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ADS1291 EMG circuit design help

young chul song
Prodigy 30 points
Other Parts Discussed in Thread: ADS1291

Hi all

I have some confusion designing EMG circuit with ADS1291. Please share your valuable opinions.

Application: EMG measuring with 2 electrodes

Chip: ADS1291
Analog power: +1.5~-1.5V(bipolar power supply AVDD:1.5V, AVSS:-1.5V)
Digital power: 3.3V

Questions
1. IN2N, IN2P, PGA2N, PGA2P, RESP_MODN, RESP_MODP
Do these pins should be pulled up high to AVDD or DVDD? <-AVDD
Whate value of a resistor is recommended for weak pull up?

2. Is it necessary to put 1M ohm and 1.5nf across RLDOUT and RLDINV for this application(EMG measuring with 2 electrodes)?
If so do I have to decouple incomming signals with capacitors and biased to RLDOUT with high resistors?

3. Do I have to pull up the VCAP2 as IN2N?

4. If I pull up the PWDN and START pins to DVDD, would it be ok without writing any register and read data as the DRDY falls? I mean reading data at default mode(500SPS).

  • 0
    TI__Guru 63146 points

    Hi Young,

    Thanks for your questions.

    1. The unused analog pins can be tied to AVDD with a weak pull-up resistor (~10k ohms should be fine).
    2. If you are only using 2 electrodes, you will need to AC couple the inputs and then provide a DC bias. The RLDOUT can provide the DC bias to each input through a 1M resistor. The 1M || 1.5nF in the RLD loop limits the bandwidth of the RLD amplifier to about 106Hz. The benefit of using RLDOUT vs. pure DC bias is you get some added CMRR at 50Hz/60Hz power line frequencies.
    3. VCAP2 should have a 1uF bypass capacitor to AVSS.
    4. /PWDN and START can be pulled high to DVDD at start-up. The device will initialize and start pulsing /DRDY with the default register settings. If you prefer to use the MCU to control START, tie the START pin low in the beginning.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 30 points

    Hi Ryan

    Thank you so much for the information. It really helps a lot.

    Bests,

    Young