Hey this opamp specifies 5uA bias current (out of opamp input) at 150mV common mode voltage in typical case. So if you have a resistance of 150K driving the input, you gain 750mV. So you shouldnt be surprised if you see 900mV at the output. With this opamp, the source impedances(on both the inputs) need to be very low.
With this opamp., you cant get away with this error. The offset current is of the order of bias current so even if you try to match the resistances on both arms., you'll still observe the error. You need to reduce the driving impedance or change to an opamp that has bias current in the pA range.
Yes, oscillations may be reason for the DC offset. Generally, this kind of high speed amplifiers can't be operated without effective bypassing directly at the part. Also, a capacitive load or longer traces shouldn't be connected without a series termination (e. g. 50 ohms) at the amplifier. Oscillations would usually show by increased current consumption, you may burn your fingers when checking the device temperature.
................
The inverting input is connected to opamp output and noninverting input is directly connected to the output of source.
The output impendence of source should be very low. I don’t it is the issue with input bias current.
When input is 3.5V the op amp output is correct, it is 3.5V.
But when input is 150mv, the op amp output is 850mV.
Do you have any ideas how to cancel this error voltage?
Thanks.
excuse my (probably) silly question, but did you bias the noninverting terminal correctly ? (There is no mentioning about it). You have a single supply of 5 volts and you get 3.5 volts at the output ? What do you mean - peak value or rms or even dc ???
The noninverting input is directly connected to the output of sensor and also connected to the ground by a 47K resistor in parallel connection according to reference design.
But opamp LT1807 I used is different from reference design.
LvW
Thank you for your question.
bias the noninverting terminal correctly.
I really have no idea about how to bias the voltage follower, please let me know.
First check output voltage when input is 2V. If output is not 2V then you introduced some offset and attenuation to the "follower". When follower is working correctly up to certain input level and then goes wrong then check stability at this level.