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AD7380 Analog Input Range

I'm not following what the allowable ground referenced voltage is for the analog inputs on the AD7380.

The 7380 specifies an "absolute input voltage" of -0.1 to Vref+0.1.

I read that as telling me that I if I have a 2.5V reference the analog inputs must be between -0.1 and 2.6V.  That means realistically I need a common mode voltage of exactly 1.25V to realize full +2.5/-2.5V dynamic range.

On the other hand that conflicts with my reading of the common mode range which is specified as 0.2 to Vref-0.2 or 2.3V?

This is an important point considering no buffer amplifier can drive all the way to ground without a costly negative supply.  If possible I'd choose a common mode input of Vcc/2=1.65V and a 2.5V reference meaning my analog inputs would swing between 0.4V and 2.9V.  That's realistic for a driver amp.

Finally the section on the input structure implies that anything between the rails is ok from the perspective of the diode protection circuitry.

So what is it?

(I asked this in another discussion but am now making it its own question).

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  • I just encountered exactly the same issue with the absolute input voltage range that asdf28 ran into.  I was using an LTC2324 driven by an LTC6362 with a common mode input ~1.65V and didn't notice that the AD7380 restricts the absolute input range when I switched to the AD7380.  This is very inconvenient as asdf28 noted but I plan to solve it the same way, by using a ~1.35V common mode input.  I tried to put a resistive voltage divider on the REFIO output (910 and 1.07k in series to ground) but this ~2k load on the REFIO output pulled it from 2.505V down to 2.275V (and it also seemed to pull REFCAP from 2.502V down to 2.469V).  I couldn't find any information in the AD7380 datasheet about just how much current the REFIO pin can source, can you give me any additional information about this?

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  • I just encountered exactly the same issue with the absolute input voltage range that asdf28 ran into.  I was using an LTC2324 driven by an LTC6362 with a common mode input ~1.65V and didn't notice that the AD7380 restricts the absolute input range when I switched to the AD7380.  This is very inconvenient as asdf28 noted but I plan to solve it the same way, by using a ~1.35V common mode input.  I tried to put a resistive voltage divider on the REFIO output (910 and 1.07k in series to ground) but this ~2k load on the REFIO output pulled it from 2.505V down to 2.275V (and it also seemed to pull REFCAP from 2.502V down to 2.469V).  I couldn't find any information in the AD7380 datasheet about just how much current the REFIO pin can source, can you give me any additional information about this?

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  • There is a VREF Load Regulation specification of -106ppm/mA but it's not consistent with your results.  This is important to me as well.  I currently have a 10k/12k divider on REFIO but haven't taped out the board yet.

    Note that it makes some sense that the REFIO buffer isn't very strong since the method of supplying an external reference is to inject it into REFIO.  Yes there is a register bit to turn off the buffer but until that happens you have two references fighting each other.  It's unfortunate that external references can't use the internal buffer and therefore need their own.

    EDIT: Also the application circuit diagram shows a 10k hanging off REFIO (though it might be a misdrawn 10k+10k divider)

  • Yeah, I saw that 106ppm spec but also couldn't reconcile it with what I was seeing and thought I might be misunderstanding it.

  • RefIO pin is not meant to draw current. You will have to buffer it and then draw current from the buffer.