Why DC offset offset output.

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#21
The OP could do some measurements too.

a) measure the DC across the input pins; that's the input offset voltage;
b) measure the DC across the 47k R, which gives the input offset current.

Combined give the output offset voltage, as the input offset voltage X gain, plus the offset as a result of the input offset current. They may be with same polarity (which I expect for 200mV output offset) or opposite polarity and cancel partly.

Karl you could check the same on your breadboard.

Jan
 
#22
OK, so same set up as before... can't guarantee they were all the same devices as before though.

From the start then. First readings with 910 ohm not connected, second set with. Vin in voltage between non inv and inv inputs. Meter neg on non inverting. VR is voltage across 47k, meter neg at inv input:

Code: 
LF356
Vout = +1mV
Vin  = +1mV
VR   = 0.00V
 
Vout = +51.1mV
Vin  = +1mV
VR   = +50.3mV

---------

4560
Vout = -1.1mV
Vin  = +0.8mV
VR   = -1.8mV

Vout = +39mV
Vin  = +0.8mV
VR   = +38.1mV

----------

LM833
Vout = -13.2mV
Vin  = +0.3mV
VR   = -13.5mV

Vout = -0.2mV
Vin  = +0.3mV
VR   = -0.5mV

----------

4558
Vout = -2.6mV
Vin  = +0.5mV
VR   = -3mV

Vout = +15.2mV
Vin  = +0.4mV
VR   = +14.9mV

----------

1458
Vout = +1mV
Vin  = -0.8mV
VR   = +1.5mV

Vout = -25.7mV
Vin  = -0.7mV
VR   = -28.9mV

----------

NE5532 (Philips)
Vout = +6.7mV
Vin  = -1.5mV
VR   = +8.1mV

Vout = -68.6mV
Vin  = -1.5mV
VR   = +68.2mV
 
#23
I think we can conclude that the offset is caused mainly from the amplified input offset voltage, except maybe for the LM833. That is an odd one. I must check its datasheet.

Back to the OP, he stated his 200mV output offset doesn't change with different opamps. Did he measure the DC at the opamp input pins?

Jan
 
#24
The LM833 was an old original National Semiconductor one from the early 90's I guess.

We could do with knowing the set up for OP's original issue because something in his set up sounds very amiss. Pretty sure only output voltage was measured.
 
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