The 5B34 is a single-channel signal conditioning module that amplifies,protects, filters, linearizes and isolates a wide variety of two- and three-wire RTDs. For true four-wire RTD measurements please refer to the 5B35
The 5B34 protects the computer side from damage due to field-side overvoltage faults. The module withstands 240 V rms at the input terminals without damage thereby shielding the internal computer-side circuitry from field-side overvoltage conditions. In addition, the 5B34 is mix-and-match and hot swappable with all other 5B Series input modules, so can be inserted or removed from any socket in the same backplane without disrupting system power.
Inside the 5B32 Series Module
Two identical sources provide excitation current for the RTD. For three-wire RTDs, the second current flows through the third RTD lead so as to cancel the effects of (equal) lead resistance; this current also flows through a stable resistor laser-trimmed to the RTD value that sets the differential amplifier input and (module) output to zero volts at that scale point. The current sources and the amplifier input are protected to withstand input overvoltage up to 240 V rms.
A differential chopper-stabilized input amplifier provides stable gain and exceptionally low drift. This allows the use of very low RTD excitation current to minimize self-heating and preserve measurement accuracy. Along with module gain and zero settings, a feedback linearizer is laser-trimmed. Custom versions of 5B34 can be laser-trimmed to meet special requirements.
Internal multi-pole lowpass filtering with a four-Hz cutoff (-3dB)enhances normal-mode (noise on signal) and common-mode (noise on signal return) rejection at 50/60 Hz, enabling accurate measurement of small signals in high electrical noise.
Signal isolation by transformer coupling uses a proprietary modulation technique for linear, stable and reliable performance. The differential input circuit on the field side is fully floating, eliminating the need for any input grounding. A demodulator on the computer side of the signal transformer recovers the original signal, which is then filtered and buffered to provide a low-noise, low-impedance output signal. The output common must be kept within ±3 V of power common.
A series output switch eliminates the need for external multiplexing in many applications. The switch is turned on by an active-low enable input. If the switch is to be on at all times, the enable-input should be grounded to power common as it is on the 5B01 and 5B08 backplanes.