What is the advantage of using an ARB instead of an Analog Output module?

[broskyar]

I have been given the task of choosing between a PXI-6713 Analog Output and a PXI-5411 ARB module. First, I need to find a module that will change frequency and amplitude dynamically with out seeing distortion or discontinuity. For example, we have had distortion from NI ARBs in the past when dynamically changing amplitude caused by output amplifier gain changes. My application cannot tolerate this type of behavior. My application requires a sine wave of frequency 10-70 Hz from 0 to 10 V min and max amplitude. The frequency and amplitude must be controllable from a front panel. I figure I could just build a LabVIEW application using NI-DAQ and an analog out module. What do you think? Do t
he function generation waveforms exhibit distortion while dynamically changing the frequency or amplitude?

[BrianA]

By what you wrote, it sounds like the analog output board may be the best fit for your application. When changing amplitude on the fly, an NI ARB may switch to a higher (or lower) gain amplifier path during generation. This causes glitching at the output for approx 10 ms while the relays settle. When changing the frequency on the fly in function generator mode, the frequency change is phase continuous without any glitching.

For an analog output board, you will not have the output glitching when changing the gain because you'll be directly scaling the value of the digital data and not the amplification or attenutation of the front end electronics (like the ARB does). The drawback to this method is that if you output a waveform that has an amplitude less than
the full scale amplitude of the DAC, you will loose resolution and the waveform quality will suffer. In some applications this is acceptable.

Another key specification to consider, is frequency resolution. It is difficult (but not impossible) to obtain good output frequency resolution with an analog output board whereas the ARB has approx 9 mHz frequency resolution out of the box.

[Steve Parus]

Some waveform boards accept an External Reference voltage to which
they scale their output voltage. This reference voltage could come
from a separate DAC on that same board, either a static voltage or as
a separate waveform itself which is used to modulate the main
waveform. This approach allows the output amplitude to be changed
without having to re-scale and re-load the entire waveform.

On Wed, 15 Oct 2003 09:01:56 -0500 (CDT), BrianA wrote:

>For an analog output board, you will not have the output glitching
>when changing the gain because you'll be directly scaling the value of
>the digital data and not the amplification or attenutation of the
>front end electronics (like the ARB does). The drawback to this
>method is that if you out
put a waveform that has an amplitude less
>than the full scale amplitude of the DAC, you will loose resolution
>and the waveform quality will suffer.

[BrianA]

Good point Steve. For the NI arbs, we use both methods. For small changes in gain, a gain DAC is used to adjust the output voltage, but when the gain change exceeds what the gain DAC can handle, we switch in either higher gain amplifiers or attenuators. Although some generators (typically benchtop) allow you to provide an external voltage to modulate the signal, the NI arb and most other computer based generators do not have this capability.

For some applications, we can can attain "glitchless" on the fly gain changes by restricting the change in gain to what the gain DAC can handle (thereby never switiching electronics in the output path).

Brian Anderson
Product Manager
Signal Generators
National Instruments

[Steve Parus]

Brian,

A related question on changing a portion of a large wavefrom on the
fly during continuous generation, perhaps on a standard E sreies card
or maybe a specific arb if that matters. If I have a 1 million
sample waveform generating at 1 MHz, can I change a portion of it by
simply writing the new data values to the waveform's buffer array at
the appropriate memory location ? I'd like to have that new portion
be used on the next waveform iteration cycle. Also, if it matters,
not have to worry about which half-cycle of the waveform is currently
being generated when I write the new portion to memory (perhaps a
question of whether double buffering is being used).

This actually would be done under Measurement Studio, but since it
per
haps is a question of NI-DAQ's capability and it relates to this
thread, I thought I'd ask here.

Thanks,

Steve

On Thu, 16 Oct 2003 09:03:25 -0500 (CDT), BrianA wrote:

>For some applications, we can can attain "glitchless" on the fly gain
>changes by restricting the change in gain to what the gain DAC can
>handle (thereby never switiching electronics in the output path).