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AD7176-2 REFOUT while using an external reference

Hi,

I am designing a board to read +-10V signals using the AD7176-2. I want to drive it by a AD8475 as on page 24, but to be able to use a single 5V supply, I will use an ADR444 supplying 4.096V as an external reference.

Now I am unsure about the state of REFOUT, and I cannot really find this specific part of information in the datasheet.

The example circuit on page 24 of the datasheet suggests that although an external reference is used, there is still 2.5V on the REFOUT pin. On the other hand, on page 27 of the datasheet it says:

The internal reference is output on the REFOUT pin after the REF_EN bit in the ADC mode register is set

So to be specific, here is my question:

When using an externel reference voltage, i.e. REF_EN is NOT set, will the 2.5V internal reference voltage still be available on the REFOUT pin?

Many thanks in advance,

DRies

  • Hi DRies,

    Upon AD7176 power-up, the internal reference voltage is enabled by default and is output on the REFOUT pin. If internal reference voltage is not used, disable this by writing to the ADC mode register (controlled by REF_IN) upon power-up. If this is done, the 2.5 V internal reference voltage will not output on the REFOUT pin.

    Regards,

    Johnny

  • Hi Johnny,

    thank you for your reply. I don't understand, why Fig. 30 is in the datasheet then, as it is obviously wrong. Either, the whole external reference part is not used at all, or the common mode adjustment of the AD8475 is defunctional. Any idea which is the case?

    Cheers,

    DRies

  • Hi DRies,

    I think that the circuit only emphasizes driving 2 differential inputs of the device but also shows how an external reference should be set up. But you are right - this may lead to confusion as it may look like the external and internal reference voltage are working together at the same time. Actually, if you look at its evaluation board, there are solder links before the REFOUT and REFIN pins. However, these are not shown here.

    Anyway, thank you for pointing this out. This will be noted.

    Regards,

    Johnny

  • Hi DRies,

    There is no problem with that schematic.

    I have to clarify that there are two places related to references:

    1. REF_EN in ADC Mode register

    2. REF_SEL(x) in Setup configuration (x) Register.

    The 1st one is to determine whether internal 2.5V reference is enabled, and if yes, it will be output to REFOUT pin.

    The 2nd one is for the ADC core to select a reference (internal, external or AVDD) for conversion.

    The two are actually independent.

    So you don't have to select the internal reference for conversion when you have enabled it.

    And you can select it even if it is not enabled (ADC will give wrong conversion of course, same thing if you selected external and don't connect a proper reference to the REF pins).

    The configuration to make the schematic work is like this:

    REF_EN set to 1, so a 2.5V will be output to set common-mode for the diff-amps.

    REF_SEL(x) set to 00, to select external reference ADR445 for ADC conversion.

    This isn't the first time somebody had a question on AD7176's references.

    And if I don't have a EVB near me, I'll get confused myself.

    So thank you for the question and we do need to figure out a better way to explain this in datasheet.

    Regards,

    Cheng

  • Hello

    I have heard that REF OUT does not output 2.5V when REF_EN is disable.

    Can I select bipolar mode of 2.5V in the case of using external reference 5V ?

    Best Regards,

  • Hi sin,

    You can use an external reference of 5V. In this setting, you can accept a differential input range of -5V to +5V. Pseudo differential input will be limited to -2.5V to AVDD, assuming you set AVSS to -2.5V. Also take note that with pseudo differential inputs, the INL specification is degraded.

    Regards,

    Jarwin

  • Hi Jarwin

    Thank you for your quick reply.
    In using an external reference of 5V, should I set REF_EN to be disable ?
    And how much volt is outputed in the REFOUT pin, 2.5V or 5V ?
    I cannnot understand Figure 30.

    Best Regards,

  • Hi sin,

    You can set REF_EN to be enabled in order to get a 2.5V output in the REFOUT pin even when using an external reference of 5V, which is set using the REF_SEL(x).

    Regards,

    Jarwin

  • Hi Jarwin

    Thank you for your help.

    You have been very helpful.

    Best Regard,

    Sin