| Multiplexing in PC-Based Data Acquisition Systems |
Fred R. Schraff,
P.E., IOtech Inc.
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Multiplexing is vital to PC-based data acquisition. Here’s a closer look at some fundamental design considerations.
PC-based data acquisition has been made practical by the widespread use of multiplexing or scanning. Multiplexing is defined as the sequential transmission of several signals on the same circuit or channel. The high speeds attainable with today’s components have made it practical to multiplex analog input channels at rates exceeding 1 MHz. The terms “multiplexing” and “scanning” have almost identical meanings in the data acquisition content, but the term scanning has a long history in reed relay based systems, as well.
A simple way to describe a multiplexing data acquisition system is to say it is a single analog-to-digital converter with a high-speed selector switch accessing signals from multiple sources or channels. As soon as a reading is taken from one channel, the switch moves on to the next. In turn, the A/D converter processes each reading. On the digital side, the data from the different channels can be directed into different storage locations to sort the consecutive channel readings—much like dealing cards to multiple players around a table.
Multiplexing is not a new concept. The first data acquisition systems used manual selector switches connected to a single analog meter. Electrically operated switches and relays soon followed. Reed-type relays quickly became the standard because their sealed contacts provide long life and switching speeds exceeding 300 Hz. Solid-state switching networks started with field-effect transistors (FETs) and evolved into IC switching arrays and multiplexers. In recent years, most of the progress has been increased switching speeds and improvement in the parameters of non-ideal behavior. The most prominent of these parameters are on-resistance, static, dynamic and sequential crosstalk, charge injection, off-leakage, input signal range, and power-off behavior.
The effects of the non-ideal parameters generally degrade overall performance as multiplexing frequencies increase. Some of the effects are obvious and others are more subtle. On-resistance, which is the equivalent of a resistor in series with an ideal switch, can range from 10 to 2000 ohms. The circuitry beyond the switch has resistance and capacitance in parallel. Consequently, there is a DC error term and a minimum settling time for the switched voltage to reach the actual input signal level. If a multiplexed input stage needs 10 microseconds to settle to within 0.01% of the true value, idealized system switching frequency can approach, but not exceed, 100 kHz for accurate 12-bit measurements.
Crosstalk denotes a situation where the signals on other channels couple with the signal on the channel being measured. There are several forms of crosstalk, all inherent to the multiplexing process, and all worsen as the multiplexing frequency and signal frequencies increase. Static crosstalk occurs because parasitic capacitance across each open switch couples a portion of each channel signal to the output to mix with the desired signal. As the impedance of each parasitic capacitance drops with increasing signal frequency, the higher signal frequencies exhibit more static crosstalk. Dynamic crosstalk is primarily a function of the multiplexing speed and the effects of the parasitic capacitance in the system. Sequential channel crosstalk is generally the worst component in any sampled data system. Some portion of the signal from the previous channel is stored in the multiplexer-output-side capacitance and thus alters the value of the next selected channel. The effect is most serious when the previous signal was much larger than the present signal.
Charge injection, in which a portion of the multiplexer switching-logic pulse appears in the output signal, is more serious when source impedances are higher. Continuing advances in multiplexer ICs are slowly decreasing the magnitude of these problems, but it is still a good practice to minimize signal source impedances where possible. Off state leakage is similar to static crosstalk, as signals from all connected channels couple to the multiplexer output when no input channel is selected.
With solid-state IC multiplexers, input-signal range is seldom more than -10 to +10 volts, except in a few systems, which can handle up to 20 volts in the positive polarity only. There is also some degree of over-voltage protection, usually up to about 35 volts on protected devices. These values are generally true only when the data acquisition system is powered. If the data acquisition inputs have substantial signal connected to the inputs while they are powered off, input signals can be loaded down or clamped; and damage to the data acquisition can occur in some cases. Some multiplexer chips are available with protected inputs which allow up to 35 volt signals whether the system is powered or not, but it is a good system design rule to avoid applying signals to analog inputs while powered off.
The reed-relay approach generally avoids most drawbacks of the IC multiplexer chips. On-resistance is negligible, crosstalk is not a problem, there is no charge injection, off-leakage is minimal, input-signal range can be 10 times higher, and the powered-off state is not a problem to the signals or the data acquisition system. The inevitable drawbacks are operating speed and finite scanning life. Reed relays have operating life of 10-50 million operations depending on voltage and load. For these reasons, slower systems generally employ reed relays.
A reed-relay scanner can also be employed with a digital voltmeter under computer control—an arrangement that allows it to change not only range, but function in the course of a data acquisition sequence. While not as fast as data acquisition systems with individual signal-conditioning circuits, this combination is very flexible. A drawback to this approach is the integrating nature of digital voltmeters which generally process signals prior to the A/D conversion; an AC signal, for example, is converted to a single DC RMS (root-mean-square) value. If an AC signal is to be captured for examination of an aspect other than the RMS value, the voltmeter-based systems is the wrong choice. Although the systems speed difference between a solid state system and reed-relay can easily exceed 100 to 1, IOtech’s MultiScan/1200 is a reed relay system with the capability of “sitting” on a single channel and reading the channel at the system conversion rate of 20 kHz.
The rate at which a system scans the channels is important for two reasons. A single channel can be read continuously to a very high degree of time resolution. Large numbers of channels can be digitized to acceptable resolution of signals up to half the frequency of the individual channel readings. For example, a 1-MHz system can scan 10 channels at effectively 100 kHz each. Signals of frequencies up to 50 kHz can be digitized adequately on each of the channels. Many signals of interest at ATE (automatic test equipment) systems are less than 50 kHz in frequency; and if the frequencies are proportionately lower, many more channels can be effectively multiplexed.
If the system has the additional benefit of channel-by-channel switching, as a result of a programmable gain amplifier, it is then possible to measure milli-volt levels and volt levels within the same system. If an ambient temperature sensor is available for cold-junction compensation (CJC), thermocouples may be scanned and temperatures computed by the host computer. Today, the sensor industry provides an ever expanding variety of sensors, and individual signal-conditioning circuits frequently have standardized outputs in the 0- to 5-V or 0- to 10-V range. Thus, a multiplexed system can be configured for virtually any mix of signal types.
Specifically, today’s PC-based multiplexed data acquisition systems
differ primarily by multiplexing speed, 12-bit or 16-bit resolution, maximum
number of system channels, packaging and connection provisions, and the
PC-interface. For example, Plug-in cards, such as IOtech’s DaqBoard™
series, often serve desktop PC users. External parallel-port devices, such
as IOtech’s DaqBook® series and the WaveBook/ 512™
are better suited to portable or frequently re-configured applications.
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