Sample, Convert Simultaneously

The first data-acquisition (DAQ) system I designed and builtused a modular 12-bit analog-to-digital converter with 16 differentialchannels. A multiplexer switched each pair of inputs to one sample-and-holdamplifier (SHA) that sampled a voltage and "held" it steady until the ADC couldperform a conversion. The multiplexed analog inputs created a problem - themultiplexer had to switch from channel to channel, so a measurement for each ofthe 16 channels took considerable real time. Thankfully, this DAQ equipmentmade measurements on voltages that varied only slightly over long periods, soacquiring data over several seconds didn't affect results.

In many cases, engineers need to make simultaneousmeasurements, which a multiplexed ADC module could not. While the cost of ADCsremained high, instrument manufacturers added an SHA to each channel, so DAQequipment sampled all channels simultaneously and then performed an A-to-Dconversion in sequence on the sampled signals. But this approach complicatedcircuit design and data handling.

First, each SHA had to "hold" the sampled voltage to within±1/2 ADC least-significant bit for a period equal to the maximum time needed toperform all the A-to-D conversions. If the held signal started to droop, itaffected measurement accuracy.

Second, although the SHAs grabbed voltage samplessimultaneously, the ADC still measures each SHA output in sequence, thusskewing data values with respect to time. By knowing the interval between dataconversions, engineers could use math to realign the samples so they appear asif converted simultaneously.

Third, you decrease the throughput of your ADC as you addchannels because you still perform one conversion at a time.

New simultaneous-conversion DAQ equipment provides an ADC andsignal-processing circuitry for each analog-input channel. But the added ADCs,higher-speed sampling and higher converter resolutions can increase the priceof equipment. On the other hand, the use of USB, PCI Express, PXI Express andoff-the shelf drivers and software simplify engineers' measurement jobs.National Instruments, for example, offers cards with as many as 16simultaneous-conversion channels at 2 Msamples/sec for each channel. DataTranslation sells a series of USB-based simultaneous-conversion DT9832 andDT9836 modules for single-ended analog inputs.

When you must digitize signals simultaneously, remember thedifference between simultaneous sampling and simultaneous conversion.Manufacturers don't always make it clear.