current output DAC's

[Jamie Morken]

Hi,

I noticed some DAC's are current output instead of voltage output,
what is the practical difference when using these in a circuit and
in what situations would it be better to use a current output DAC?

cheers,
Jamie

 

[Tony Burch]

Hi,

I noticed some DAC's are current output instead of voltage output,
what is the practical difference when using these in a circuit and
in what situations would it be better to use a current output DAC?

cheers,
Jamie

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
http://www.SuperSolderingSecrets.com Video tutorials show prototype
soldering techniques

 

[hrh1818]

Hi,

I noticed some DAC's are current output instead of voltage output,
what is the practical difference when using these in a circuit and
in what situations would it be better to use a current output DAC?

cheers,
Jamie

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.

 

[Joerg]

Hi,

I noticed some DAC's are current output instead of voltage output,
what is the practical difference when using these in a circuit and
in what situations would it be better to use a current output DAC?

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.

 

[John Larkin]

Hi,

I noticed some DAC's are current output instead of voltage output,
what is the practical difference when using these in a circuit and
in what situations would it be better to use a current output DAC?

cheers,
Jamie

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

 

[Joerg]

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.

Sometimes the RF boys use cascodes

 

[Jamie Morken]

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

 

[Joerg]

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.

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.

 

[Jamie Morken]

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

 

[Joerg]

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

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

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.

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 ...

 

[Jamie Morken]

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

 

[whit3rd]

I noticed some DAC's are current output instead of voltage output,
what is the practical difference when using these in a circuit and
in what situations would it be better to use a current output DAC?

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.