7B35 Isolated, Process Current Input with Loop Power


7b34-01-1The 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.

Input and Output Ranges

 7B35 Functional Block Diagram
Figure 1. 7B35 Functional Block Diagram

7B35 Input Field Connections
Figure 2. 7B35 Input Field Connections

Input Types  Output Options
Two-Wire Transmitter, 4 mA to 20 mA
+1 V to +5 V
0 V to +10 V


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Input Range  Output Range 
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+4 mA to +20 mA1
+1 V to +5 V
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+4 mA to +20 mA1
+2 V to +10 V


7B35 Specifications
(typical @ +23°C ±5°C and Vs = +24 V dc)

Model 7B35
Input Range1
 Standard Range
4 mA to 20 mA     
Custom Ranges  Not Available*
Standard Range 4 mA to 20 mA
 Custom Ranges  Not Available*
 Input Loop Supply Voltage, Isolated
+24 V dc @ ILoop = 20 mA 
Output Range (RL > 2 kΩ
+1 V to +5 V or 2 V to +10 V
Initial @ +25°
±0.01% Span (±0.1% Span, maximum)
Nonlinearity2 ±0.02% Span, maximum
Input Offset vs. Temperature  N/A3
Span vs. Temperature
±40 ppm/°C
Output Offset vs. Temperature  ±0.003% span/°C
 Output Noise
5 MHz Bandwidth  6 mV peak
10 Hz to 100 kHz Bandwidth
0.9 mV rms
0.1 Hz to 10 Hz Bandwidth
4 µV peak
Bandwidth, -3 dB 100 Hz
Output Rise Time, 10% to 90% Span
5 ms
 Common-Mode Voltage (CMV)
Input-to-Output and Power
1500 V rms continuous 
Common-Mode Rejection (CMR) 
Input-to-Output and Power @ 50/60 Hz
105 dB
Input Protection 120 V rms, continuous
±35 V dc, continuous
Input Transient Protection ANSI/IEEE C376.90.1-1989
IEC 255-4, Class II
Output Resistance < 1 Ω
Voltage Output Protection Continuous Short to Ground 
 Power Supply
Voltage Range, Operating +14 V dc to +35 V dc
+25 mA, maximum
±0.0001%/% of Vs
Mechanical Dimensions 1.663" x 2.11" x 0.563
(42.24 mm x 53.6 mm x 14.3 mm)
Weight 60 grams
Temperature Range
Operating -40°C to +85°C
Storage -40°C to +85°C
Relative Humidity, 24 hours
0 to 90% @ +60°C noncondensing
ESD Sensitivity
IEC 801-2, Level 2
RFI Susceptibility ±0.5% Span error @ 400 MHz, 5 Watt, 3 ft

Warm-up time required to meet specifications is approximately 10 minutes.
* Contact factory for OEM requirements.
1 Includes the combined effects of repeatability, hysteresis, and nonlinearity.
2 Nonlinearity is calculated using best-fit straight line method.
3 Input offset term is included in the output offset specification.
Specifications subject to change without notice.