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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  • Hello John,

    a R-C filter is required on the analog inputs to the ADC for anti-aliasing purposes. A typical filter would be a 1K resistor in series with each analog input, a 0.1 uF cap from AIN+ to AIN- and a 0.01uF cap from each input to GND. With this network, it is possible to see a signal at the ADC input due to a differential error introduced by the bias voltage.

    You could use a part such as the AD7794, which  is a 6-channel version of the AD7793. You would have 2 channels for the thermocouples and a third channel could be used to bring out the bias voltage for the thermocouples. You would also have an extra channel which could be used for cold junction compensation.

    If you prefer to stay with the AD7793, then I suggest an external bias voltage as you suggested. I do not understand your suggestion regarding IOUT. IOUT is an excitation current. You could set up the bias voltage by using 2 resistors to generate a voltage of AVDD/2 which could then be used a the bias voltage. Another option is to use a reference such as the ADR380. This is a 2.048V reference. The 1K price is $0.78.

    Your error analysis is correct. The FS error includes the error due to the offset (gain error is the error with the offset removed). At a gain of 16, the AD7793 has a full scale error of +/-250 ppm max after performing an internal full scale calibration. This spec is over the complete temp range from -40 to +105 degC.

    Regards,

    Mary.

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  • Hello John,

    a R-C filter is required on the analog inputs to the ADC for anti-aliasing purposes. A typical filter would be a 1K resistor in series with each analog input, a 0.1 uF cap from AIN+ to AIN- and a 0.01uF cap from each input to GND. With this network, it is possible to see a signal at the ADC input due to a differential error introduced by the bias voltage.

    You could use a part such as the AD7794, which  is a 6-channel version of the AD7793. You would have 2 channels for the thermocouples and a third channel could be used to bring out the bias voltage for the thermocouples. You would also have an extra channel which could be used for cold junction compensation.

    If you prefer to stay with the AD7793, then I suggest an external bias voltage as you suggested. I do not understand your suggestion regarding IOUT. IOUT is an excitation current. You could set up the bias voltage by using 2 resistors to generate a voltage of AVDD/2 which could then be used a the bias voltage. Another option is to use a reference such as the ADR380. This is a 2.048V reference. The 1K price is $0.78.

    Your error analysis is correct. The FS error includes the error due to the offset (gain error is the error with the offset removed). At a gain of 16, the AD7793 has a full scale error of +/-250 ppm max after performing an internal full scale calibration. This spec is over the complete temp range from -40 to +105 degC.

    Regards,

    Mary.

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