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AD7794 intead of ADUCM360

Hello 

I want use AD7794 instead of ADuCM360 in CN-0359 Circuit Note.

But I am confused about which pin configuration I should use.

I have indicaetd the ADC pins in the picture.

What should ADC[3:0] pins pole configuration be?

For example : ADC0, ADC1, ADC2, ADC3 respectively (+)(-)(+)(-)

What is the proper congfiguration for these pins?

  • Hi,

    I would suggest to move or post this thread to Reference Circuits as they may have better understanding regarding the circuit. Regarding the use of AD7794 as replacement, it would be great if you will define first what will be your target specifications. What will be the input signal requirements, what is the required accuracy and resolution, ODR and timing conditions, does it required dynamic specs, drivers, external references and PGAs, etc.

    Thanks,

    Jellenie

  • Hello JellenieR,

    Thank you for feedback:) I did what you suggest.

    In CN-0359 at the page 8, it has been specified that if I want to use a discrete ADC, the AD7794 is a good choice. So I want to use it. But I don't know how I configure these ADC[0:3] pins.

    I hope I am clear enough.

  • Hi cevdet29,

    Each of the ADC channels being sent to ADuCM360 in the CN0359 are single-ended inputs.  So you'll want to send each input (ADC[0:3]) to the AIN[0:3] (+) inputs and ground the AIN[0:3] (-) inputs of the AD7794 within this application.

    You'll also need to ensure that you have an excitation current available for the RTD input on the AD7794 that is available.  So you might need to ensure the RTD section of the design is set properly and then set the rest of the channels to read the analog voltages.

    Hope that helps

    Brandon

  • Hi cevdet29,

    The ADuCM360 has pseudo differential capability and can accept the 4 signals as either single ended or differential inputs. The AD7794 has fully differential inputs and what Brandon suggests will be enough for your application. But, you can also connect them as a differential signals with ADC3 and ADC2 connected to a positive and negative differential input, respectively (e.g. ADC3 to AIN1(+) and ADC2 to AIN1(-) ). In this way, you use only two differential input pairs.

    You do have to set the register settings for the excitation currents in AIN5 to use the RTD.

    Also, the ADC signal inputs from the CN0359 are at 1.65V +/- 1.65 V. You will need to use an external reference of 3.3V to fully capture the signal. The ADuCM360 accomplishes this through an AVDD/IOVDD of 3.3V.

    I would also like to suggest you use the AD7124-4. It has pseudo differential operation and can use 3.3V reference. It also is a 24-bit sigma-delta ADC and is for low power applications which is similar to the AD7794.

    Anyway, the AD7794 will still work for your application. Hope this helps,

    Nikko 

  • Hello ACatapang / Brandon

    These was really helpful:)Thank you.. 

    My configuration is gonna be like this:

    ADC3 -> AIN1+

    ADC2 -> AIN1-

    ADC1 -> AIN2+

    ADC0 -> AIN2-

    One more thing; Should I use an external reference? Beacuse AD7794 has its own internal reference. I want to use internal reference for 3.3V.

    I think if I use the internal reference in AD7794, I can only get 1.17V, am I right?

  • Hello ACatapang / Brandon, 

    Could you please reply the question before this one?

    You suggested to use an external voltage reference of 3.3V. Which voltage reference component you suggest me to choose?

    And should I connect the REFIN- pin to the GND? 

  • Hi cevdet29,

    Sorry for the late response, the notifications for this thread were junked by my email application.

    Yes, you should use the external reference. And yes, the internal reference has only 1.17V.

  • Hi cevdet29,

    The AD7794 suggests the use of ADR391 and ADR381 which are 2.5 V micropower low noise voltage reference.

    For 3.3V, I suggest you use the ADR4533 which is ultra-low noise but has Is=950uA compared to the ADR391 with Is=120uA.

    Yes you can connect the REFIN- pin to GND.

    Btw the external reference can only accept 3.3V when AVdd is powered at 3.3V

    Hope this helps.

  • Thank you ACatapang, This is really helpful.

    Thanks for support...