The 5B39 is a single-channel signal conditioning module that converts a high-level analog input voltage into a floating, isolated proportional output current of 4 to 20 mA or 0 to 20 mA, across loads from 0 Ω to 750 Ω. The module provides high accuracy of ±0.05%, low nonlinearity of ±0.02%, and the protection of 1500 V rms isolation between output-to-input and output-to-power supply. The input common must be held to within ±1 V of power common.
Inside the 5B39
Signal isolation by transformer coupling uses a proprietary modulation technique for linear, stable and reliable performance. A demodulator on the output side of the signal transformer recovers the original signal, which is then filtered and converted to an accurate current output by a current (V-to-I) converter output stage. Output protection enables the 5B39 to withstand accidental connection to 240 V rms power lines without damage, while isolating computer-side circuitry. In addition, the 5B39 is mix-and-match and hot-swappable with all 5B Series modules, so can be inserted or removed from any socket in the same backplane without disrupting system power.
Track-and-Hold for DAC Applications
In applications where a single system digital-to-analog converter (DAC) is used to drive a number of current output channels, the 5B39 includes a track-and-hold input circuit. Selected by a high Write Enable input, the hold mode exhibits an output droop rate of 80 µA/s. This corresponds to a refresh interval of 25 ms for 0.01% span droop. On power-up, the module's output remains at 0 mA for about 100 ms to allow the user to initialize the track-and-hold circuit.
In applications using one DAC per channel, where the track-and-hold feature of the 5B39 is not used, the Write Enable input should be set to low by grounding it to power common, as on the 5B01 and 5B08 backplanes. The module current output will then track its input signal.
Generating A Voltage Output Signal
The 0 to 20 mA output of the 5B39-04 and the 5B39-03, can produce a 0 to +10 V output by connecting a 500 Ω conversion resistor across the module's output terminals. This approach should be used with caution because the output lacks the low impedance of a true voltage source. This means that the tolerance and size of the load impedance relative to the conversion resistor can introduce significant error. For example, a load impedance < 500 kΩ would contribute < 0.1% error.