I found an article with EMG circuit using INA128 instrumentation amplifier with gain of 400. I couldn't understand how will it work considering the dc-offset. Can anyone explain me if this circuit is valid?
http://advan.physiology.org/content/31/2/242/F1.expansion.html

 

You are seeing one of the problems with instrumentation amplifiers. They have [relatively] large offsets. You didn't say which flavor of INA128 you were going to use, but I'll assume it's the INA128PA, which could have the highest offset.

Worst case, the input offset at 25C is given as ±125±1000/G uV. For a gain of 400, the output can be 50mV + 1mV, and it can be positive OR negative.

So the question is - what does your input signal look like, and what is its maximum value? If you cannot adjust the gain to give the required performance, then find a better amplifier, or design one yourself.

Also note that there is a 10nA bias current. If the impedance is too high, this will also increase the offset.

As always, specify your requirements before selecting hardware.

 

I was doing some research for my EMG amplifier. Since ECG/EMG signals are around 5mV but have around 300mV DC offset, the most of the instrumentation amplifiers used for EMG, have gain of less than 10.
But this article mentions gain of 400 by having capacitor in series of gain-resistor. I haven't seen this method before. So, I wanted to know how it works.

 

Okay, I misunderstood your question.

I have never seen this with instrumentation amplifiers. At DC, the gain is maximum, whatever that is. I doubt it is controlled very well, and it may not be stable (it may oscillate).

Maybe you should think about a multistage amplifier with a high impedance front end (like pA of bias current). If your signal is millivolts, you shouldn't have a problem with voltage noise. Many op amps today have 1uV or less of input noise voltage.

You can null out the offset in the first stage. Also, maximize gain in the first stage, but don't go hog wild. The op amp still has a limit on the output voltage.