions for capacitors values?

[Tommy1984]

I hope the input signal is DC because it is not biased properly to be an AC amplifier.

Thanks for your reply!
Ya it’s DC!

[Tommy1984]

Yes, it should work.

What did you measure the output? Did you check it reads zero when the probes are shorted together?

Thanks for your reply!
The output is around 150 mV.
How do you mean by shorting the probs? The input terminals?

[Zero999]

Yes, it should work.

What did you measure the output? Did you check it reads zero when the probes are shorted together?

Thanks for your reply!
The output is around 150 mV.
How do you mean by shorting the probs? The input terminals?

Presumably you were using a multimeter? If it's a really cheap model, it's possible or has wandered off calibration it's possible it has an offset voltage. See what it reads, when it isn't connected to your circuit and the probes are shorted together.

[Tommy1984]

Yes, it should work.

What did you measure the output? Did you check it reads zero when the probes are shorted together?

Thanks for your reply!
The output is around 150 mV.
How do you mean by shorting the probs? The input terminals?

Presumably you were using a multimeter? If it's a really cheap model, it's possible or has wandered off calibration it's possible it has an offset voltage. See what it reads, when it isn't connected to your circuit and the probes are shorted together.

Aha ok! Now I understand! No it’s not a cheap one and yes it’s shows ZERO when I short the probs!
I’m using VC 880
https://www.conrad.com/ce/en/product/124609/Handheld-multimeter-digital-VOLTCRAFT-VC880-Calibrated-to-Manufacturers-standards-no-certificate-Data-logger-CAT-III

[Zero999]

Yes, it should work.

What did you measure the output? Did you check it reads zero when the probes are shorted together?

Thanks for your reply!
The output is around 150 mV.
How do you mean by shorting the probs? The input terminals?

Presumably you were using a multimeter? If it's a really cheap model, it's possible or has wandered off calibration it's possible it has an offset voltage. See what it reads, when it isn't connected to your circuit and the probes are shorted together.

Aha ok! Now I understand! No it’s not a cheap one and yes it’s shows ZERO when I short the probs!
I’m using VC 880
https://www.conrad.com/ce/en/product/124609/Handheld-multimeter-digital-VOLTCRAFT-VC880-Calibrated-to-Manufacturers-standards-no-certificate-Data-logger-CAT-III

What do you get when you short the input of your amplifier to 0V?

Where did you get the AD8628 from? Could it be fake?

[daqq]

The voltage is VERY close to the negative power rail. Are you sure that it would work at those levels? While it is true that it's a rail to rail op amp, that just means that it can operate closer to those rails than a non rail to rail op amp.

[tszaboo]

1.) Try powering the amplifier from +/- 2.5V. It might be a rail to rail problem.
2.) Try decreasing the 1M resistor to 100K and the other one also. It could be an offset voltage due to bias and offset current.

[2N3055]

That opamp has less than 100 pA input bias current... Non inverting input of opamp is "in the air" with 10s of gigaohms of input loading and will function as a nice electrometer....
You need to provide guarding for the input and also clean board and area around the input very, very good..

[Tommy1984]

That opamp has less than 100 pA input bias current... Non inverting input of opamp is "in the air" with 10s of gigaohms of input loading and will function as a nice electrometer....
You need to provide guarding for the input and also clean board and area around the input very, very good..

Thanks for your reply!
I'm thinking of using a 100 ohm resistor for the input, do you think it's ok?

[2N3055]

It depends what do you connect to the input.
It will be correlated to output impedance of what you connect to input.

What do you plan to connect to input?

[Tommy1984]

It depends what do you connect to the input.
It will be correlated to output impedance of what you connect to input.

What do you plan to connect to input?

As shown in the picture, it's 0.045 mVDC

[2N3055]

I read that, and didn't mean that..

I meant: what, as in is shunt resistor, output of some sensor, NTC, strain gauge... What?
So we can determine interaction of that and opamp input...

[Tommy1984]

I read that, and didn't mean that..

I meant: what, as in is shunt resistor, output of some sensor, NTC, strain gauge... What?
So we can determine interaction of that and opamp input...

A shunt resistor!

[tszaboo]

That opamp has less than 100 pA input bias current... Non inverting input of opamp is "in the air" with 10s of gigaohms of input loading and will function as a nice electrometer....
You need to provide guarding for the input and also clean board and area around the input very, very good..

Yes. 100pA x 1Mohm x 1000 gain is 100mV

[2N3055]

AAh now we're talking... :-)

in that case, if you can be sure that opamp input will be always connected to shunt, you don't need anything, except shunt.

Did you try to ground noniverting input and check output then? Any changes? What is output then?

Regards,

Sinisa

[Tommy1984]

AAh now we're talking... :-)

in that case, if you can be sure that opamp input will be always connected to shunt, you don't need anything, except shunt.

Did you try to ground noniverting input and check output then? Any changes? What is output then?

Regards,

Sinisa

Hi Sinisa,
can you explain the wiring more please? Grounding the non-inverting and?
Regards, Tommy

[Tommy1984]

That opamp has less than 100 pA input bias current... Non inverting input of opamp is "in the air" with 10s of gigaohms of input loading and will function as a nice electrometer....
You need to provide guarding for the input and also clean board and area around the input very, very good..

Yes. 100pA x 1Mohm x 1000 gain is 100mV

I didn't get your formula well!
I have 0.045 mV input and the gain is about 1000 so the output must be something close to 45mV.... What's the relation between input voltage, Rf, output voltage and the current?

[2N3055]

He is talking about feedback resistors... It is all about currents... and voltage drops over resistors..

But I think input offset caused by input offset current would be 100pA over 1000 Ohms.. That would be 100 nV multiplied with 1000 times amplification that gives error of 100 uV on output... Not 100 mV....

As for wiring, I meant grounding input, meaning connecting + input to ground. That should give you close to zero on input and close to zero on output... I said close to zero on output because there will be internal opamp voltage offset multiplied with amplification.. But it should be less then or about 1mV max....

[Tommy1984]

He is talking about feedback resistors... It is all about currents... and voltage drops over resistors..

But I think input offset caused by input offset current would be 100pA over 1000 Ohms.. That would be 100 nV multiplied with 1000 times amplification that gives error of 100 uV on output... Not 100 mV....

As for wiring, I meant grounding input, meaning connecting + input to ground. That should give you close to zero on input and close to zero on output... I said close to zero on output because there will be internal opamp voltage offset multiplied with amplification.. But it should be less then or about 1mV max....

I understand it, non-inverting input (the + input) to ground. And then the input voltage to the inverting input through Rf and Rg?

[2N3055]

No. Your schematic is OK.  Something is wrong with circuit as soldered....

So I recommend to short input to ground so you have approx. zero on input  and then measure output.
You should have less than 1 mV in that config... If you have more, something is wrong...

Look, my advice to you is to download LTSpice and play with that.
I attached schematic of your circuit... It works fine...

There are many good free resources for opamp theory... And many more books on the topic... It's light reading time... :-)

Good luck!

Sinisa