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ADS1282 SYNC Issue

Darren Gan
Prodigy 50 points
Other Parts Discussed in Thread: ADS1282

Hi All,

Currently working with 3 ADCs... the problem I faced is there are a few hundred micro volts difference between the 3 ADC even though the input signal is the exactly the same.

Prior to acquiring the data, I had performed a SYNC command to the 3 ADC by -> Taking the SYNC pin HIGH for 50us ( SYNC pin was LOW initially after powering up ).. then taking the SYNC pin LOW.

Double check on the DRDY signal after SYNC... ALL 3 ADC's DRDY signal went LOW at the same time.. NO problem with synchronizing the 3 ADC.

However, there are a difference of a few hundred micro volts between the 3 ADCs... depending on the input signal voltage level.

For 1.5V DC input... the difference is 800uV

For 1V DC input....... the difference is 500uV

For 0.5V input.......... the difference is 300uV

As you can see... as the input voltage gets lower... the difference gets smaller...

Does anyone of you faced the same issue before?

800uV difference is actually quite big...

Thank you!

Best Regards

Darren

  • 0
    TI__Guru* 92715 points
    Darren,


    Since the difference varies with the size of the input voltage, what you are describing is a difference in gain error in the three devices.

    I'm not sure of what PGA gain you are running the ADS1282 in, but if you look in the Electrical Characteristics Table of the datasheet, you can see that the DC gain error is somewhere between -1.5% to -0.5% (before calibration). The PGA imparts a systemic gain error of -1% between the PGA output impedance and the modulator input impedance also described in the datasheet on page 16. The variation you are seeing between the 3 devices is about 0.5%.

    I would try to run some sort of calibration between the devices. In the datasheet, the commands are labeled OFSCAL and GANCAL. Much of this is described on pages 30, 31 and 34. First run an offset calibration and then a gain calibration and I would guess that the three measurements would line up.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 50 points
    Hello Joseph,

    Yup. I did both Offset and Gain calibration. But the converted value read back is still difference by several hundreds micro volts.
    The Offset Register read back is =>
    ADC1 = FF F4 A6 (LSB)
    ADC2 = FF F9 8F
    ADC3 = FF F0 61

    The Gain Register Read Back is =>
    ADC1 = 42 B9 E2 (LSB)
    ADC2 = 42 DA 89
    ADC3 = 42 A9 A4

    The value read back from the Gain Register seems odd....

    Did you try to set up 3 ADC as well? And the value converted is EXACTLY the same??

    Best Regards,
    Darren
  • 0 in reply to Darren Gan
    TI__Guru* 92715 points
    Darren,


    The Gain Register seems a bit high. I would have expected 40 Ax xx for a value.

    I'll admit that I didn't try to set up 3 different ADCs. I did manage to find three ADS1282EVMs though and I've gathered the proper equipment. I'll need some time, but I should be able to set up a test to see that the calibration should get the devices to about the same point.


    Joseph Wu
  • 0 in reply to Darren Gan
    TI__Guru* 92715 points

    Darren,

    This test ended up being a bit harder to do than I thought. The real problem came because the ADS1282EVM isn't really set up to run a calibration in a convenient way. The EVM has a front end buffer consisting of a set of OPA1632s on the front end. I have to make a measurement to make sure that I know what the input to the ADC is, and there's no convenient measurement point. In the end, I soldered some points on to get both the ADC inputs and the reference inputs to get a proper reading from the multimeter.

    Also, the references for all three EVMs do not match, so I calibrated each to the equivalent value of the reference on board each of the three EVM. I would note that I put in half of the reference value as the input to run the calibration. I'll explain how I did that.

    First, I used a Data Precision 8200 to generate my input voltage. This is basically a precision DAC that outputs an adjustable output voltage to 10uV precision. I use this as the input to the buffer stage, but remember that I read back the voltage at the input of the ADC.

    I set the input to the buffer so that the output of the buffer and the input to the ADC is 0V. Then I run an offset calibration command. Then I measure the reference input with an Agilent 3458A and calculate 1/2 that value to get my full scale value. I then set the Data Precision so that the buffer output is this value and run the gain calibration so that this is full scale.

    I could have calibrated this to the a different value, but in this case I wanted to show that the reference and the input could be calibrated to match. In any ADC, you compare the input with a known reference to get the digital output code. Note that you should always calibrate the offset first and gain error second. If you reverse the order, then your gain error could be off by the amount of offset there is compared to the full scale.

    Now that the device is calibrated, I can change my input and get data for these three devices. I made measurements with the inputs at about 0V, 1V, 2V, and 2.5V (the last near full scale). I record the measured input and then the output code. With this data I can calculate the offset and gain error that I have after calibrating the device. I've put together a spreadsheet that I'll attach to this post. Each column has a description and you should be able to repeat the measurement.

    I did have a small offset of about 1000 codes. This is the equivalent of about several micro-volts which I described earlier. I'm not surprised by this because I only have 10uV or resolution with my input Voltage setting. In the end, the gain error I got was about 0.002% to 0.005%. The specification says I should be able to get better. I did get to about 125uV of accuracy and less with calibration. In my setup, I'm sure I could do better with wiring to lower the noise and get better accuracy. I had the setup in a noisy environment, and wires strung all over to get a quick result.

    The thing that I would take away from this measurement is that you need to consider both the input and the reference to get a proper measurement. I've calibrated the offset and gain, measured back both inputs and reference inputs to make sure that the ADC gives back accurate data. Read through the spreadsheet and see if it makes sense and we can discuss how your setup can be made to calibrate the ADC.

    Joseph Wu

    ads1282_data.xlsx