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ADS131M08: Using higher External Vref than datasheet recommands

Josel Go
Expert 7355 points

Part Number: ADS131M08
Other Parts Discussed in Thread: ADS131E08

Hi Experts, 

I'm Using the ADS131M08IRSNR and I've used an external voltage reference of 3V.
After sampling my device with the ADC I've wanted to use the external voltage reference so I've disconnected the 3V and changed it to an internal voltage reference.
When I'm testing the voltage on the REFin line im getting 1.176V [instead of 1.2V +/-0.1%] and the reading that I'm getting from the ADC are wrong.
The reason that the internal reference voltage I'm measuring is wrong is because I've set high voltage reference? because in the datasheet it says that you can set the REFIN as high as VDDA+0.3V which in my case is 3.3V.

The 2nd thing is why am i getting the readings so wrong? for GAINx1 the calculations should be (VALUE*1.2V)/2^23, am i getting it wrong? The OSR is at max value [16,384], The exrenal clock set for 8.182MHz.

Another question about filtering the signals when I'm designing my board - 

I saw when you want to use anti-aliasing you should connect the differential inputs like that - 

image.png

Those values are stay the same for any current I want to measure? because in my case I want to measure small currents from 1uA to 200uA [R_L is 1kOhm] and i would like to know what is the best application in my case that the currents are measuring will be as quiet as they can be with this ADC.

Regards,

Bar

  • 0
    TI__Mastermind 49505 points

    Hi Josel Go, Bar,

    You already had an open query about ADS131M08, can you keep all your questions about ADS131M08 in the same thread in the future?

    First, you should not use 3V reference voltage for ADS131M08. AVDD+0.3V is the absolute limit for Vref on this ADC, any voltage higher than AVDD+0.3V applied to REFIN could damage the device. You can find the normal reference voltage in 6.3 Recommended Operating Conditions table in ADS131M08 datasheet, the Vref should be limited to 1.1V~1.3V.  

    For your reference voltage question, as you already saw my colleague's answer in the following thread, "The typical internal voltage reference value when measured on the REFIN pin will be closer to 1.18V, as shown in Figure 6-29 data sheet plots.  However, there is internal scaling applied to the reference voltage, so that the full scale input range will be +/-1.2V/gain, and this corrected voltage will have an accuracy of +/-0.1% typical and temperature drift of 8ppm/C typical as shown in the datasheet specifications."  1.176V you measured is a right reference voltage.

    ADS131M08: DS shows it internal reference typ is 1.2V ,but curve shows is 1.18V

    I can check whether the conversion code from your ADS131M08 is correct or not if you can share your raw data and also let me know your test conditions including input signal and gain setting.

    Your image was not uploaded successfully, so I can not answer your question about anti-aliasing filter. Please clarify your question as well.

    BR,

    Dale

  • 0 in reply to Dale Li
    Prodigy 10 points

    Hi Dale Li,

    Thank you very much for that answer. About the open query - I didn't open the thread, I've just add set a new question in the data converters forum and I've been waiting for confimation.

    I have one more following questions:

    1. The AVDD in my case is 3.3V so if i use Vref of 3V is'nt it OK? [less than AVDD+0.3V].So if it's not fine and you can't set 3V Vref as you've said, Its not the standart application, you're right about that but would it damage the component?
    2. Another question about that, if the ADC set to the internal voltage and also the external voltage is set to 3V as I already did, would it damage the device if the registers in the device set to internal voltage and not to external [that as you've said, it would ruin the device]?

    3. About the anti-aliasing.
      I'm trying to sample small DC voltage of my transistors drain and i wonder if it would help with filtering some of the noise or any AC noise that interrupts my signal. When I'm looking at the application infromation [section 9], in the section of antialiasing [9.2] will this application help me reduce AC noise? are the components they've set are right for that kind of application? [figure 9.1] meaning adding for both terminals 1kOhm resistor in series and between the 10nF capacitor? or i need to change this filter for my signal.
      adding for both ends 1kOhm might ruin my signal? because i need to measure really small voltages changes [uV].
      For this samples i would like to use gain x1 and gain x8. The input signal needs to be as close to a DC signal around 0.1V for the negative and maximum voltage on the positive would be 0.3V.

    If I'm lacking any information please let me know.
    Thanks,

    Bar.

  • 0 in reply to Bar Elias
    TI__Mastermind 49505 points

    Hi Bar,

    Please see my answers below:

    1. As I already explained, 3V Vref will not damage the device, but it has exceeded the recommended Vref so ADS131M08 will not operate properly and you will not be able to get the conversion code as expected, so you have to limit your Vref to the range of 1.1V~1.3V, typically use 1.25V Vref.

    2. As I said, it will not damage the device because the 3V voltage does not exceed the absolute maximum Vref rating of ADS131M08 which is AVDD+0.3V, but 3V Vref will not work for normal operation. If you really want to use a high Vref, you can use ADS131E08.

    3. It depends on the characteristic of AC noise. The anti-aliasing filter is used for anti-aliasing purpose. Check the details in the following links

    BR,

    Dale