DAC range limited?

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Freaks,

I did a brief search but could not find a thread that covered this issue.

I am using the DAC of a 128a1u. I need an output from 0-2.5VDC. Even with no load, I cannot get the output above 2.3VDC. I have tried both an external reference of 2.5VDC and the AVCC (3.3VDC) as reference.

I recall some discussion of this being an inherent problem. Is this the case?

Thanks,

A.

AVR Studio 4 Ver. 4.18 684
avr-gcc Ver. 4.3.0
ISIS 7
ELECTRA

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I've never had a problem getting the xmega DAC to output close to the top rail. My issue is usually with the lower rail. It almost never swings below 50mV and typically I can't get it below 150mV. I always buffer the DAC immediately and then drive it back into an A2D port for self calibration using a precision 2.048V reference. An additional gain stage can be used to get the desired range.

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Quote:
typically I can't get it below 150mV

There is DAC linear output voltage range in 128A1U datasheet, which is in range of 0.15v to AVcc-0.15v.

Ozhan KD
Knowledge is POWER

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electronic.designer wrote:
Quote:
typically I can't get it below 150mV

There is DAC linear output voltage range in 128A1U datasheet, which is in range of 0.15v to AVcc-0.15v.

That's why I brought this up. He's trying to go between 0 and 2.5V. He's not going to hit 0, so he has a problem on both ends right now.

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Freaks,

I am sorry it has taken so long to respond.

I was not aware of the residual voltage at the bottom end of the D/A's range. Guess I should read a little more.

With the O/P from the D/A isolated, I am still unable to get more much more than 2.25VDC from the O/P. I have confirmed the REF voltage on the AREF pin as well as the 3.3VDC on AVCC.

As far as I can tell, it must be an issue internal to the 128u. When generating a ramp, the wave is perfect until the REF is above 2.3VDC. At that point the top of the wave becomes completely flat. If I lower the reference the top of the waveform returns.

I can get around the problem with a bump in gain for a buffer amp I have at the O/P from the D/A but this issue is still puzzle.

I should note that these chips are fairly old. I do not have the revision code in front of me but they could be as old as two years.

Thank you for your input.

A.

AVR Studio 4 Ver. 4.18 684
avr-gcc Ver. 4.3.0
ISIS 7
ELECTRA

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I was recently playing around with the DAC on a 192A3U (I think a fairly recent revision, but am not able to check right now) and I was having no problem outputting up to the reference voltage.

I kind of think that when the datasheet says that the linear output range is from .15V to AVcc - .15V they mean just that: the output is linear in that range. That doesn't imply that it can't output outside of that range, just that outside of that range the output is nonlinear.

It's possible that I wasn't measuring the output correctly and there is actually a small error right at the ends.