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ADS131E08: Understanding OFFSETCAL operation

Ken Bass
Prodigy 90 points
Part Number: ADS131E08

We are using a ADS131E08 and I am issuing an OFFSETCAL command. While it does do something, I was expecting it to 'zero out' the offset.  I am not seeing that.

I believe I am following guidelines...

My config sequence is:

  • Assert chip RESET - wait 20usec
  • Send SDATAC
  • Issue RREG addr 0 to verify chip present
  • Configure registers WREG
    • CONFIG1=0x96
    • CONFIG2=0xE0
    • CONFIG3=0x40
    • CH1SET to CH8SET = 0x10
  • Assert START line - wait 5ms to allow 1ksps to settle
  • Issue OFFSETCAL - wait 160ms to allow to complete
  • Start sending RDATA after DRDY to read samples

Prior to OFFSETCAL all 8 of my channels are negative.

After OFFSETCAL completes, I am reading the following values:

336 235 -204 798 2929 626 418 3279

Why would I even get a negative number? Why some numbers so high? Taking the channel 8 above with the value 3279 - if I monitor that channel the value varies maybe +/- 40 ADC counts - it is as if 3279 should have been chosen as the '0' offset.

And if set CH1SETtoCH8SET to 0x11 (short inputs and set mux to midsupply), I am reading values:

313 249 259 176 82 378 288 325

I am confused if the OFFSETCAL is even working properly.

  • 0
    TI__Guru* 92655 points

    Ken,


    In general, I think the configuration sequence looks good. What you're getting as output data may depend on what you have connected to the input of the device.

    Let me go through a couple things though. First you set up the device, run the OFFSETCAL command, and then take some data. This data looks like it has some large variance (including one negative value). Then if you set CH1SET to CH8SET to 0x11 with the input shorted to mid-supply, you get good values.

    The last part doesn't surprise me. This is basically what the OFFSETCAL command uses for it's calibration. This sequence does three things. 1. Shorts the inputs to mid-supply. 2. Resets the digital filter. 3. Takes 16 samples for averaging. That is likely why you have the small 100-400 code values for offset. These values are all on the order of 100uV of positive offset.

    After the OFFSETCAL is run, the inputs are disconnected from mid-supply and measure the input. Are you using any input filtering before the inputs? Is there any active amplification before the device? I just want to make sure the the inputs are fully settled. It's not unreasonable to have a negative input in the measurement depending on what is connected. Regardless, can you share even a partial schematic for your circuit?

    It's also possible that there's some EMI pickup in your circuit. This would depend on the layout of your board or any input wiring that you have it connected to. Another thing to try would be to not run the calibration and see what values you get over time. I'd like to see if there is additional settling in your system and if the output values change after a time.

    Read through this and see if any of these comments are of any help. Post back with a response and we can continue to work on this.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 370 points

    We are measuring 8 currents and 3 voltages using two of the ADCs. For the currents, we are measuring the voltage across a 1 ohm resistor. That node goes directly into the positive input of the ADC port. For the voltages, we have a high impedance voltage divider which goes into a buffer amp and that goes directly into the positive input of the ADC port. All IN-N pins are tied to circuit ground. Both voltage and current have low impedance drives into the ADC.

  • 0 in reply to Joseph Wu
    TI__Guru* 92655 points

    Ken,

    You didn't respond to my post and I was just checking to see if you had resolved your offset issue. As I mentioned, the offset calibration shorts the inputs internally to AVDD/2. This might give you different results if you have your inputs connected differently external to the device. 

    I'll close this thread for now. If you continue to have problems, post back and we can continue to work on this.


    Joseph Wu