5B35 ISOLATED, LINEARIZED 4-WIRE RTD INPUT

The 5B35 is a single-channel signal conditioning module that amplifies, filters, isolates and protects a wide variety of four-wire RTDs. The module provides a linearized, highly accurate output of 0 to +5V.

Accurate and Stable Performance
The four-wire configuration of the 5B35 supplies RTD excitation from a precision, protected current source: two RTD wires carry the excitation current and two wires serve as signal-input leads that measure the voltage across the RTD. Because no excitation current flows through the signal-input leads, the length or resistance of the leads creates little or no RTD measurement error. A low-drift chopper-stabilized differential amplifier allows use of very low excitation current to minimize error due to RTD self-heating. The amplifiers low input offset drift of ±0.01°C/°C and gain drift of ±30 ppm/°C assure accuracy over the full operating temperature range.

True Three-Port Isolation
The floating, differential input circuit on the field side eliminates the need for any input grounding. Signal and power isolation by transformer coupling uses a proprietary modulation technique for linear, stable and reliable performance. 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. True three-port isolation (Input-Output-Power) includes common-mode ratings of : 1500 V rms between input and output and input and power; 250 V rms between power and output - no return path is required between the power and signal output commons.

Filtering and Protection
An optimized five-pole Butterworth filter (with a 4-Hz bandwidth) provides 116 dB of normal-mode rejection (noise on signal) at 60 Hz and 108 dB at 50 Hz. Output noise is an exceptionally low 0.3 mV p-p in a 100 kHz bandwidth and 6 mV p-p in a 5 MHz bandwidth.

The 5B35 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 5B35 is mix-and-match and hot swappable with all other 5B Series modules, so can be inserted or removed from any socket in the same backplane without disrupting system power.

Convenience Features
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 connected to output common.

Figure 1. 5B35 Functional Block Diagram

Input Types 100 Ω Platinum RTD 10 Ω Copper RTD 120 Ω Nickel RTD Output Range 0 to +5 V

Figure 2. 5B35 Input Field Connections