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General question offset Shifting with the ADS1248

Inspiron
Prodigy 30 points
Other Parts Discussed in Thread: ADS1248

I have a general, I think very easy question concerning ADS1248 analog frontend. I want to use the ADC on a Wheatstone Bridge.

The configuration should be as follow:
AVDD = 5V, DVDD = 3V, AVSS = 0V
Sensor Supply AVDD
Differential Measure
Voltage Reference Extern (Voltage Divider on AVDD): 2.5V
PGA 128

The datasheet says that the FS Input Range is +/-Vref/PGA, so in my case it is +/-19.53mV.
Assuming that I have a Signal Range from 0mV to 39mV, is it possible to shift the signal with the Offset Shift Register to -19.5V...19.5mV to use the complete ADC Input Range?

2nd question: is the Voltage Divider a possible, good solution to generate the external reference source?

Thanks for your help!

  • 0
    TI__Guru* 97465 points

    What you're describing won't work on the ADS1248. The input of the ADS1248 will get over-ranged at about 10-15% of the full-scale (either positive or negative) and get limited by the output of the ADC's modulator. This is based on the ratio from the size of the input to the size of the reference. So if you imagine (with a 2.5V reference, and a gain of 128) the input range to be +/-19.5mV, then the max input that could be corrected for would be about 22-23mV. The offset is a digital correction to the output of the digital filter and can't correct for that much difference. If you want to shift the input range, then you might be able to alter the Wheatstone bridge. You can add resistors to change the range and get it within the range of the ADC.

    For the second question, I assume that your question means you want to construct a voltage divider from the supply. The answer is yes, it is possible, but the risk is that your noise might be higher. References are generally designed to be low noise and low drift, stable voltages. Supplies are not, in addition, you would have noise coming from digital clocking and any supply noise coming from the supply generation.

    However, if you set up the measurement as a ratiometric system you can limit the effect of the noise. If you have a source used to drive the bridge and the reference at the same time, then the noise on the reference is the same as the noise on the sensor. This would cancel the noise in the measurement since the ADC reading is a direct ratio from the reference voltage.

    Regardless, you could use a voltage divider to drive the reference from the voltage used to drive your bridge.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 30 points

    Thank you for the fast answer.

    Joseph Wu said:

    What you're describing won't work on the ADS1248. The input of the ADS1248 will get over-ranged at about 10-15% of the full-scale (either positive or negative) and get limited by the output of the ADC's modulator. This is based on the ratio from the size of the input to the size of the reference. So if you imagine (with a 2.5V reference, and a gain of 128) the input range to be +/-19.5mV, then the max input that could be corrected for would be about 22-23mV. The offset is a digital correction to the output of the digital filter and can't correct for that much difference. If you want to shift the input range, then you might be able to alter the Wheatstone bridge. You can add resistors to change the range and get it within the range of the ADC.

    Ok, that means if I alter the bridge to get a signal range from -19.5mV to 19.5mV instead of 0 to 39mV I can use the full input range, is that correct? The 10-15% overranging you talk about, is it safe to use it, or do you recommend not to overrange the input?

    Because the offset is a digital correction to the output of the filter, it would take the same effect if I add the offset to the output code in my uC. So the correction of the offset in the ADC will has no affect to the analog input, right?

    Joseph Wu said:

    However, if you set up the measurement as a ratiometric system you can limit the effect of the noise. If you have a source used to drive the bridge and the reference at the same time, then the noise on the reference is the same as the noise on the sensor. This would cancel the noise in the measurement since the ADC reading is a direct ratio from the reference voltage.

    Regardless, you could use a voltage divider to drive the reference from the voltage used to drive your bridge.



    Yes, I will drive the bridge with AVDD and using a voltage divider to drive the reference voltage to set up a measurement as a ratiometric system.

  • 0 in reply to Inspiron
    TI__Guru* 97465 points

    Yes, if you alter the bridge a little, you should be able to shift your input range to get something bipolar (-19.5mV to +19.5mV). When it comes to the over-ranging, it won't cause any damage to the part and should be safe. I would generally not operate it here, just in case there is some delay caused by over-ranging the gain amplifier.

    The offset can also be performed by the uC, functionally it is the same as if it were done by the ADC. The offset correction is a digital implementation has no effect on the analog input.

    Let me know if you have any other questions.

    Joseph Wu

     

  • 0 in reply to Joseph Wu
    Prodigy 30 points

    Ok, thank you for your answer!

    Regards
    Marco