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ADS114S08: the resolution of ADC of 2.4 of sbaa275 configuration

Toshihiro Watanabe
Genius 10235 points
Part Number: ADS114S08

Hello,

When using 2.4 of sbaa275 configuration, I think analog input and reference input voltages are always positive.

ADS114S08 input range is +/- Vref/gain so it seems half of input range doesn't use.

Does this mean ADS114S08 loose 1bit because the half of input range doesn't use?

Is there any way for using full scale of ADC?

We would like to use ADC full scale range for RTD measurement.

Best regards,

Toshihiro Watanabe 

  • 0
    TI__Guru** 112705 points
    Hi Watanabe-san,

    You are correct in that the circuit in section 2.4 of the application guide SBAA275 for RTD measurements will only use the positive portion of the full-scale range. As only one current source is used, current will only flow in a single direction with the voltage drop measured across the RTD as a part of the voltage divider of the RTD and the reference resistor.

    It is possible to measure in both directions of the full-scale range if 2 current sources are used and an additional biasing resistor is used in series with the second current source. This would be similar to the circuit in section 2.3, but with a resistor (such as 100 Ohm) placed between the terminal block and lead 2. However this method, though possible is not recommended as there will be additional error created by drift of the resistor as well as added noise of the second current source in the reference.

    The concept of reduced measurement resolution by limiting the measurement to 15 bits (as opposed to 16 bits) is not necessarily accurate. Although it would be best to capture the entire full-scale range in the design, this is often not practical due to actual operating parameters.
    Measurement resolution is a factor of circuit noise, the IDAC current, the voltage value created for the reference, any PGA gain applied and the temperature range to be measured. The actual voltage output to be measured can be fine tuned to maximize as much of the full-scale range as possible. but quite often the factors of gain, reference and current will have some limiting factors. For example, the reference resistor to be used is based on precision and low drift, and the value calculated for maximizing the full-scale range may not be a standard value. The amount of gain that can be applied will be a factor of the input common-mode voltage for the analog inputs and the reference/IDAC current combination used.

    Attempting to use both positive and negative sides of the full-scale range will produce a smaller voltage to be measured. This results in more gain to be required and a reference voltage that may not be an optimum value to fill the full-scale range now created. In the end there may be no benefit to the measurement resolution but will add error of the additional resistor and current source.

    Best regards,
    Bob B