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ADS1298 Gain

Bruno Mazzarotto
Prodigy 30 points
Other Parts Discussed in Thread: ADS1298

Hello there.

So the problem is about the ADS1298 gains. For a 2x theorical gain, I got a real 1.33 gain. (4x - 1.6, 12x - 1.844, for example).

Am I missing something?

  • 0
    TI__Intellectual 1600 points

    Hi Bruno,

    You should be getting the gain that one would expect, i.e. going from 4x to 8x should double the signal.  However, the ADS1298 is a pretty advanced device.  In order to make any recommendations, we would need to know a lot more about your system.  Namely, we would need to know what the network driving the ADS1298 looks like, where the signals are coming from, what the ADS1298 supply rails look like, and what commands are being used to configure and read from the device.  The more information you can provide, the more likely we could figure out the issue.

     

    Regards,

    Rudye

     

  • 0 in reply to RudyeM
    Prodigy 30 points

    Well, actually what I did was to set the gain to 1, apply a dc voltage from a power-supply (0.94V) and write down the result from the ADS. After that, I just set the gain to 2x, and note down the value, and so on, until 12x without changing anything else. The circuit designed was the onde suggested at the ADS's datasheet for a unipolar power source (5V for analogical and 3,3V for digital). The registers changed from default were just for the high resolution, sample rate and ad-reference. All channel were set to normal electrode input, changing only the gain values. Hope this can help. Regards

  • 0 in reply to Bruno Mazzarotto
    TI__Intellectual 1600 points

    Hi Bruno,

    There are two concerns I have about running your test the way you described.

    First, it seems like you are starting with too large a signal.  The default reference for the ADS1298 is 2.4V, and the maximum differential range between INP and INN changes according to the PGA setting.  This is covered on page 23 of the datasheet - Max(INP-INN) = Vref/Gain.  So for a gain of 2, the differential input range is only 1.2V.  Any gain above 2 is going to have too small an input range for your 0.94V signal.

    Second, I'm unclear on how you are connecting the power supply to the inputs.  Figure 22 in the datasheet shows two distinct way to connect a signal to the inputs.  It sounds like you are only connecting the power supply directly to the inputs, i.e. power supply (-) to INN and power supply (+) to INP.  This would have the effect of giving  you a common mode (CM) of 0.47V and a differential signal of 0.94V.  If you consider Equation 2 on page 23 of the datasheet, you'll see that that CM isn't large enough for this differential signal.  Since you are using the device in a single supply configuration, your AVSS is 0V.  That means that you need a common mode of 0.47+0.2=0.67 for a gain of 1.  Otherwise the INN input is going up against the input range of the PGA.

    A better way to setup this experiment would be to use 2 power supplies.  One would be setup between ground and INN and would set the CM.  The other would be between INN and INP to set the signal.  This would put you in the 'Single-Ended Input' configuration on page 21 of the datasheet.  You'd still need to be careful about what common mode and signal levels you were providing for each gain setting, but in this configuration you'd have complete control of both parameters.

    Hope this helps!

    Rudye

     

  • 0 in reply to RudyeM
    Prodigy 30 points

    Hi RudyeM,

     

    I've follow your advices and now the ads1298 is working the way it should.

    Thanks for your support.

     

    Regards