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ADS1148 0-20mA precision sensing

Mayank Gupta
Prodigy 240 points
Other Parts Discussed in Thread: ADS1148, ADS1248

Hi,

I am using ADS1148 for sensing 0-20mA sensing. I am trying to attain 4 digits precision with the same. Problem I am facing is that out varies with different ADS1148 chips.

4-20mA Loop Calibrator Input applied: 20mA

Voltage at burden resistance (100ohm 0.1%): 1.995 (as measured by Fluke 15B+)

Value read by different ADS1148 (multiplier 10): 19.918, 19.950

With precision of ADS1148 I am expecting values to be précised upto 4 decimal places

We are unable to trace out why different ADS1148 chips are giving different result.

Vref: On chip 2.048

Power Supply: Unipolar 0V - 4.5V

Noise level on supply rails: +- 20mV

7215.Sheet1.pdf

ADC Value Conversion

ad_code = bytes[1] << 8 | bytes[0]

val = ad_code & 0x7FFF

sign = (ad_code & 0x8000) == 0x8000;

if (sign) {
val *= -1;
}

ADC_VAL = val 

Conversion Formula

VOLT = (ADC_VAL * 2.048) / 32767

SIGNAL = VOLT * 10

Regards

  • 0
    Prodigy 240 points
    Further more

    Sampling rate is 5SPS with 50/60 Hz digital filtering
    Channel Selection is differential
    MUX0.MUXSP_AIN0 | MUX0.MUXSN_AIN1
  • 0 in reply to Mayank Gupta
    TI__Guru* 97555 points
    Mayank,


    Are you asking why one device measures a value of 19.918 and the other measures 19.950?

    It is likely that the error that you are seeing is just the gain error of the device. Even if the reference value is exactly 2.048V (and there is likely some error due to that as well) the gain error of the ADC is still 0.5% max. This is listed in the Electrical Characteristics table in the datasheet on page 3.

    As an example if you are making a measurement with your conversion you still have a 0.5% additional error.

    VOLT = (ADC_VAL * 2.048)*10*(1±0.005) / 32767

    Assuming you are supposed to get 19.95, just based on the gain error you could be off by as much as 10mV.

    Note that the error in the reference is also a contributor in this equation. As an example, if the Reference is really 2.047V instead of 2.048V, then there may be additional error. If you measure the reference to be 2.047 and use it in the equation, then there isn't any error from the reference. However if you have a reference of 2.047V and you use 2.048V in the equation, then every measurement will be 0.05% high.

    Again just looking at the result, it is likely the discrepancy in the measurement is from the gain error.



    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 240 points
    Thanks for reply.

    I am using PGA: 1 (a gain error 0.5% will produce close result). In terms of production - do we need to measure the gain error of individual IC and and include that as an adjustment factor? or there is any better approach for same?

    Also, I have noticed that Vref is 2.044. Reading is taken by Fluke 15B+, so I am not sure about accuracy of last digit displayed on multimeter. What is the best practice to measure the Vref so that we can substitute the actual Vref value and get more accurate results?

    Regards
  • 0 in reply to Mayank Gupta
    TI__Guru* 97555 points
    Mayank,


    Since the gain error varies from device to device, you would likely have to measure each device and include some sort of customized adjustment factor.

    Also, since the VREF is variable as well, you would need to include that in the adjustment factor as well. However, you would need something more accurate that the Fluke 15B+ as your multimeter. Note that on the 4V scale of that multimeter, your accuracy is 0.5%+3 counts. In this case the multimeter has a similar accuracy as the ADC as well.

    I would note that the ADS1248 has a better accuracy (0.02%) than the ADS1148. However, the reference accuracy for both is ±10mV. If you want to avoid having to calibrate the gain, you may need to find an external, low noise reference, with better accuracy.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 240 points
    Sounds good Joseph
  • 0 in reply to Mayank Gupta
    TI__Guru* 97555 points
    Mayank,


    One other thing that I'd forgotten was that there may be some error depending on how you have the device hooked up.

    Note that the input common-mode range extends from AVSS+0.1V to AVDD-0.1V. If you are making a low side measurement, the input range does not extend to ground on a unipolar supply. This is because of the front end PGA (even in PGA=1). There are limitations on the amplifier's input range and output range in gain so you can't reach either rail.

    I thought I'd mention that just in case this is another source of error for you.


    Joseph Wu