current output DAC's

Hi Jamie, When designing with current output DACs, the practical difference is that you will usually need to select an appropriate op amp to work with the DAC to convert the current output to a voltage.

Current output DACs don't put the "output op amp" on the chip so that you can select your own output opamp, depending on your bandwidth, noise, accuracy (dc precision), settling time and power requirements.

When selecting opamps you often trade off bandwidth for DC precision, so you don't want to select an opamp with excessive bandwidth.

Sometimes you need to use a current output DAC to meet your design requirements. At other times, you might select a DAC because of it's other specifications (such as high sample rate / bandwidth), and it will also happen to be current output.

Anthony Burch

In Instrumentation applications 1 to 5 milliamp and 4 to 20 milliamp signals are used to transmit over long distances to eliminate attenuation caused by line resistance. Hence in any situation in which you want to make sure the amplitude of the signal at the receiver is the same as the amplitude of the signal at the transmitter a current signal is preferred over a voltage signal.

Makes it much easier to do summation and "analog math". In a current design you just tie the outputs together (minding the compliance ranges, of course). Some can even be used as digital potmeters when they offer direct access to the resistor ladder.

The issue is mostly that the faster dacs only come in differential current-output flavors, so one generally doesn't get a choice.

Turning the dc diff current into an accurate single-ended, ground-referenced voltage can get seriously non-trivial at higher speeds. The comm boys just use transformers, but some of us want response to DC.

There are tons of diff-out amps designed to drive diff-input adc's; I know of no equivalent parts to condition the diff dac outputs. Strange.

John

Sometimes the RF boys use cascodes :-)

how about for a DAC used on a SMPS, would there be any advantage to a current or voltage DAC in this case? I guess a noisy environment can disrupt current and voltage equally though.

cheers, Jamie

You mean when its supplies come from a switcher? I do that all the time, got to keep the switcher nice and quiet. You've got to keep the reference clean. Once that gets dirty it's all over, no matter which kind of DAC.

Ya my supply is from a switcher too, using a high PSRR regulator and some LC filtering to clean up the supply/references.

Also I was wondering for EMI from other surrounding electronics (switchers) what DAC/ADC architectures are best. Also if a DAC is micropower I wonder if it is more susceptible to noise over one that uses more power.

cheers, Jamie

As long as that high PSRR regulator is not one of those low dropout deals.

Obviously, if you have long traces coming off the DAC you'll see more noise. As for the ADC resist the temptation to split grounds no matter what professors and others say.

Now wait, now I spilled the beans and reduced my number of potential fix-it jobs ...

Thanks for sharing the beans :)

At high frequency, current output is a good match to low-input impedance amplifiers (like a transimpedance amplifier or the emitter of a grounded-base transistor). At lower frequencies, op amps rule and voltage output is near-ideal for their high input impedance.

Your money, your choice.