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1. The datasheet recommends a CM voltage range of 1:0V to 2.3V. At the same time Figure 82 in the datasheet shows a Wheatstone Bridge sensor with 5V excitation. This sets a CM voltage to 2.5V. Can you please clarify?

2. Is it possible to increase resolution by oversampling, specially with thermocouples? If so, what will be the suggested oversampling rate?

3. Can you please provide reference to your app notes (published or internal) that covers conversion of thermoelectric emf to temperature (Cold junction compensation is relatively straight forward)? A table lookup for the entire range of thermocouples will be much appreciated.

Thanks,

Sudhir Gupta

• 1.  The common mode range is different for PGA mode vs. PGA bypass.  PGA bypass has the widest common mode range (AVSS-0.1V to AVDD+0.1).  I assume you will be using the PGA.  When using the PGA you can calculate the common mode range based on the equation on page 22.  These equation depend on AVSS and AVDD as well.  In general the optimal common mode input voltage is 1/2 AVDD.  So, for a Wheatstone Bridge with a 5V supply, the output is 2.5V.  This same supply is used for AVDD so the optimal common mode voltage is 1/2 AVDD or 2.5V.  Apply your system requirements to the equations on page 22 and confirm your common mode range.

2.  Page 16 shows how the Effective number of bits is impacted by sampling rage.  Reducing the sampling rate effectively increases the oversampling, and reduces noise.  You will get the lowest noise and consequently the most Effective Number of Bits (ENOB) by choosing the slowest slow sampling rate (i.e. 20 sps).  Rather than thinking about resolution, it is easer to consider the noise error.  If you look at table 1, you can see that for a sampling rate of 20 sps, and a gain of 128 the RMS noise is 0.09uV, and the peak-to-peak noise is 0.41uVpp.  You can think of this as an error introduced on your thermocouple signal.  If conversion time is not an issue, just choose the lowest sampling rate for best results.  If faster sampling is needed you will have to choose the best compromise for your system for achieve faster sampling and acceptable noise levels.   See this Video on ADC Noise for more details on the subject.

3. Here are three different resources that may be helpful to you:  A Basic Guide to Bridge MeasurementsA Basic Guide to Thermocouple MeasurementsA Basic Guide to RTD Measurements

I hope this helps,  Art

• Hi Sudhir,

You will find conversions from thermoelectric emf to temperature for all different types of thermocouples in there both using look-up tables and polynomials.

Regards,
Joachim Wuerker

• Thank you Art for the detailed explanation.

Regards.

Sudhir

• Thank you Joachim,

The information is precisely what I have been searching for sometime.

Regards,

Sudhir