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ADS1261EVM: ADS1261 CONVERSION DATA

Ismihan Gül Gürbüz
Prodigy 30 points

Part Number: ADS1261EVM
Other Parts Discussed in Thread: ADS1261

Hi,

I am using below register configuration for ADS1261EVM in my application.

MODE0: 2400 SPS, sinc 1

MODE1: Normal mode, CONVERT= Continuous conversion, DELAY= 50us

REF: REFENB = Internal Ref Enabled, RMUXP = Internal Positive, RMUXN = Internal Negative

PGA: PGA bypass, GAIN=1

INPMUX: MUXP = AIN3, MUXN = AIN4

INPBIAS: VBIAS disabled, Pull-up mode

AVDD and AVSS are connected to 5V and Gnd respectively.

With above configuration I tried to measure the voltage between AIN3 and AIN4.

Firstly, 2V was connected to AIN3 and AIN4 was connected to ground.

When I read the conversion data, I read 666241 hex value. I calculated the necessary operation (conversion data * ( Vref / 2^23) ),
I found 1.9997 V. This value is very close to 2V. There was no problem.

But, when -2V was connected to AIN3 and AIN4 was connected to ground.

When I read the conversion data, I read 9adb99 hex (sign) value. I calculated the necessary operation (conversion data * ( Vref / 2^23) ),
I found -1.9754 V.

Why is the positive value very close to 2V while the negative value is not very close to -2V?

I tried sending the system offset calibration command and reading conversion data again. But the results did not change.

Thanks,

İsmihan.

  • 0
    TI__Guru 58781 points

    Hi Ismihan,

    You mention that AIN3 = -2V in your post. The ADS1261 cannot measure true negative voltages (below ground) using a unipolar supply (AVDD = 5V, AVSS = ground). This can damage the ADC.

    You can read more about how an ADC can read negative voltages here: https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/1048636/faq-can-my-adc-measure-negative-voltages?tisearch=e2e-sitesearch&keymatch=faq%3Atrue

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 30 points

    Hi Bryan,

    The input voltage was connected from an isolated source. Therefore, a negative voltage is not applied to the ground of the ADC. Differential input voltage was applied to AIN 3 and AIN 4. The negative voltage was applied from a differentially isolated source. For example, we connected the ground of the isolated source to AIN3(MUXP) and we connected 2V of isolated source to AIN4(MUXN). We did not connect the ground of the ADC any AIN port.  Do I still need to use a bipolar supply (AVDD = 2.5V, AVSS = -2.5V) ?
    Thanks,
    Ismihan
  • 0 in reply to Ismihan Gül Gürbüz
    TI__Guru 58781 points

    Hi Ismihan,

    Thanks for the clarification. Typically, saying that "-2V was connected to AIN3" implies that this is a true negative voltage with respect to ground. It seems what you have actually done is ground AIN3 and then apply 2V to AIN4, which gives you a differential measurement of -2V. This configuration is fine using a unipolar supply.

    Another way to test this measurement would be to hook up the voltage source the way you had it initially (AIN3 = MUXP = 2V, AIN4 = MUXN = ground). Then, simply swap the inputs internally (MUXP = AIN4 and MUXN = AIN3). This should give you the exact same results as your tests, without having to modify the connections to the ADC in any physical way. Please try this test and report back the outputs.

    Also, why are you manually performing the code conversions? The EVM software already does this for you, and reports the noise as well. Can you provide screenshots of the GUI for both measurements so I can see how the noise changes between each?

    -Bryan