ADS124S08EVM: Data converters forum

Hello Kiyoshi Kogure,

When making RTD measurements it may be helpful to look at the information and various circuits in A Basic Guide to RTD Measurements.

KIYOSHI KOGURE said:

Question 1 　　　9.6.1.5 About Digital Filter Selection
With Three-Wire RTD Measurement, there is no need to scan multiple channels in a minimal amount of time.
In this case, I would like to know if a third-order sinc filter (sinc3) is recommended instead of a low-delay filter.

You will lower the noise slightly when using sinc3 filter.  As you mentioned, the low-latency filter shortens the overall time when cycling through the mux.  If you use a different configuration where more than one measurement is made, the low-latency filter will be the quickest method.

KIYOSHI KOGURE said:

Question 2 　　　9.6.1.6 About Reference Control Register REFP_BUF/REFN_BUF
If 0: Enabled (default), will the reference buffer be disabled?
Also, is it necessary to disable the reference buffer if REFSEL[1:0] is set to 10: Internal 2.5-V reference(1)?

When using a high-side reference, you want to enable both the REFP and REFN buffers.  By default, only the REFP buffer is enabled (bit set to 0).  So you will need to change the default setting for the REFN buffer from disabled (1) to enabled (0) when using the high-side RTD reference in the reference control register (REF - 0x05).

KIYOSHI KOGURE said:

Question 3　　　 9.6.1.6 About Reference Control Register REFSEL[1:0]
For Three-Wire RTD Measurement, High-Side Reference 　

About Reference input selection
　　　　01 : REFP1, REFN1
　　　　10: Internal 2.5-V reference　　　　

　　　　I would like your advice on which one to set.

On the EVM, the high-side reference is connected to the REF1.  So you would choose the the REF1 reference so that the measurement becomes ratiometric.

KIYOSHI KOGURE said:

Question 4　　 9.6.1.6 About Reference Control Register REFCON[1:0]
Set the operation of the internal reference voltage. It is stated that
Could you please tell me more about setting the operation of the internal reference voltage?

and
　　　　 If I set it to Internal 2.5-V reference, do I need to set it to one of the following?
　　　　01 : Internal reference on, but powers down in power-down mode
　　　　10: Internal reference is always on, even in power-down mode

You need to turn on the internal reference as the internal reference is used to establish the IDAC excitation current used for the measurement.  So either option that turns on the internal reference can be used.  In practice, leaving the internal reference always on is the easiest solution but will burn more current as it is always on.  However, if you choose the other selection, the internal reference will power down when in power-down mode.  This conserves power, but you also need to consider that there will be startup delay and analog settling when the reference is powered back up.

Best regards.

Bob B

Hello Kiyoshi Kogure,

Yes, when using the IDACs you must use either 01 or 10 setting depending on your desired use case.  I suggest starting with the 01 setting first to see how that works for you.

Best regards,

Bob B