Switch or No Switch

At one time, a 12-bit ADC cost more than $100, so data-acquisition (DAQ) equipment vendors routinely provided a multiplexer that let users switch one of several signals to the expensive ADC. But, as costs for high-precision ADCs have dropped, more DAQ equipment provides one ADC per input signal. Which approach works better?

If, for example, you plan to use an expensive DMM to make 4-wire resistance measurements at several points, multiplexing makes sense. Likewise, if you plan to measure slowly-changing signals, multiplexing works well.

In some cases, multiplexers can cause problems. First, because a multiplexed DAQ system makes one measurement at a time as it “rotates” through connections, measurements lack time alignment. If you plan to make sound and vibration measurements on an automatic transmission, for example, you will need time-aligned measurements so you can see relationships between signals. Second, switching between n channels reduces your measurement bandwidth to BW/n. Thus, a 16-channel system that can take 100 ksamples/sec has a maximum bandwidth of 6.25 ksamples/sec/channel. Not only do you lose phase relationships, you reduce measurement bandwidth.

Multiplexers can introduce electrical measurement errors, too. The switches in a multiplexer always introduce some stray capacitance in a measurement circuit. When connected to a signal, that capacitance charges or discharges accordingly. When the multiplexer switches from channel x to channel y, some of the charge accumulated from signal x remains on the stray capacitance. Thus, the multiplexer needs a short settling time so the voltage across the stray capacitance can “catch up” to the voltage on channel y. A high-impedance signal source aggravates this problem by introducing a longer RC time constant, which translates into a longer settling time between measurements.

If you switch a multiplexer and measure the new input signal too soon, you end up measuring some of the previous signal, too. The resulting measurement appears to have a small amount of crosstalk from the previous channel. You also can observe some interference from high-frequency signals that simply pass through the stray capacitance across a switch.

As an alternative to multiplexing, DAQ equipment now offers ADC-per-channel or simultaneous-sampling ADCs. Because each channel provides its own ADC, samples align in time and preserve phase information. The lack of stray switch capacitance removes the need to adjust measurements for settling times. And, isolated channels eliminate crosstalk or artifacts of signals on adjacent channels.

Multiplexers still have an important role. Engineers use them when they must acquire data from hundreds of sensors and cannot afford an ADC per channel. Multiplexers also can work in reverse and provide test signals to many points from one source.

Thanks to Fred Molinari, president and CEO of Data Translation and Charles Stiernberg, product marketing engineer for data acquisition products at National Instruments for information used in this column.

	Two switches in a multiplexer include stray capacitances that can affect measurements. The 10-pF capacitances exist in parallel, so a 16-channel multiplexer would have an input capacitance of 160 pF.