# DAC differential output signal problem?

I'm trying to use a MAX5183 DAC to generate an analog voltage from a digital input. I have got it working in as far as the outputs follow the shape of the input digital waveform (just a sawtooth input to test for now), however, the differential outputs don't appear to be behaving as I expect.

The first differential output, OUT1P, shows a sawtooth wave from 0 to 0.4 V, which seems fine. However, the OUT1N output also shows a sawtooth wave from 0 to 0.4 V. OUT1N is exactly out of phase with OUT1P (when OUT1P is 0.4 V, OUT1N is 0 V, and vice versa), but is never opposing polarity. That means the differential amplifier I'm trying to use to combine them into a single-ended output gives me a signal that's -0.4 to 0.4 V, rather than 0 to 0.8 V as I expected.

Am I misunderstanding what the DAC should output, or is there a problem? I was expecting that the differential outputs should be opposite polarity rather than out of phase with each other.

My circuit is basically copied exactly from Figure 5 in the data sheet

And here is an example of the outputs observed from OUT1P (top) and OUT1N (bottom):

• are you sure it can output negative values without any negative supply?
– Ilya
Commented Feb 17, 2022 at 14:55
• I think this is a classic case of "the devil is in the details." The outputs are differential, so the datasheet doesn't lie, per se. Any common-mode noise coupled to those differential signals should be effectively squelched by the CMRR of the op-amps. So the diff pair are effectively still good for "long runs", despite not being true zero-crossing. Commented Feb 17, 2022 at 15:47

Yes, you are misunderstanding the DAC output.

DAC has no negative supply, so it can only have voltages between GND and supply on analog outputs.

The outputs are differential, and OUTP voltage minus OUTN voltage is negative.

The op-amp after the DAC output converts the differential output to single-ended.

• Ah, thank you, that makes sense. I had assumed that "differential outputs" would always provide two outputs with opposite polarity, but hadn't considered that the DAC would need a -ve supply to do so. Commented Feb 17, 2022 at 15:16
1. Absolute maximum ratings:

OUT1P, OUT1N, OUT2P, OUT2N, CREF1, CREF2 to AGND...................................................-0.3V to +6V
(no negative voltage, -0.3V is normal absolute maximum (minimum) for anything that works from positive rail and GND)

Also

It is clearly not supposed to output negative voltage on any rails.