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Using ADS124x

Saumitra Deshpande
Prodigy 210 points

Other Parts Discussed in Thread: ADS1241, ADS1240, ADS1234, ADS1232

Hi,

I designing a new load-cell based system which have 4 load-cells connected in parallel, each having 350ohm resistance. Excitation is +5V & load-cell output is 2mV/V. I looking through datasheets of ADS1240 & ADS1241 & have couple of doubts.

1) I need to measure individual output of each of 4-load-cells. How the psuedo-differential inputs i.e. using Ain(x) - Ain(com) pairs for each channel of ADS1240 perform with respect to true differential inputs of ADS1241 or ADS1234 ? In other words, how much would it affect to make sense(-) of all 4-load-cells common, connecting to Ain(com) ?

2) Datasheet specifies ENOB=18.5 BITs (Ref=2.5V, PGA=128, RANGE=0, DR=2). From this I calculated the RMS noise voltage as follows,

FSR = +/-2.5/128 = +/- 19.53mV or 38mV for ENOB calculations

ENOB = (log(FSR/RMS Noise)) / log 2) ---> RMS Noise = 105nV (approx)

Pk-pk Niose = 6.6 x 105nV = 693nV

Are these calculations ok ?

Thanks in advance

_Saumitra

  • 0
    TI__Guru** 113765 points

    Saumitra,

    You do not want to connect all (-) connections of the four load cells together as there will be interaction between each bridge.  Either the ADS1241 or ADS1234 will work, but there are differences between the parts in regards to data rates, digital filter response and clocks.  The ADS1234 will run from an internal oscillator, while the ADS1241 requires a crystal or some other clock source.  The ADS1234 operates at a potentially higher data rate (80sps) while the maximum for the ADS1241 is lower (15sps.)  The latency for the ADS1241 (single cycle) is much less than the ADS1234.  You can use the excitation voltage as the reference for the ADS1234 for ratiometric measurements, whereas the reference is limited on the ADS1241 to 2.5V.

    Your noise calculations for the ADS1241 appear correct.  Make sure you compare these values with the ADS1234 which are significantly better.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 210 points

    Hi Bob,

    Thanks for the reply.

    Isn't the the sampling rate for ADS1234 @ 80SPS actually reduces to 20SPS when using all 4-channels, since settling time increases to 51mS ?

    regards,

    _Saumitra

  • 0 in reply to Saumitra Deshpande
    TI__Guru** 113765 points

    Saumitra,

    Yes, the single channel data rate is a maximum of 80sps for the ADS1234.  You have a similar issue with the ADS1241.  Even though the ADS1241 is single cycle settling, you have some extra overhead with mux switching that requires a SPI command, although this could be done at the same time you read.  ADS1232 can be driven with GPIO which changes the MUX more quickly. 

    With the worse case scenario with the ADS1234 you need to take 5 samples.  With a data period of 1/80 (0.0125 seconds) after five periods your data is truly valid (0.0625 sec).  This makes the total throughput rate of reading all 4 channels closer to 15sps after the data is read.  The ADS1241 has a maximum data rate of 15 sps for a single channel.  If you divide this by 4 and if you switch while reading data your total throughput is around 3.75 sps.

    Best regards,

    Bob B