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ADS1294 common mode range

Other Parts Discussed in Thread: ADS1294

Hello,

We are using the ADS1294 in a 1 and 2 channel EMG amplifier application. We have a dedicated front-end and use the ADS1294 merely as a high-end AD-converter. The gain of the input stage is set 1x, supply voltage is 2.8V, high res mode, 8kSPS. Input signal is symmetrical. Internal ref voltage is 2.4V.

The maximum differential input voltage is 2V (4Vtt). According to eq 5, page 56, the CM range is 1.6V > CM > 1.2V. So initially we set the CM voltage to 1.2V. But with this CM voltage the undistorted input range is only 2 Vtt. Only when we increased the CM voltage to 1.6V the undistorted input range is 4Vtt.

How to explain this?

 

  • Hi Sake, 

    It sounds like you are still violating the common-mode input range somehow. If the CM voltage = 1.6 V, a 4V peak-to-peak signal would range from  -0.4V to 3.6V (CM - 2V to CM + 2V ) which would exceed the absolute maximum input range for a 2.8 V supply.

    Can you please clarify whether you are using a single-ended input (INN is fixed at the CM voltage) or differential input configuration? 

    Any scope captures you can provide will also be helpful.

    Thanks,

  • Hello Ryan,

    The INN and INP inputs are driven differential. With reference to figure 50 in the manual, the driving voltages are: CM voltage = 1.2V (which must be increased to 1.6V), the INP peak-to-peak voltage is CM - 1V to CM + 1V  and the INN peak-to-peak voltage is CM + 1V to CM - 1V (180 degr. phase difference). This results in a differential input peak-to-peak voltage (AINP - AINN) of +2V to -2V (= 4 V peak-to-peak). This within the 2*Vref range and within power supply limits.

    Reg,

    Sake 

     

  • Hi Sake, 

    Thanks for the clarification.

    The intention of equation (5) is to allow for enough voltage headroom above AVSS and below AVDD for the PGA to work properly. When your CM = 1.2 V, the voltage on each input pin ranges from 0.2 V to 2.2 V. This is borderline OK for the negative supply. It makes sense that when you increase the CM a little, your distortion goes away.

    You may ask, "well, if my CM = 1.6 V, and the voltage on the inputs ranges from 0.6 V to 2.6 V, why is this not causing distortion? My max input is still within 200 mV of the supplies?" Perhaps the PGA can actually operate a little closer to the positive supply than the negative, but I'm not sure.

    Two things you may want to try: reduce your CM to 1.4 V, so that you are safer on both ends, and ensure that your CM voltage, power supplies, and input signals are referenced to the same GND.

    Regards,

  • Hello Ryan,

    With a CM = 1.4 V the distortion free differential input voltage is approximately -1.5V to +1.5V.

    I think you should reconsider eq. 5 in the datasheet.

    Regards,

    Sake