Input offset voltage and current selection

[katanoma]

Dear All,
Kindly asking about what offset voltage and current to choose from the datasheet of the operational amplifier AD8571. It is supposed to work at a temp. of 25C and under 5V supply.

attached is an image of the datasheet.
What is meant by the common voltage, or Vcm??

Best Regards

 

[Harald Kapp]

For a reliable circuit you should always design using the worst case values . In this case 10µV and 200pA. Only if you can narrow down the operating conditions to exact specifications should you use 5µV / 70pA as marked in the image.
The common mode voltage is the voltage that is common to both inputs of the OpAmp. The OpAmp itself is a difference amplifier. So theoretically it shouldn't matter whether for example U+=0.1V, U-=0.0V or U+=4.1V and U-=4V. In both cases the difference is 0.1V. However, due to technical limitations this is an imperfect function, the imperfection being expressed as the common mode rejectio ratio. Plus this works only for a limited range of input voltages. In your case 0 ... 5 V. For voltafes outside this range, the common mode voltage will not be suppressed as expected.

Regards,
Harald

 

[katanoma]

Thanks Harald,
But still have question about the common mode voltage,,, I am planning to use op-amp AD8571 to amplify a signal of Micro Volts and in the datasheet i have two specifications:
1. Vs=5V & Vcm=2.5V.
2. Vs= 2.7 & Vcm=1.35V.
so which one to choose? the amplification is 100

Best Regards

 

[Harald Kapp]

In your 1st post you stated that your supply voltage is 5 V. So Vs=5 V, no question there.
As to Vcm: that depends on the concrete circuit you are using. You don't necessarily have to use Vcm as stated in the datasheet. Vcm is not a property of the OpAmp, but a property of your circuit and can be adjusted to your needs (e.g. from Vcc using a suitable voltage divider).
If you work with a single supply (as I assume from your post), then it can be a good idea to offset the input signal from GND by some amount. This offset voltage (Vcm) can be anywhere between 0 ... 5 V. There are different circuits that can do that (see e.g. "op amps for everyone", here: http://www.ti.com/lit/an/slod006b/slod006b.pdf).
If your signal is AC then a simple capacitor will decouple the offset voltage at the output of the OpAmp and leave you with the amplified AC signal.
If your signal is DC, however, you have to subtract the offset voltage from the amplified signal in order to reconstitue the signal voltage without offset.

Regards,
Harald