AD7793 noise and accuracy

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

CN0206.pdf
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  • Thank you very much, Mary.

    May I please run another question by you?  The AD7785/AD779x have two differential inputs (not counting the reference inputs). This made me think that it wouldn't be hard to develop a two-channel thermocouple system, as long as we had another way to do the cold-junction compensation. However, there is a AD779x Frequently Asked Questions Doc that warns that using the internal bias circuit creates differential noise on the input when filtering components are present. The magnitude of this noise isn't dicussed, but since it's brought up in the document, I'm assuming it's a problem and should be avoided. Would you agree?

    One suggestion to get around this is "if the ADC has an unused channel, the bias voltage can be made available on this unused channel, then applied to both AIN+ and AIN-". That's fine, but now I won't be able to implement a two-channel thermouple.

    So I had this idea instead. What if I used one of the Iout channels to turn on a low noise 1.2V reference. Then, this 1.2V reference could be used as the bias voltage for each channel. Do you think using the same reference for both channels might cause a problem? I don't think I'd have to pay for an accurate reference, since it's just being used as a bias, and it's the differential thermocouple voltage I'm looking for. I'm thinking it's much more important to use a low noise, low tempco reference. Is this a good idead? Can you recommend a reference?

    Thank you very much!

     

Reply
  • Thank you very much, Mary.

    May I please run another question by you?  The AD7785/AD779x have two differential inputs (not counting the reference inputs). This made me think that it wouldn't be hard to develop a two-channel thermocouple system, as long as we had another way to do the cold-junction compensation. However, there is a AD779x Frequently Asked Questions Doc that warns that using the internal bias circuit creates differential noise on the input when filtering components are present. The magnitude of this noise isn't dicussed, but since it's brought up in the document, I'm assuming it's a problem and should be avoided. Would you agree?

    One suggestion to get around this is "if the ADC has an unused channel, the bias voltage can be made available on this unused channel, then applied to both AIN+ and AIN-". That's fine, but now I won't be able to implement a two-channel thermouple.

    So I had this idea instead. What if I used one of the Iout channels to turn on a low noise 1.2V reference. Then, this 1.2V reference could be used as the bias voltage for each channel. Do you think using the same reference for both channels might cause a problem? I don't think I'd have to pay for an accurate reference, since it's just being used as a bias, and it's the differential thermocouple voltage I'm looking for. I'm thinking it's much more important to use a low noise, low tempco reference. Is this a good idead? Can you recommend a reference?

    Thank you very much!

     

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