Multifunction Analog Input/Output USB Data Acquisition Products

If voltage mode is used, analog input connections can be either single-ended or differential. Low per-channel cost is a major advantage of using single-ended inputs because only one multiplexer switch is needed per channel and there is less system wiring. Single-ended inputs all share a common return or ground line. The measurement is the difference between the signal and the ground. Only the "high" signal wires are connected through the amplifier and multiplexer. The "low" wires return through the system ground connections. That is, both the signal source and the amplifier are referenced to ground. This arrangement works fine as long as the ground potential difference is very small. However, if there is a large difference in ground potential, extraneous currents can flow and generate errors. This is called a "ground loop". Ground potential differences (ground loops) can cause larger interference problems than electromagnetic fields. Ground loops occur if the sensors are tied to a local ground. That local ground can be many volts different from the ground potential at the receiving circuit. Single-ended measurements are also more susceptible to noise than differential measurements due to differences in the signal paths and the tendency to pick up surrounding electrical activity. With single-ended inputs you have no way of distinguishing between the signal and the noise.

To avoid this problem, a differential-input connection can be used with an amplifier that has a high common mode rejection ratio. A differential arrangement connects both ends of the signal source to the inverting and non-inverting inputs of the amplifier. Measurement is the difference in voltage between the input and the reference, which helps to reduce noise and any voltage that is common to both wires. Twisting wires together will ensure that any noise picked up will be the same for each wire. In this way, voltage due to ground loops appears as a common mode voltage and can be rejected by the common-mode-rejection properties of the amplifier. An instrumentation type amplifier (rather than a simple operational amplifier) is required and two multiplexer switches are required per channel. An obvious disadvantage of differential inputs is that you need twice as many wires, so you can connect only half the number of signals, compared to single-ended inputs.

If you have signals larger than 1V, short signal wires, and close together signal sources, single-ended inputs should be fine. If your application falls outside these boundaries, differential inputs would be the wise choice.

In industrial applications, where wiring can be very long, it is common to use "current transmitters"; i.e., devices located at or near the transducer which convert the sensor output to an analog current signal (e.g. 4 to 20 mA). Current signals are sometimes used because they are relatively immune to errors such as induced noise and voltage drops due to lead resistance. and have become a de facto standard for critical process control measurements. Another plus is that it saves money due to reduced wiring cost. Current mode operation is somewhat susceptible to magnetically-induced errors. The simplest solution is to use twisted-pair signal lines and to keep those lines remote from power lines or other sources of magnetic induction. Also, at the receiving circuit, the signal usually must be converted into a voltage and level-shifted to a zero-based range so that it can be digitized using the full input capability of the digitizer.

Resistance measurement is most commonly associated with direct inputs from temperature sensing devices, such as thermistors and resistance temperature detectors (RTDs). RTDs are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature. They are slowly replacing the use of thermocouples in many industrial applications below 600°C, due to higher accuracy and repeatability. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements. Thermistors differ from RTDs in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range (usually -90°C to 130°C).

Signal conditioning is an essential requirement in many applications and systems which demand accurate and precise measurements. Signal conditioning circuits convert one type of electronic signal into another type of signal that can be used to improve and enhance the data acquisition system and application. Sometimes the data acquisition card that you plug into the computer provides necessary conditioning; sometimes that signal conditioning must be done external to the computer. In general, signal conditioning consists of amplification, isolation, filtering, sensor excitation, cold-junction compensation and more.

Signal inputs accepted by our signal conditioners include DC voltage/current and AC voltage/current. Sensor inputs can be accelerometer, thermocouple, thermistor, resistance thermometer (RTDs), strain gauge or bridge, and LVDT/RVDTs. Signal conditioned outputs include voltage, current, frequency, relay, isolation, and more.

ACCES offers a wide variety of external signal conditioning products which include analog, digital, and serial signal conditioning. Sometimes the data acquisition product you plug into the computer provides the necessary conditioning; and sometimes that signal conditioning must be done external to the computer.