Hello,

I have a couple of questions abou the AD7793. On page 3 of CN-0206, the resolution of the system is calculated to be 0.014C. The p-p noise is stated as 6.6*0.088uV=0.581uV. But isn't the 0.088uV an output noise spec (per table 8 of the datasheet)? With a gain of 128, the sensitivity of the thermocouple is now 128*40uV/C=5.12mV/C.

Shouldn't the output noise of 0.581uV be compared to a sensitivity of 5.12mV/C? I'm saying this because Table 8 doesn't say this noise is "referred to the input" of the device.

Also, I'm trying to figure out what the accuracy of a thermocouple system would be using the AD7793. For now, I'm trying to calculate the error caused by only the 0.01% reference. Please check my math and methodology: For 40uV/C and a gain of 16, the ADC input signal is now 640uV/C. For a K-type thermocouple, the signal at 1370C is 54.819mV. The gain of 16 brings this to 0.877104V (I've chosen a gain of 16 because if I increase the gain to 32, then the ADC input will be above 1.17V, and I won't be able to use the internal reference). With a max signal of 0.877104V and a 0.01% reference, the error due to the reference is 87.7uV max. 87.7uV/640uV/C = 0.137C. I'm thinking this would be the error just due to the internal reference for a K-type thermocouple at 1370C. Have I done this correctly? I'd appreciate any feedback you can send my way.

Thank you very much,

John Araujo

Hello John,

the rms noise numbers listed in the datasheet are input referred. This is not stated clearly in the datasheet.

I agree with your error calculations regarding the internal reference.

Regards,

Mary.