Maximum input voltage to ADC with 0.5x gain

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Normally the highest voltage that the ADC can measure is AREF. Say you select the 0.5x gain setting. Could you measure voltages above AREF as long a they are scaled down below it by the gain stage?

And what is the absolute limit? For AREF it's Vcc-0.6. The pin can take up to Vcc safely.

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Last Edited: Mon. Aug 1, 2016 - 09:09 AM
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I don't think you can do that.

 

The AVcc pin can be 3.6 V under general operating conditions.

The Absolute Max for Vcc is 4.0 V, while any other pin can be Vcc+ 0.5, so perhaps 4.5 V, but you shouldn't be operating at that set point.

 

The input limits apply to the front end sample and hold which then feeds the gain stage, before the ADC unit.

 

So "scaling" with the gain has no impact on the amplitude of the signal fed into the sample & hold, and the gain stage.

 

JC

 

 

 

 

 

 

Last Edited: Fri. Jul 29, 2016 - 11:25 PM
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Sure, what I mean is, if I say connected that pin to Vcc, which we will say is 1.8V. My AREF is 1.0V, and gain is 0.5. So can I measure 0.9V with the ADC?

It satisfies the limit for the pin relative to Vcc, but exceeds AREF. If the gain op-amp uses AVCC as a rail it should work.

The reason I ask is that the internal divide by 10 Vcc seems to be fairly inaccurate. For battery powered applications it's inadequate.

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You mean you have an amplifier outside the chip with a gain of 1/2? (Like a resistor divider)

 

With a 1v ARef, you can measure the pin at 0.9v. Should give you about 3686 counts.

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Torby wrote:

You mean you have an amplifier outside the chip with a gain of 1/2? (Like a resistor divider)

 

I mean I set the XMEGA's ADC's gain stage to 0.5x. So the amplifier is internal to the XMEGA, part of the ADC.

 

 

 

It doesn't show where that amplifier's rails are coming from. Presumably one is GND and the other is... AVcc?

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With a gain of 1/2, "full scale" would be twice your reference. The max input can be reference * 2 or the max voltage on the pin, whichever is lowest.

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