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ADS1220 Single-ended shorted input

Samuel Tal
Prodigy 130 points
Other Parts Discussed in Thread: ADS1220

Hi, 

Using the Olimex BB-ADS1220 tiny board with the ADS1220 for quite some time, I measure B-Type thermocouple 5-13000 uV in differential mode between channels 0-1, gain 128, 20 SPS with very good results.

I added now two single ended channels AN-In2 and 3 to Vss, gain 4, pga disabled to measure other higher voltages..

While channel 2 reads OK, when power is applied to the board channel 3  shorts to ground  (I tie the 2 inputs together for testing)
With no power, the input is high..
(Measuring the inputs with Ohmmeter at no power shows approx 2.4MOhm on both) 

I thought the problem was with the board (chip), but the same problem occurs with a new one.

I can suspect my complex program, but I could not find in the ADS1220 specifications and manual any register setting that will short AN3 to Vss.

Is this a known issue, can it depend on bad register setting? (I manipulate the registers all the time, including internal temp measurement, offset calibration and channels' scanning).

Thanks

  • 0
    TI__Mastermind 23275 points
    Hi Samuel,

    I would suspect that you set the low-side power switch bit (Bit PSW in Configuration Register 2) incorrectly by accident. You need to set this bit to 0. Otherwise the device will short AIN3 to AVSS whenever a conversion is ongoing.

    BTW: We recently released an updated ADS1220 datasheet to the web where we tried to clarify the low-side switch connection a little better.

    Regards,
  • 0 in reply to Joachim Wuerker
    Prodigy 130 points

    Thank you Joachim for your reply.

    I overlooked the PSW bit, and probably did an error when converting my command to binary, and, as you wrote, had that bit set.

    The problem is fixed now thanks to your support.

    Sam