Current loop converters receive signal inputs from devices such as sensors and produce analog outputs in the milliamp (mA) range. Typically, these outputs are standard current loops with ranges of 0 to 20 mA, 4 to 20 mA, or 0 to 50 mA. Current loop converters use optical isolation, capacitors, and magnetic induction to isolate loops of transmit and receive data. They use separate transmit (T+ and T-) and receive (R+ and R-) loops, each of which can be active or passive. Current loop converters are used in a variety of industries and applications, especially with computer peripherals. For example, RS232 connectors are current loop converters that connect printers to personal computers (PC) by using current on and current off communications to transmit binary digits.
Current loop converters receive three types of inputs: signal inputs, sensor inputs, and inputs from special devices. Signal inputs include DC voltage, DC current, AC voltage, and AC current. They also include frequency and charge, signals that often require filtering and amplification. In terms of sensors, current loop converters receive inputs from accelerometers, thermocouples, thermistors, and resistance temperature detectors (RTD). They also receive inputs from strain gauges, many of which use a Wheatstone bridge, and from both linear and rotary variable differential transformers (LVDT and RVDT). Inputs from special devices include not only signals from encoders, but also signal per cycle counts from counters or tachometers. Timers, clocks, relays, and switches can also provided special inputs to current loop converters.
There are several device specifications and form factors to consider when selecting current loop converters. Some devices use differential channels where the input is the difference between two signals and common mode is filtered out. Other devices use multiplexers that combine several inputs into one output for transmission. Additional device specifications to consider are bandwidth, accuracy, maximum output voltage, vibration rating, and shock rating. In terms of form factors, some current loop converters mount on integrated circuits (IC) or printed circuit boards (PCB). Others mount on standard racks or DIN rails, dock in bays or slots, bolt into walls or cabinets, or fit in stand-alone cases or cabinets. Current loop converters can also include a front panel and display or a touch-screen interface. Some handheld units provide remote operation features. More complex units interface with supervisory or host computers.
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Bridge conditioners are instruments that provide excitation and support for strain gages, Wheatstone bridges, load cells, and sensors. They also include circuitry for signal conditioning, amplification, and processing.
Data acquisition is the digitizing and processing of multiple sensor or signal inputs for the purpose of monitoring, analyzing and/or controlling systems and processes. Signal conditioning includes the amplification, filtering, converting, and other processes required to make sensor output suitable for rereading by computer boards.
Digital-to-analog converters (DAC) transform a digital number into a corresponding analog voltage or current.
Signal filters block or decrease (attenuate) unwanted frequencies or signal wave characteristics.
Temperature Signal Conditioners
Temperature signal conditioners accept outputs from temperature measurement devices such as resistance temperature detectors (RTDs), thermocouples, and thermistors. They then filter, amplify, and/or convert these outputs to digital signals, or to levels suitable for digitization.
Voltage-to-frequency converters accept a voltage signal and convert its analog level to a signal with a corresponding frequency.