DAQ Board Analog Outputs: Features that Matter

The outputs of DAQ boards may be critical for performance, given your application. Look for these features and /or their rated specifications in the product literature of a board you're thinking of purchasing:

• Independent DACs for each channel
• Excellent settling time (aka update time)
• De-glitched outputs
• Reconstruction filters

Discussion

Most high-quality DAQ boards offer analog output circuits that have a separate (independent) D/A converter (DAC) and data buffer per channel. This allows outputs to be updated one-at-a-time or simultaneously (e.g. to change both channels of an XY chart recorder or plotter). When two DACs are updated simultaneously the total throughput may be twice the single-channel throughput.

The primary component in DAC throughput is settling time (aka update time) . While a DAC’s output begins to change as soon as it gets a new data value, the output is valid only after the analog circuitry has stabilized. Settling time (update time) is the amount of time a DAC needs to reach its rated accuracy after receiving an output change (say from a sensor). Full-scale settling time is actually a worst-case-scenario value. Smaller than full-scale changes settle in less time. Settling time varies inversely with the size of the output change; its units are given in microseconds (µs).

To provide cleaner, distortion-free analog output signals, some DAQ boards use de-glitching circuits. DACs produce glitches: rapid, high energy pulses or spikes created by small timing differences (skew) in the converter’s data switches. The de-glitching circuit holds the output constant for a brief period, then reconnects the DAC output after the glitch has passed.

Another method of reducing distortion or noise from an analog output is to pass the output through a low-pass "re-construction" filter. e.g., a delta-sigma-type DAC used found in certain DAQ boards constructs an analog output signal by using an 8x interpolation filter followed by a 64x oversampling converter. This method automatically interpolates between data points to smooth the resulting analog output, giving the effect of much higher resolution. It also offers very steep “brick wall” filtering of unwanted output noise.