Cortex M4 ADC

Discussion in 'General Electronics Chat' started by Dritech, Nov 19, 2015.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    Hi. I am going to use the Cortex M4 to process ECG signals. The reson of using an MCU is to reduce external components as much as possible. The only external components I wish to use is the instrumentation amplifier to remove any CMR and for a small signal amplification before entering the ADC. Since an ECG signal has both positive and negative cycles, it it possible to set the ADC reference from -2.5V to 2.5V?
  2. JohnInTX


    Jun 26, 2012
    I doubt it. Look at the Electrical Specifications for the specific Cortex A4 - based chip you are going to use. All of the ones I pulled up (TI, ST, NXP) have Absolute Maximum Ratings with very little negative voltage (Vss - 300mv or so). Pretty standard.
    Last edited: Nov 19, 2015
  3. BR-549

    AAC Fanatic!

    Sep 22, 2013
    Hello Dritech,

    Read this page. Some things to mull over. And read your chip data sheet.

    It’s a great chip. I am studying the MSP432. The sheet is 150 some pages.

    The reference manual for chip is over 800.

    Still reading mine.

    After getting a feel for adc mating, check out this. These articles are for medical device interface.
  4. eeabe


    Nov 30, 2013
    Since you are using an external amplifier, it might not be too many more components for a very simple resistor network that allows + and - input. I recently did a +/-10V input to a single ended op amp that was powered by 3.3V. It took 3 resistors...

    .. and some algebra... here's my circuit:

    ADC buffer.png
    Last edited: Nov 20, 2015
  5. crutschow


    Mar 14, 2008
    Most standard instrumentation amps have a bias pin to set the output DC level.
    You just add a stable DC voltage to that pin equal to 1/2 the dynamic range of the ADC input (say 2.5V for a 0-5V input).
    That way plus and minus signals at the amp input will be translated to voltages between 0V and the maximum ADC input.
    Dritech and eeabe like this.
  6. eeabe


    Nov 30, 2013
    Do you mean an amplifier with differential output? If so, that is a very good solution. Another cool thing about fully differential input and output amplifiers is that they self bias, so you can have completely AC coupled inputs without having to worry about the DC bias of the inputs. That could save some components in some cases.
  7. crutschow


    Mar 14, 2008
    Look at this instrumentation amp, for example.
    It has a REF input which sets the DC level of the output.