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ADS1261: Reference divider, two load cells with the same AC-excitation, single-ended resolution

Hal Mas
Prodigy 220 points
Part Number: ADS1261

Hi,

I have a load cell connected to an AC-excitation using a H-bridge. The conversion looks good but using the max GAIN = 128 I am still not using the full range of the ADC.

I am currently using a load cell 2mV/V with a 5V Excitation. With a PGA GAIN = 128 it gives 1.28V for the whole range of the load cell. Can I use a 10V excitation instead and use resistor divider of the reference P and N (AIN0 and AIN1) as below:

Do you have any recommendations? Should I add filter, high impedance Op-Amp between the divider and the ADC input? I am scared to add more noise and get worst performances than keeping my 5V excitation.

Second question, I am also planning to add a second load cell measurement. Instead of using a second AD1261, could you confirm I can use the same excitation, REFP (AIN0) ,REFN (AIN1) and use the last two differential inputs AIN8 and AIN9 to measure my second load cell:

I will have to switch the MUX back and forth between:

writeSingleRegister(REG_ADDR_INPMUX,(INPMUX_MUXP_AIN6|INPMUX_MUXN_AIN7));

and 

writeSingleRegister(REG_ADDR_INPMUX,(INPMUX_MUXP_AIN8|INPMUX_MUXN_AIN9));

How can I prevent reading old data values while switch the mux? Should I oversample and discard the first conversion? 

Finally, my last question is about single ended measurement. If I use REFP: AVDD and REFN: AVSS and I want to read AIN0 in single ended mode, what should I use for the Negative Input Multiplexer? I would have think to connect it to AVSS but it is not an option:

I tried to use AIN1 as Negative input and connect it to the ground on my PCB, the result when I short AIN0 to AVDD is something close to 8388000 which make me think I have 2^23 resolution not 2^24.

Thank you

  • +1
    TI__Guru 52396 points

    Hi Hal,

    You have identified the challenge with moving to a higher excitation voltage: is it worth it to get more output from the bridge if this requires adding more components to your system?

    Are you getting enough performance from your system with the current configuration? There is no need to maximize the ADC's full-scale range if you are able to resolve your target number of noise-free counts with the existing system.

    When you change the mux, the conversion in progress restarts automatically. This is described in Section 9.6 in the ADS1261 datasheet. So after the commands are issued, wait until DRDY goes low and this will be settled data from the currently-selected channels

    In order to measure a single-ended input (one referenced to ground or AVSS), you would need to connect another analog input to ground or AVSS and then select this channel as MUXN.

    And as you have noted, the ADS1261 has a binary 2's complement coding scheme, and as such extends from -FS to +FS. If you are only measuring from 0 to +FS, then you are only using half of the ADC's code range, resulting in one less bit of resolution.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 220 points

    Thank you Bryan for the explanations.
    Well I feel I need to try different configuration (SPS, filter ect...) and measure the performances before trying to get something better.