I am using the AD8428 in-amp and it is very sensitive to input resistance and hence the classical DC coupling as seen in the datasheet will not suffice because the thermal noise will become so high that it renders the signal undetectable. How can I get rid of this DC-bias without using a DC coupling with high-value resistors? Any ideas are greatly appreciated.
All the best.
May I know the source of the input signal and what applications you are working on? I want to know the sensor's characteristics so that I could recommend suitable parts for your application.
Thank you for your reply.
I'm measuring the Debye effect i.e. the measurement will be taken in a 0.9% NaCl solution under a 500 KHz ultrasound transducer. The ultrasound has an effect on the density of the saline solution so currents will flow on the same frequency as the ultrasound. The 500 KHz constraint is only because of the availability of an ultrasound transducer. The desired frequency range is from 250 KHz - 3.5 MHz.
I am wondering if a DC servo loop could be the answer to my problem?
Can I implement a servo loop that subtracts the DC component before it enters the terminals of the AD8428 or would some other in-amp be better for this?
What components do you recommend I should use in the servo loop?
I am thinking of limiting the gain of the first stage instrumentation amplifier to probably less than 10V/V (enough not to saturate the part by the dc offset) and followed by high pass or a band pass filter stage to removes dc effects and to reamplifies the signal. I would suggest to use the AD8421 for the instrumentation amplifier stage and AD8626 or ADA4898 for the active filters. Also feel free to use our tool for easy filter designs.
I also agree that you could implement DC servo and there are several threads here that discusses this in details.
Please see the thread below:
Would you be so kind to show me an example of an active bandpass filter with the passband narrow around 500 kHz? With a gain > 100 V/V, using the op-amps you recommended?