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Part Number: ADS124S08
I'm using a unipolar analog supply. If I make a ratiometric measurement where I use an external reference as an input (such as from an RTD reference resistor), should I enable the reference buffer for REFxN? What about REFxP?
The external reference inputs without the reference buffer are approximately 250k Ohm input impedance. When using a high impedance voltage source or when creating a reference voltage by using the IDAC(s) of the ADS124S08, care must be taken when adding any resistance on the input path of the ADS124S08 reference inputs with the buffer disabled. The default configuration for the ADS124S08 is to use the external REF0 input pair. Also, the default configuration for the REFP input is configured with the reference buffer enabled, and the REFN input is configured with the buffer disabled.
Normally for this type of operation the analog supply will be unipolar (for example +5V and AGND). If the default reference configuration is used for an RTD measurement using a low-side reference, the reference resistor would be tied to AGND. One side of the reference resistor would connect to the REFP input and the other end of the resistor to the REFN input. If a reference input filter is added between the reference resistor (as shown in application note SBAA201) and the reference input, then a unique occurrence can be observed where a significant voltage drop will be seen across the REFN input filter resistor. The input configuration is shown below, and the resistor in question is shown circled:
The voltage drop across the resistor is due to the lower input impedance of the disabled REFN buffer and an error will be seen in the measurement as the reference is sampled. Depending on the value of the resistor, an error of 1% or more can be seen in the measurement result.
The ADS124S08 datasheet recommends disabling the REFN buffer when connected to AGND (AVSS). This means that the filter resistor should be removed in the case where REFN is tied to AGND to eliminate the measurement error. A similar occurrence will also take place on the REFP input if the REFP buffer is disabled.
Tying the REFN input directly to AGND is one option. Another option where both REFP and REFN buffers can be enabled is by using a high-side reference to make the measurement. An example of the high-side reference is shown in the ADS124S08 datasheet and discussed in section 10.2 as shown below:
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