Hello,
I am developing a surface EMG data acquisition device.
Requirements are as follows:
High input impedance (>>GOhm)
CMRR >=120dB
Gain of 1000-1300
Expected signal is ~1mVrms with a variable +/-200-300mV DC offset.
I am planning to use an ADS131M08 differential delta-sigma ADC and to beam the data out using an nRF52840 or similar. This device also has an internal PGA w/ 128 max gain (would it be worth using this amplifier as well?).
My intuition was to keep the signal differential even in this "pre-amplifier," using an instrumentation amplifier because of its very high input impedance and CMRR. I selected the INA821 and referred to this AD document as a guide:
https://www.analog.com/en/resources/analog-dialogue/raqs/raq-issue-161.htmlHowever, I realized that I would need a way of AC-coupling the inputs because of that DC offset. I found references to DC servo circuits that I could use so that I do not sacrifice my input impedance target.
My questions are:
- How can I provide a path for input bias current while maintaining my gigaohm impedance target?
- Is it worth using the topology seen in the AD blog post?
- Is there an alternate way to AC couple the signal?
- Is it reasonable to use the in-amp with a Gain ~1000? Should there be previous/subsequent parts of the filter chain that add gain?
I have attached a block diagram for reference.
Thanks! Please let me know if I have made any fundamental mistakes or am misunderstanding something.