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AC coupled op-amp : output offset voltage

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col_implant

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I wanted to shift and gain 1Khz signal generated with a Vp-p of 625mV and a Voffset of 625mV to an AC coupled 1.6Vp-p signal. To do this i have used a simple active high pass filter shown in the figure...i use the TSH80 amplifier from ST.My gain is 2.5 and the cuttoff frequency is about 200Hz. Pretty simple.. right?

For some reason the output signal is giving an offset of about -160mV. The offset appears to decrease slightly as i increase the cuttoff frequency (by decreasing capacitance). Even when I set the input signal to DC i still get an offset of about -100mV even though i do not measure any offset at the amplifiers input terminal. Apart from this output offset the amplifier appears to be working perfectly. There is no mention of an output offset voltage in the amplifier datasheet

Is there some silly error i am making? Is my amplifier broken? Is there a better circuit or a simple inprovement i can make to the existing circuit?

Thanks
 

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Thanks for the simulation... but i know how the circuit should work.

I initially had the cap first.. switching was one of the many things i tried. I switched back now and no difference in the effect.
 
Thanks for the simulation... but i know how the circuit should work.

I initially had the cap first.. switching was one of the many things i tried. I switched back now and no difference in the effect.

hi,
You could 'inject' a low level +v offset voltage into the non inverting pin of the OPA, also if you are AC coupling the output the 100mV offset should not be a problem.

This will give a +100mV offset at the output
 

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The output voltage is due to the input bias current flowing through the 20kΩ feedback resistor. From the data sheet (amazing document), the maximum op amp input bias current is 15µA at room temperature. You can reduce this effect by about a factor of four by adding a 20kΩ resistor from the (+) op amp terminal to ground, since the maximum input offset current is 3.5µA at room temperature.
 
Hi there,

There is a spec for input offset voltage, but there is also a spec for input offset current, which can be as high as 5ua.
5ua flowing though 20k will cause 0.1v of output offset, or as Carl suggests it can be partly due to bias current too.
The solution is to lower the feedback resistor value if your drive source can stand it and go from there, or better yet, choose a better op amp part as that op amp is an overkill on GBW for a 1kHz signal with gain only 2.5 or so.
 
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