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problem with ADS1248 and DAQ (DT9826) measure the same voltage

Houssem Khouja
Prodigy 90 points
Other Parts Discussed in Thread: ADS1248

Hello,

I am working with the ADS1248 to take measurement. I developped the driver and it's ok.

Now I have to make comparison between ADS1248 measures and a DAQ (DT9826) to prove that ADS1248 measures are good. I made two tests and the results was the following:

- Test 1:

I connected a calibrator to both ADS1248 (ch0,ch1) and DT9826 inputs (same calibrator output).

The calibrator outputs a voltage equal to 500mV. 

The result is that I get exactly 500mV on the DT9826, but I get 497mV on the ADS1248.

The strange part is that when I only connect the ADS1248 to the calibrator I get 500mV like I am supposed to receive.

When I put back the DT9826 in parallel the ADS measures 498mV again.

-Test 2:

I connected a PT100 sensor in 4 wires configuration. On the ADS1248 the measurement channels are ch0 and ch1, and the excitation channel is ch4. 

On the DT9826 I connect ch0 input at the same points ch0 and ch1.

on the DT9826 I get around 110 mV (110ohm*1000µv excitation) wich is good, on the ADS1248 I get full scale measurement.

When I disconnect the DT9826, the ADS1248 measures around 110 mV.

Could this problem be related to the input impedance of one of the DAQs? Otherwise why am I getting full scale when I connect the DT9826?

  • 0
    Prodigy 90 points

    It turns out that the DT9826 inputs are not differential,so for the second test when I connected the DT9826 inputs to ch0 and ch1 of the ADS1248, I connected the ch1 input of the ADS1248 to the ground thats why I get full scale measurement.

    But I still don't know why measurement is not exact for the calibrator. I hope someone have a suggession.

  • 0
    TI__Guru* 92715 points

    Houssem,

    I'm not sure what is wrong, but I would suspect the problem is with the DT9826. When you do not connect the DT9826, it looks like your measurements are correct.


    Try Test #2 again. When you run this test, you say the ADS1248 gives full scale when the DT9826. When you have this case set up, measure the voltage across the 110Ohm resistor to make sure that it is really at 110mV. Do this with an external multimeter. If this is much higher, it might be that the DT9826 is sourcing current into the measurement. This might explain the change in Test #1. If there is extra current, the load regulation may cause the output voltage to change as well. You can also measure the output voltage with with another multimeter to check that the calibrator has changed voltage.


    I'm not to familiar with the DT9826. Is there a setting to output a source current that you accidentally have left on? You could just attach it to a resistor and measure it back to see if there is extra current.

    Joseph Wu

  • 0 in reply to Houssem Khouja
    TI__Guru* 92715 points

    Houssem,

    Sorry, I missed this added post. Regardless, you should be able to take another multimeter and measure back the output of calibrator for test 1 or even use it for a test similar to 2.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 90 points

    Hi Joseph,

    Thank you for the answer. I made these tests and the multimeter mesures the right value (110mV).

    Actually as the DT9826 works in common mode, I have to work in common mode two. I also noticed that in common mode the ADS1248 can't measure negative values.

    Maybe supply voltage to ADS1248 should be different for common mode. Now I am supplying the ADS1248 this way: AVDD=5V, AVSS=GND, DVDD=3.3V and DGND=GND.

    Could this be the reason?

  • 0 in reply to Houssem Khouja
    Prodigy 90 points

    To be more clear, I meant by common mode, single ended mode. So AIN1 is connected to the ground.

  • 0 in reply to Houssem Khouja
    TI__Guru* 92715 points

    Houssem,


    That may be part your problem. If you look at page 3 in the electrical characteristics, you can see this line:


    If you ignore the gain, you can read this as the inputs should be between AVSS+0.1V and AVDD-0.1V. If you have a single 5V supply, the inputs should be between 0.1V and 4.9V. It is outlined a little more on page 26, with Figure 53. If you consider that A1's and A2's can only go to 0.1V to the negative rail, it shows the basic idea of how the inputs work.

    If you choose +/-2.5V for the analog supplies, you can use GND as one of the inputs.

    However, in your first post you said that the input was correct only when you attached the DT9826. I'm not sure how this would make the reading worse. I would use a precision multimeter to measure the input and the reference to get an accurate reading of the gain error. Regardless, with a 5V AVDD, an input cannot be tied to GND for accurate readings.


    Joseph Wu