Estimate of Accuracy/Resolution of ADS1248

Hello Frank,

The accuracy that you may achieve is dependent on mainly three factors:  Accuracy of the Cold Junction compensation Measurement, accuracy of the Type B thermocouple, and accuracy of the Analog to Digital converter performing the TC measurement. There are many factors in your system that will affect each of the 3 items above; the following is an estimate/approximation based on some assumptions about the system and typical specifications of the ADS1248 device.

A quick estimate/example:

1)  Accuracy of the Cold Junction Compensation.  Let's say that the thermocouple is connected through a thermocouple connector/isothermal block and the cold junction compensation temperature measurement is performed using class A PT-100 RTD. Class A RTDs can achieve accuracies of +/-0.15C.

2)  Accuracy of the B-Type Thermocouple.   Lets assume that the Class B thermocouple guarantees +/-0.5C accuracy.  (This info provided by the thermocouple manufacturer).

3)  Accuracy of the A to D system.  The DC accuracy will be primarily a function of the Offset error, Gain error and INL error, and accuracy of the voltage reference.

For example:

A Type B thermocouple @ 700C produces a voltage of 2.431mV at the inputs of the ADS1248.  The user sets up the ADS1248 with the extenal Reference REF5020=2.048V and with a PGA=128V/V.  We can do an estimate of errors at a few points in temperature.  The seebeck coefficient (uV/C) of the thermocouple changes as function of temperature:

Errors due to the ADC (based on t typical specs):

offset error = ~2uV (typ)

Gain error = 0.005% => 2.431mV*0.005%= ~0.1215uV

INL error = 6 ppm => 6 ppm * VREF/PGA = 0.096uV

Reference REF5020 Initial Accuracy Error = 0.05% => 2.431mV*0.05%=~1.215uV

Measurement Error of ADC = square root( INL Error²⁺Gain Error²+offset Error² + Reference Error²)

Measurement Error ADC = 2.345uV at 700C

We know that the Seebeck coefficient of a B-type thermocouple at 700C is approximately  6.8uV/C; using this coefficient we can do a conversion to temperature: 

Measurement Error ADC in Celsius = 2.3415uV/6.815uV = 0.344 C

Finally the total error in the system:->

Total Error @ 700C = square root( ADC Errror² + CJC Error² + TC Error²)

Total Error @ 700C = ~+/-0.6C

We can repeat the same estimate calculations using a spreadsheet for different temperatures to obtain an extimate, please see below.  At 1200C, the calculated error in this example is approximately +/-0.8C.   

 Thank you and Best regards,

Luis