Problem : input offset voltage abnormaly high

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titinicolas77 19 years ago

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

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Mike Monett 19 years ago

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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Jim Thompson 19 years ago

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

...Jim Thompson

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Fred Bloggs 19 years ago

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

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Jim Thompson 19 years ago

I missed that. So he has a "singer" ?:-)

...Jim Thompson

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colin 19 years ago

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

Colin =^.^=

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Spehro Pefhany 19 years ago

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

"it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com

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Problem : input offset voltage abnormaly high

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