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Analog sig processing using AD8236 and AD8609


I am hoping someone can help?

I have designed a simple ECG analog processing path feeding in to one of the available A/D modules of a PIC microcontroller (24FJ128GA310).

My issue is that one of my analog rails for the analog path and the PIC A/D module (AVss) will be significantly different from my digital Vss for the PIC. My PIC data sheet informs that neither AVdd and Vdd nor AVss and Vss can be more than 0.3V different.

My analog rails for the AD8236 and AD8609 are AVdd = +2.5V  to  AVss = -2.5V but my digital rails for the PIC are Vdd =+2.5V and Vss = 0V.

Vss and AVss will therefore be 2.5V apart thus violating this rule.

I realise I can level shift the AD8236 output with the Vref pin so theoretically I could raise the output of this Inst amp to 1.25V rather than leaving it at ground.

My signal processing ICs in the analog path are Inst Amp (AD8236) followed by (AD8609) Hi Pass  with Gain, Low Pass with Gain, Notch, Final Gain and have rails of +/-2.5V so 1/2 the possible span (0V to -2.5V) would never 'see' the signal as it would be crunched in the 0 to 2.5 V span?

*Noting the passives forming the filtering and gaining around the 2nd order filter would be referenced to analog ground set at midway between the +2.5v and -2.5v and this analog ground would connect to digital ground Vss back at the PIC. I suppose then that AVss also would have to be connected to Vss.

So a long way of asking can anyone advise on a more elegant solution to accommodate these AVss and Vss max separation criteria?

Rgds Michael

ps I apologise in advance for needing to draw in a part (16bit pic)) which is not an analog devices part.

  • Hi Michael,

    I think your plan should work.

    Connect the AVss of the PIC to the ground only.  Make sure that we have a common ground.

    You can definitely use the REF pin to level-shift the output of the AD8236 by applying a voltage at the non-inverting pin of the AD8609. I would think this would be the simplest solution.

    Another implementation is to use the AD8609 to level shift the output like figure 67 on the AD8220 datasheet.

    Best regards,