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# AD8479 Single Supply with ground references

First off, I'm putting together a battery stack monitor for a 20-string pack with an absolute maximum common mode just south of 250V, where my end goal is 16 single ended measurement points for some data acquisition hardware in a lab environment.

I attempted to operate these AD8479s with a single 15V supply, and with the reference pins tied to ground, the output rails either high or low (seemingly randomly between chips). If I power the device with a split 9V supply, with the reference pins tied to the virtual ground, the output follows the differential on the inputs pins perfectly. The problem is, as I have narrowed through experimentation, there must be some non-zero reference voltage on the REF pins when the V(-) pin is tied to ground for single supply operation.

So my question then is: what exactly are the limitations of this chip with regards to single supply operation? What limits are imposed upon the reference voltages when the negative supply is tied to ground for single supply operation?

I have used the LT1990 for this exact same purpose in the past, and it functions exactly as expected in single supply mode. I was hoping to utilize the AD8479 in order to leverage the higher common mode rejection ratio as well as my academic pricing at Mouser (Mouser doesn't carry LT parts).

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• Hi Matthew,

Thank you for using ADI products on your applications. With regards to your question, you are right that there is a proper way of using the AD8479 in a single supply operation. You can read the discussion on page 12 of its datasheet. It actually says that the input common mode voltage range of the device is reduce when it is on a single supply operation. You can actually compute for the allowable common mode range using the equation below:

So you may want to adjust the voltage on the Vref pin to get your desired common mode range to be able for the device to operate correctly on a single supply operation.

Best regards,

Emman

• Hi,

I want to used the AD8479 with +5V only, VREF pins to GND.

Using your formula, gives VCM from +240Vdc (Vx= 4V) to +60Vdc (Vx=1V).

Is it correct? If yes, it's not in accodance with datasheet, figure 14. Input Common-Mode Voltage vs. Output Voltage, Single Supply, VS = +5 V, VREF = 0 V.

In this figure, VCM is from 220Vdc to 0Vdc.

Why? How to calculate VCM?

I want to used AD8479, from VCM = 2V to 60V. (differential 2V to 4.5V) Is it possible?

Best regards,

David

• Hi,

I want to used the AD8479 with +5V only, VREF pins to GND.

Using your formula, gives VCM from +240Vdc (Vx= 4V) to +60Vdc (Vx=1V).

Is it correct? If yes, it's not in accodance with datasheet, figure 14. Input Common-Mode Voltage vs. Output Voltage, Single Supply, VS = +5 V, VREF = 0 V.

In this figure, VCM is from 220Vdc to 0Vdc.

Why? How to calculate VCM?

I want to used AD8479, from VCM = 2V to 60V. (differential 2V to 4.5V) Is it possible?

Best regards,

David

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