7B34 ISOLATED, LINEARIZED RTD INPUT

The 7B34 is a single-channel signal conditioning module that interfaces, amplifies and filters input voltages from a wide variety of two- and three-wire platinum, copper and nickel Resistor Temperature Detectors (RTDs) and provides a protected precision output of either +1 V to +5 V or 0 V to +10 V, linear with RTD temperature. Three-wire lead resistance compensation is provided and 2- or 3- wire RTDs may be used. RTD excitation current, and a predictable upscale open circuit indication provide a complete signal conditioning solution. Model 7B34 features a nonlinearity of ±0.05% maximum (Pt RTDs). To accurately measure low level signals in electrically noisy environments, 1500 V rms of galvanic transformer-based isolation with a common mode rejection (CMR) of 160 dB @ 50/60 Hz and a normal model rejection (NMR) of 60 dB @ 50/60 Hz are provided. Rated to operate with a nominal +24 V DC supply, Model 7B34 is mix-and-match and hot-swappable with other 7B Series input modules, so it can be inserted or removed from any socket in the same backplane without disturbing system power.

The three input pins of Model 7B34 are fully protected up to 120 V rms line voltage. A 250 µA excitation current is provided to create an input voltage to the 7B34. This current also provides the upscale open circuit indication. A one-pole 3 Hz filter preconditions the RTD signal prior to amplification, provided by a low drift input amplifier. Amplitude modulation is used to implement transformer isolation (1500 V rms input-to-output and power). Isolated front-end circuitry power is supplied by a DC/DC converter. The output section contains a two-pole low pass filter (-3 dB @ 3Hz), a buffer amplifier and a power oscillator. The two-pole output filter and subsequent buffer ensures that a low noise, low impedance (<1 Ω) signal is available at the output to drive loads to 2 kΩ minimum.

Figure 1. 7B34 Functional Block Diagram