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Old 23rd December 2012, 09:07 AM   #1
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Default OPA548 unity gain

Hi,

I have build OPA548 in unity gain, but I have huge 25V dc offset at output. I canīt find any wiring mistake. Any clues what can be wrong?

Thanks.
Standa
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Old 23rd December 2012, 09:52 AM   #2
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When I ground the Vin+ there is no dc offset. I should lower the 47k input resistor, right?
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Old 23rd December 2012, 11:11 AM   #3
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It could be oscillating. Is this device unity gain stable ? I can't just see it mentioned in the data sheet.
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Old 23rd December 2012, 11:16 AM   #4
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I have read at various places that OPA548 is unity gain stable. In the datasheet there is square wave response at unity gain.
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Old 23rd December 2012, 11:21 AM   #5
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Fair enough

I would definitely scope it and see to be sure. I searched the pdf for "unity" and can't turn anything up on that.
http://www.datasheetcatalog.org/data...nts/opa548.pdf
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Old 23rd December 2012, 11:25 AM   #6
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Quote:
Originally Posted by kacernator View Post
When I ground the Vin+ there is no dc offset. I should lower the 47k input resistor, right?
... or put the same 47k in the feedback loop. If it is bias current that is the cause, it will then balance out.

To check for oscillation, try parallelling the input 47k with a cap, see if that changes things. Do you have a scope?

How does it work with a load, say 100 ohms to check?

Does it do this with no input connection?

jan
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Old 23rd December 2012, 11:34 AM   #7
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Putting 47k in the feedback loop doesnīt change anything.
I already have 220pF parallel to input 47k. But offset is there witnou input connected.
When I load the output with 10R 2W, the resistor starts to smoke immediately.

Unfortunately I do not have a scope.
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Old 23rd December 2012, 11:38 AM   #8
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Difficult without a scope.

Remove the two diodes just in case something is amiss there. The circuit will work OK without for testing.

Our you running this on a true split supply where both rails rise at the same time ?
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Old 23rd December 2012, 11:44 AM   #9
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I have already tried disconnecting the diodes, still there is dc offset.

Yes, I am running it at true split supply +-28V.

If it is oscillating, how can I tame it? I have power suppy caps soldered directly to chip pins.
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Old 23rd December 2012, 11:53 AM   #10
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Without seeing what its doing (scope ) its imossible to say what the problem and cure may be.

Is the zobel network present ?

Not sure it will reveal much but have you measured the DC voltage on the two input pins and the difference in voltage across the two input pins. The difference should be zero. If you 25 volts on the output, then that same voltage must be on the inverting input as well. So what voltage on the non inverting input ?

If nothing shows up I would try altering for a voltage gain higher than unity by suitable feedback resistors just to see what happens.
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