ADS1118: 14 bits on internal temperature and 16 on thermocouple

Pablo Guzman

Part Number: ADS1118

Hi Guys

I have a question related to the temperature measurement, on the data sheet al least for the internal temperature sensor into the device it would show as 16 bit conversion, but we only use 14 bit left justified, I do on that way and it works. My question from here is how was calculated the the value 0.03125 to convert the code to temperature using the formula code*0.03125= temp.

Now using a thermocouple we need to consider the 16 bits or 14bits only as the internal temperature sensor. My setup is AIN2_AIN3 as inputs, FSR to 2.048, 8SPS I would like to know how to convert that code to temp

over 5 years ago

0 Bryan Lizon86 over 5 years ago

TI__Guru 56671 points

Hi Pablo,

Both of your questions are answered in the datasheet in section 9.3.6: one 14-bit LSB is equal to 0.03125C, and section 9.3.6.2 provides a comprehensive explanation regarding how to convert codes to temp.

-Bryan

0 Pablo Guzman over 5 years ago in reply to Bryan Lizon86

Prodigy 30 points

Thanks for your reply.

Ok based on that in both cases the adc and internal temperature sensor temperature data are represented as a 14-bit result that is left-justified within the 16-bit conversion result.

Regards

0 Bryan Lizon86 over 5 years ago in reply to Pablo Guzman

TI__Guru 56671 points

Hi Pablo,

The internal temperature sensor is separate from the analog inputs on the ADC. The temp sensor has a 14-bit result that is left-justified within the 16-bit output. The signal you measure from the ADC inputs is a 16-bit result.

-Bryan

0 Pablo Guzman over 5 years ago in reply to Bryan Lizon86

Prodigy 30 points

Thanks for your reply

Now I have another question regarding the code convertion into a voltaje, what I'm doing is converting the hex 16 bit result form the themocouple into a Dec value, them I multiply that value by the LSB acording with my FSR. That is correct?

HEX= DEC

DEC*(FSR/2^16)= Voltaje measure by the thermocouple.

If that is correct I have an issue cause in ambient temperature I read a value in DEC of 22 if I convert that into a voltaje based on the previous formula it would be 22*7.81uv =171.875uv using a FSR of ±0.256V, that's an increament of 4.3 °C when I add that value to the CJC value for the total result of the temperature.

0 Bryan Lizon86 over 5 years ago in reply to Pablo Guzman

TI__Guru 56671 points

Hi Pablo,

I would recommend you review the following reference design: https://www.ti.com/lit/ug/slau509/slau509.pdf?ts=1611758962308&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FTIPD109

The code-to-temp information in the ADS1118 datasheet is specific to that ADC's temperature sensor, and is not applicable to the actual thermocouple measurement. Instead, this document explains in detail how to take thermocouple measurements with the ADS1118.

After reading, let me know if you have any additional questions.

-Bryan

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ADS1118: 14 bits on internal temperature and 16 on thermocouple