Audioguru
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The added resistor should be equal to the total input resistance or 10.2 Mohm (the bias current path to ground) to get proper cancellation....................
You could try to mitigate the problem inserting in the negative feedback a resistor of the same value of the series resistors you are using (i.e 5 .1Mohm). I mean: remove the short between output and inverting input of the op-amp and place between these two points a resistor of about 5.1 Mohm. The problem should disappear.
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The added resistor should be equal to the total input resistance or 10.2 Mohm (the bias current path to ground) to get proper cancellation.
Then all of our offset voltages should be halved.Sorry, I've understood the sum of the two was 5.1 Mohm and not each of them.
As already said by Audiogu the input bias current of the op-amp generates significant offset when large resistors are used. You could try to mitigate the problem inserting in the negative feedback a resistor of the same value of the series resistors you are using (i.e 5 .1Mohm). I mean: remove the short between output and inverting input of the op-amp and place between these two points a resistor of about 5.1 Mohm. The problem should disappear. In this case however you'll be prone to noise so you can insert a capacitor in parallel with this new feedback resistor. The value of the C have to be low enough not to significantly modify the transfer function.
When you wish to use high impedances, choose a better OpAmp such as the CMOS types with pA input bias currents or even much less.
If you are presently stuck with an LM358 bipolar Op AMP then recalculate the RC values to stay with ceramic XR7 or better and R with a tolerable input offset.
The LM358 is very old. It is one of the first low power opamps.Old design and low power produce a noisy opamp.
high value resistors produce thermal noise. Why are your resistor values so high and why is your frequency so low?
Lots of old opamps with horrible spec's (741?) are still manufactured for school kids to play with and for replacements.
RCA invented the CA3130 and Harris took over their semiconductor division then Intersil was formed from Harris workers.
I deal mainly with audio circuits so I use low noise, wideband and low distortion audio opamps like TL07x, LM833 and OPAx134.I have tried CA3130. it gave me the wanted result and no offset exist now.
What kind of reliable opamp families that you recommend generally? actually, I got used to use these lousy families of lm324, lm358, etc... so I hope that you tell me what do you use personally for low cost signal conditioning circuits.
CA3130 is better for noise, input current offset, Iio (0.1pA typ@25'C) and bandwidth (4MHz )
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