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ADS1261: Multiplexed sensors design check

Ibrahim Kamal
Intellectual 460 points
Part Number: ADS1261
Other Parts Discussed in Thread: ADS131M04

Hello,

We have to digitize 4 different strain gauges used in bridge configurations. So we need 4 differentials inputs, therefore the ADS1261 seems good for this (by the way, is there a better choice for simultaneous sampling of strain gauges at relatively high frequencies?).

I've used the ADS1261-60 Design Calculator Excel sheet in order to calculate the maximum data rate I'll be able to get.

Using the Chop mode, it seems to be 2912.9/4 = 728.225 SPS. Do you confirm?

I'm a bit confused regarding to the frequency response of the digital filter when multiplexing the inputs, is it the same as a continuous sampling on one channel?

And about the ADC configuration for multiplexed inputs, is this the good workflow:

- First mux on channel 1

- Start continuous conversion

- Wait for !DRDY

- Get the sample

- Mux on channel 2

- Wait for !DRDY

- Get the sample

- etc.

Thanks in advance,

Victor

  • 0
    TI__Mastermind 44680 points

    Hi Victor,

    If you need to sample four channels simultaneously, then you'll either need to use 4 different ADCs operated synchronously, or a simultaneously sampled ADC (a device that integrates multiple ADCs on-chip). You can refer to the following list of simultaneously sampled ADCs to see if any of these devices might better meet your needs: http://www.ti.com/data-converters/adc-circuit/precision-adcs/products.html?pqs=paqs&familyid=2019#p84=24;32&p1028=4;4&p3091=Simultaneous%20Sampling&p1498=Catalog

    Using the 40kSPS data rate, with CHOP enabled, 50us delay, and the internal 10.24 MHz clock, the time to 1st conversion is about 2793 SPS, Therefore, multiplexing between 4 channels results in 2793/4 = 698 SPS / channel. You'd have to provide a slightly faster external clock to increase this to 728 SPS / channel.

    The frequency response of the digital filter is the same as the single channel case. When multiplexing channels, you're running the ADC at a faster data rate (wider filter bandwidth that allows more noise to pass); however, you're not getting data out on each channel as fast as you would in the single channel case.

    You're pseudo code operations look correct.... Writing to the  INPMUX register to change the channel will automatically restart the ADC's conversion process. Then, after /DRDY goes low, you can read out the data.

  • 0 in reply to Christopher Hall
    Intellectual 460 points

    Hello Chris,

    Thanks for your answer.

    Christopher Hall said:

    You're pseudo code operations look correct.... Writing to the  INPMUX register to change the channel will automatically restart the ADC's conversion process. Then, after /DRDY goes low, you can read out the data.

    So can wee guarantee the sample will be taken at a constant frequency?

    Christopher Hall said:

    If you need to sample four channels simultaneously, then you'll either need to use 4 different ADCs operated synchronously, or a simultaneously sampled ADC (a device that integrates multiple ADCs on-chip). You can refer to the following list of simultaneously sampled ADCs to see if any of these devices might better meet your needs: http://www.ti.com/data-converters/adc-circuit/precision-adcs/products.html?pqs=paqs&familyid=2019#p84=24;32&p1028=4;4&p3091=Simultaneous%20Sampling&p1498=Catalog

    ADS131M04 parallel architecture seems to better meet my requirements. However it is in pre production right now. I'm not used to use pre release devices, so can we say they are bug free? I can see a post saying they will be available for "normal" sale in June. Is this also the case for the QFN package?

    Best regards,

    Victor

  • 0 in reply to Ibrahim Kamal
    TI__Mastermind 44680 points

    Hi Victor,

    While multiplexing, the sample rate of the ADS1261 will depend on when the write register command occurs. Therefore, depending on the software implementation, the data rate will likely be fairly consistent with some small timing variances, but probably not as consistent as a simultaneously sampling ADC that is continuously converting (which will only vary due to clock jitter).

    Preproduction devices are:

    "These are prototypes/experimental devices, and are not yet approved or released for full production. Testing and final process, including without limitation quality assurance, reliability performance testing, and/or process qualification, may not yet be complete, and this item is subject to further changes or possible discontinuation. If available for ordering, purchases will be subject to an additional waiver at checkout, and are intended for early internal evaluation purposes only. These items are sold without warranties of any kind."

    Generally, when we release preproduction devices we've evaluated them and found no major issues and expect their performance to be very close to the final product, though it is possible some specifications may still change before they are released to full production.