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# ADS1118-Q1: FSR and internal impedance

For an automotive prototype project, we use your component ADS1118-Q1 for read thermocouple(range of measure is -40°C to 900°C).

But we have problem/understanding.

We don’t understand why when we change FSR, converted values (ADC*LSB) are different.

Is there a link between the internal impedance and the different measurement impedances? Even if Rdiffa and Rdiffb are big(75k). I can't find why value are different. (Internal impedance is 710k to 22M)

See table below with thermocouple type K  (with temperature around 100°C=4mV), even if I subtract 1mV(25°C ambient). I must have 3mV. I don’t understand why I have 1.29mV for 0.512(best resolution normally).

 FSR ADC Value LSB ADC converted mV 4.096 24 125u 2.98 2.048 44 62.5u 2.76 1.024 71 31.25u 2.23 0.512 82 15.625u 1.29
• Hi Jocelyn,

A thermocouple measurement requires two measurements in order to accurately read the thermocouple temperature: the first is measuring the thermocouple, the second is measuring the temperature of the cold junction. It sounds like you are only making the first measurement, is that correct?

We have a thermocouple cookbook that describes in detail the different ways to measure TCs: https://www.ti.com/lit/an/sbaa274/sbaa274.pdf

We also have an app note specifically related to the taking TC measurements using the ADS1118: https://www.ti.com/lit/an/sbaa355/sbaa355.pdf

Please review these documents and let me know if they help clear up some of your questions.

-Bryan

• Hi Brian,

I have excel file with my different measures(different parameters).

Have you an email to send this file?

Best regards,

Jocelyn

• Hi Jocelyn,

If you are comfortable posting the data in this forum, you can do so directly into the thread.

If not, you can hover over my user name and a drop down will appear that says, among other things, "Request friendship". This will allow us to share information offline via direct message.

-Bryan