trying to understand the REF pin on a DAC

The ref pin generally scales the full scale output of the DAC. If you think of the digital value going into the DAC as a fraction that goes from zero to (2^12)/((2^12)-1), the output voltage will be that fraction multiplied by the reference voltage. In effect, the DAC is a one quadrant multiplier that that multiplies an analog voltage (the ref) by a digital fraction.

Sorry, that should have read ".../((2^12)+1)"

The output voltage is

Vout = YourDigitalCode * Vref / 4096

so it directly affects the output.

John

Arggg! Make that "((2^12)-1)/(2^12). The largest value the digital input can have for a 12 bit converter is (2^12)-1, but the converter has 2^12th states, one of those being zero.

It sets the 'full scale deflection' of the DAC. Very useful in some instances.

In other instances it may be used to filter/smooth the reference voltage so that it has very low noise.

Graham

Wow thanks - that makes sense...

I was reading the datasheet for the chip and I have a question...

At one point it says that the maximum ref voltage is Vdd + 0.3, then later is says the maximum recommended Vref is Vdd-1.5. Would it be OK just to tie this pin to the 5v I'm using to power the chip?

I want the output to be 0-5v.

Dave

Yes, that's the 'absolute maximum' above which damage will occur.

For reliable performance no doubt.

Best to filter it first. Even with just an RC.

Graham

Oops..,

^^ 10

You should have supplied the link to the data sheet the first time. ;-)

See the note under the recommended operating conditions block on page 3.

This beast has times 2 amplifiers between the ref input and the DAC, and, evidently (though they don't seem so proud of the fact, hence the fine print) those amplifiers cannot swing anywhere near Vdd, so the maximum ref input that will not cause clipping the high end digital values is Vdd-0.4/2.

For a 5 volt Vdd, that is 2.3 volts, for an internal reference voltage of 4.6 volts, and the output cannot exceed that.

You would need to power this chip from at least 5.4 volts to get a 5 volt, full scale output, with a 2.5 volt reference input.

This is the first DAC I have seen with this internal reference multiplier "feature". If they were proud of this feature they might have written a 2X inside the pair of triangles on the block diagram at the top of page 2, with the note about the output limitation of the amplifiers, there, also.

Wow - thanks for all the responses, I've been setting the REF to 2.048 (or as close as I can get) using a pot. The DAC appears to be functioning, but the output is not as expected.

1100001111101000

1100011111010000

1100111110100000

Decimal is the input pin of an Arduino "analog in" - I'm not sure if full scale is 500, or 1000, but as you can see the DAC is not returning analog voltages quite in step with the input commands.

I'm plugging the chip directly into the Arduino board, and measuring the output as soon as the CS pin goes back high.

Is is OK for me to do this, or do I need resistors and capacitors in the mix too?

Please forgive my ignorance with this - It's part of a larger software (my forte) project, and I do struggle a bit with the electronics.

If you are unsure of the Arduino ADC characteristic (and because your software reading the ADC may have problems), you should measure the DAC output with a voltmeter - that should give you a clear indication of what the DAC is doing, without the added confusion of having to read the ADC.

To confirm operation of the ADC, you could apply a variable voltage to the ADC input, and see how the readings vary as you vary the voltage.