Problem : input offset voltage abnormaly high

[[email protected]]

Hi,

I am using a OP07 as an inverting amplifier.
With Rin=1k and Rgain=100k the gain is about 100.
The output is directly connected to a voltmeter (ie, I didn't put a
Rload in the circuit).
I use 1uF and 0.01uF decoupling capacitors on the +/-15V supply.
I use a 20k pot (as specified for the OP07) to trim the input offset
voltage.
The circuit is working well on the test breadboard.

BUT after I soldered all the components on a prototyping PCB the offset
voltage increase by a factor x90 (about 180mV with the inputs shorted,
compared to 2mV on the test board ). This offset is so high that I
can't trim it!

From the datasheet, the OP07 has an input offset of about 30uV, which

should give max 3mV after the x100 gain, not 180mV.

I tried the change the capacitors and the op-amp... but did't solve the
problem.
I noticed that without the decoupling capacitors the offset is even
higher (about 5V).
I really don't see what produces this abnormal offset (the strangest
being that the circuit works very well on the breadboard).

Does anyone has an idea?

Nicolas

 

[Mike Monett]

Hi,

I am using a OP07 as an inverting amplifier.
With Rin=1k and Rgain=100k the gain is about 100.
The output is directly connected to a voltmeter (ie, I didn't put a
Rload in the circuit).
I use 1uF and 0.01uF decoupling capacitors on the +/-15V supply.
I use a 20k pot (as specified for the OP07) to trim the input offset
voltage.
The circuit is working well on the test breadboard.

BUT after I soldered all the components on a prototyping PCB the offset
voltage increase by a factor x90 (about 180mV with the inputs shorted,
compared to 2mV on the test board ). This offset is so high that I
can't trim it!
should give max 3mV after the x100 gain, not 180mV.

I tried the change the capacitors and the op-amp... but did't solve the
problem.
I noticed that without the decoupling capacitors the offset is even
higher (about 5V).
I really don't see what produces this abnormal offset (the strangest
being that the circuit works very well on the breadboard).

Does anyone has an idea?

Nicolas

Hi Nicolas,

Sounds like the pcb version might be oscillating or picking up rfi from a
local radio station. One clue is the offset increases when the bypass caps
are removed.

The op07 is fairly low bandwidth, so you might not see anything on the
output. Try the standard debugging methods. Check the bypass caps to make
sure they really are good. Trim any leads to the input pins to the shortest
length possible. See if the offset voltage shows hand effects where it
changes when you move leads or wave your hand near the circuit. Check the
pcb carefully for inadvertent shorts to adjacent traces. Make sure the
ground connections really are ground. Perhaps the ic on the pcb is bad.

Look for the obvious things. You'll find it, I'm sure. Don't let your mind
say something is OK because that can't be the problem. Check it anyway.

Regards,

Mike Monett

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Noise-Rejecting Wideband Sampler:
http://www3.sympatico.ca/add.automation/sampler/intro.htm

 

[Jim Thompson]

Hi,

I am using a OP07 as an inverting amplifier.
With Rin=1k and Rgain=100k the gain is about 100.
The output is directly connected to a voltmeter (ie, I didn't put a
Rload in the circuit).
I use 1uF and 0.01uF decoupling capacitors on the +/-15V supply.
I use a 20k pot (as specified for the OP07) to trim the input offset
voltage.
The circuit is working well on the test breadboard.

BUT after I soldered all the components on a prototyping PCB the offset
voltage increase by a factor x90 (about 180mV with the inputs shorted,
compared to 2mV on the test board ). This offset is so high that I
can't trim it!
should give max 3mV after the x100 gain, not 180mV.

I tried the change the capacitors and the op-amp... but did't solve the
problem.
I noticed that without the decoupling capacitors the offset is even
higher (about 5V).
I really don't see what produces this abnormal offset (the strangest
being that the circuit works very well on the breadboard).

Does anyone has an idea?

Nicolas

Assuming no miswire on the PCB... you overheated the OP07 while
soldering, or injected lethal ESD.

...Jim Thompson

 

[Fred Bloggs]

Assuming no miswire on the PCB... you overheated the OP07 while
soldering, or injected lethal ESD.

...Jim Thompson

Didja notice that when he removed the decoupling capacitors the output
"offset" went to 5V? Haha- ridiculous.

 

[Jim Thompson]

Didja notice that when he removed the decoupling capacitors the output
"offset" went to 5V? Haha- ridiculous.

I missed that. So he has a "singer" ?

...Jim Thompson

 

[colin]

I missed that. So he has a "singer" ?

Yeah maybe the capacitance of the breadboard is actualy making it stable,
usualy it works against you.

Colin =^.^=

 

[Spehro Pefhany]

Hi,

I am using a OP07 as an inverting amplifier.
With Rin=1k and Rgain=100k the gain is about 100.
The output is directly connected to a voltmeter (ie, I didn't put a
Rload in the circuit).
I use 1uF and 0.01uF decoupling capacitors on the +/-15V supply.
I use a 20k pot (as specified for the OP07) to trim the input offset
voltage.
The circuit is working well on the test breadboard.

BUT after I soldered all the components on a prototyping PCB the offset
voltage increase by a factor x90 (about 180mV with the inputs shorted,
compared to 2mV on the test board ). This offset is so high that I
can't trim it!
should give max 3mV after the x100 gain, not 180mV.

I tried the change the capacitors and the op-amp... but did't solve the
problem.
I noticed that without the decoupling capacitors the offset is even
higher (about 5V).
I really don't see what produces this abnormal offset (the strangest
being that the circuit works very well on the breadboard).

Does anyone has an idea?

Nicolas

You didn't put any capacitors on the output, did you? That's a good
way to turn it into an oscillator. If you're measuring with a
voltmeter you will see things similar to what you are reporting.

BTW, the adjustment range is typically +/-360mV at the output with a
gain of 90.


Best regards,
Spehro Pefhany