Voltage limits on differential ADC channels

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We've never used the differential channel features in any of our AVR aps that use the ADC. I've got a sensor that gives a bipolar millivolt output, roughly in the range of +1VDC to -1VDC.

Perfect, I thought, to use on an AVR differential ADC channel. But my colleague pointed out that the AVR datasheet for the part (like Mega32, Mega169, etc.) says in the Absolute Maximum Ratings section:

Quote:

Voltage on any Pin except RESET
with respect to Ground ................................-0.5V to VCC+0.5V

and that inputting a bipolar signal such as 5V Vcc, 5V ARef, & differential signal of +/- ARef/2 would violate the absolute maximum ratings.

Now I'm not too worried about frying the high-impedance ADC input with a millivolt signal, but can someone point me to some references on what signal levels can be stuffed directly into the AVR differential ADC channels? Does the signal need to be shifted into Gnd-to-ARef range first, and a "dummy" signal of Aref/2 be constructed for the negative ADC differential input?

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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The levels on the analog pins do need to be kept between gnd and Vcc, at least
because of the chip clamp diodes. However, you may be able to use a simple
trick if perhaps the output of your sensor can source or sink a little current. If
it's output stage is like an op-amp, this may work. For example, connect a 50k
resistor from Vcc to an analog input. Connect a 10k resistor from that input to
your sensor's output. If the sensor output is -1 volt, the voltage at the analog input
is zero. At a sensor output of +1, the analog input will be 1.66 volts. A cheesy
way to level shift with a little sacrifice in gain. By fiddling with the resistor ratio,
you could use a differential input pair combined with the Avr internal gain option
to get whatever sensitivity you need.

Tom Pappano
Tulsa, Oklahoma

Tom Pappano
Tulsa, Oklahoma

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Lee

Quote:

Does the signal need to be shifted into Gnd-to-ARef range first, and a "dummy" signal of Aref/2 be constructed for the negative ADC differential input?

Yes, no negative input voltages are alowed.

Furthermore, I seem to recall that only some AVR devices support bipolar conversion on differential inputs.

Peter

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A variation of Tom's proposal would be to replace the 10k resistor with a 1.2v voltage reference (LM385-1.2), this would shift the -1v to +1v sensor output to +0.2v to +2.2v retaining the full 2v sensor output span and minimize any noise present on Vcc. Consider that the AVR ADC input is expecting a source impedance of <10k, which may be an issue.

Stan