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ADS124S08: What's the condition of the RTD Circuit Design Parameters in datasheet

Willy Chen10
Prodigy 220 points
Part Number: ADS124S08

Hello,

1.I want to know what data rate of ADS124S08 and what mode is in table 45 in datasheet.

2.What does "code flicker" mean in 10.2.2 Detailed Design Procedure?  

and how to make the measurement result on table 45 with no code flicker ?

The key considerations in the design of a 3-wire RTD circuit are the accuracy, the lead wire compensation, and the sensor self-heating. As the design values of Table 45 show, several values of excitation currents are available. The resolution is expressed in units of noise-free resolution (NFR). Noise-free resolution is resolution with no code flicker. The selection of excitation currents trades off resolution against sensor self-heating. In general, measurement resolution improves with increasing excitation current. Increasing the excitation current beyond 1000 µA results in no further improvement in resolution for this example circuit. The design procedure is based on a 500-µA excitation current, because this level of current results in very low sensor self-heating (0.4 mW).

Many Thanks,

Willy

  • 0
    TI__Guru* 97465 points

    Willy,


    The noise in Table 45 mirrors the noise performance of the device using a data rate of 5 SPS in using the Sinc3 filter. However, Table 45 is only used to illustrate that you trade resolution in noise and self heating. That is, you want to increase the reference so that it is large compared to the ADC noise, but keep the excitation of the reference resistance low enough so that the self-heating of the RTD does not cause additional drift of the measurement. For noise performance of the device, Tables 1-8 show the noise with the ADC in various configurations and data rates.

    For code flicker, this refers to the peak-to-peak noise that you may see with a measurement when the input has no added noise. As an example, let's just say you have the inputs shorted are measuring 0V. This should guarantee that you have the least amount of noise in the measurement. The device is in a gain of 1, using the sinc3 filter, with a data rate of 20 SPS. If you measured many points (~500) the ADC would report a code that may have a small offset, but the noise (or code flicker) will be about 16 codes peak-to-peak.

    This can be found from Table 1. For the configuration (gain=1, sinc3 filter, 20SPS), the peak-to-peak noise is 4.8uV. In codes this becomes:

    (4.8uV/2.5V)*2^23 = 16.1 codes

    Additionally, you can calculate the noise-free resolution in the number of bits.

    log(2^24/16.1) / log(2) = 20.0 bits

    or you can calculate it from the noise voltage and the full scale range of the device:

    log(2.5V*2/4.8e-6V) / log(2) = 20.0 bits

    Both will get the noise-resolution for gain=1, sinc3 filter, 20SPS as shown in Table 2 of the datasheet.

    Note that when you measure anything other than inputs shorted, you will add extra noise. There will be noise from the source and the reference that comes into the ADC measurement, but shorting the input will give an indication of the noise performance of the ADC itself.

    If you have any other questions about the ADS124S08 and how it is used, please feel free to post back to the forum.


    Joseph Wu