Just Trying to Understand an ECG Front end like the ADS1298 here.
This is a 8 channel simultaneous sampling Analog Front End for ECG applications.
Now the chip can be driven from analog bipolar supplies like
AVdd +2.5V
AVss -2.5V
and has an internal reference of 2.4V that is referred to the negative supply
Vrefp +2.4V
Vrefn = Avss = -2.5V
The usable input range of the internal PGA / ADC without saturating is +/- Vref / Gain so if I consider a max gain of 12 we get a range from +200mV to - 208mV.
Now I'v attached the characteristics of an ECG signal below.
As can be seen the signal of interest is only around 5mV but it is imposed on a dc electrode offset of +/- 300mV and in addition to that a large common mode mains offset of upto 1.5V.
My question is, if I DC couple this ECG signal to the inputs of the PGA, how are such large offset voltages not saturating the PGA?
Is the high CMRR of the device (-115 dB) taking care of these offsets.