7B35 Isolated, Process Current Input with Loop Power

The 7B35 is a single-channel signal conditioning current input module that interfaces with two-wire transmitters, providing an isolated protected precision output of either +1 V to +5 V or +2 V to +10 V. Model 7B35 features an isolated 24 V DC loop power supply for driving the transmitter. 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 105 dB @ 50/60 Hz is provided. Rated to operate with a nominal +24 V DC supply, Model 7B35 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.

Inside the 7B35 Series Module

The two input pins of Model 7B35 are each fully protected up to 120 V rms . The current input signal is converted to a voltage, then filtered by a one-pole 100 Hz filter. 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 isolated transmitter loop power supply is unregulated and will provide a voltage that is proportional to the voltage used to power the 7B35 module. The isolated voltage provided by the 7B35 is between 13 V DC and 34 V DC for loop currents between 4 mA and 20 mA and supply voltages between 20.4 V DC and 26.4 V DC. The output section contains a two-pole low pass filter (-3 dB @100 Hz), a buffer amplifier and 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 down to 2 kΩ.