Figure 2 shows a functional diagram for the Model 3B37 isolated thermocouple signal conditioner. The input signal is filtered, amplified and has cold junction compensation applied before the isolation barrier. The outputs of the module are isolated from the input (up to ±1500 V peak) and factory calibrated for 0 to +10V (voltage output) and 4-20 mA (current output), which correspond to the specified input span.
The input modules are available in several factory-calibrated input ranges. Each input module includes separate screwdriver-adjustable zero and span potentiometers for both of the outputs which can be used for fine calibration within the chosen range. The voltage and current adjustments are independent and non-interactive, enabling precise calibration of both outputs.
The current output of each input module has two user-programmable jumper options. One option enables the user to program the current output to be proportional to either a 0 to +10 V or a -10 V to +10 V voltage output. The second option enables the user to set the current output span to 4-20 mA or 0-20 mA. All modules are shipped from the factory configured so that the current output is proportional to the 0-10 V output range and all current outputs are 4-20 mA.
| Figure 2. Model 3B37 Functional Block Diagram
Input Module Features
- Wide Variety of Sensor Inputs:
ac and dc Strain Gauges
- Dual High Level Outputs
Voltage: 0 to +10 V or ±10 V
Current: 4-20 mA/0-20 mA
- Mix and Match Module Capability
- High Accuracy: ±0.1%
- Low Drift: ±1µV/°C
- Reliable Transformer Isolation:
±1500 V Peak and CMR = 160 dB
- Meets ANSI/IEEE Transient Protection
- Input Protection: 130 V or 220 V rms Continuous
Each input module is a single-channel signal conditioner which plugs into a socket on the backplane and accepts its signal from the input screw terminals. All input modules provide input protection, amplification and filtering of the input signal, accuracy of ±0.1%, low drift of ±1 µV/°C (low-level input modules), and feature two high-level analog outputs that are compatible with most process instrumentation. The isolated input modules also provide ±1500 V peak isolation.
The choice of a specific 3B module depends upon the type of input signal. Input modules are available to accept millivolt, volt, process current, thermocouple, RTD, ac and dc strain gage, frequency and LVDT inputs. The voltage output of each module is available from the system I/O connector while the current output is available on the output screw terminals.
Wide Zero Suppression Capability
A wide zero-suppression capability and easy-custom ranging are available with a unique plug-on ranging card (AC1310). If a special input range is desired, it can be provided by ordering the externally-programmable version of the desired module (i.e. 3B32-00) and the plug-on ranging card, AC1310.This card houses user-supplied resistors which determine the zero and span of the desired range. RTD modules accept an additional resistor to establish linearization of the input signal, while the 3B37 thermocouple module uses an additional resistor to set the cold junction compensation level. This ranging capability enables the user to provide zero suppression of up to and beyond 100% of the input range and provides a very wide range of span modification. This enables the user to map any portion of the input signal to the full output span. For example, a user who wants to measure temperature with a thermocouple in the range of +800°C to +900°C can use this ranging card for greater system resolution within that defined 100°C temperature span. This tremendous flexibility should satisfy virtually any requirement. The resistor values are determined by equations defined for each module. A software program (3B-CUSTOM) is also available to assist in quickly calculating the resistor values, using only the user-defined input and output requirements.
Special ranges can also be factory configured. Analog Devices will provide this service when a model 3BXX-CUSTOM is ordered with the desired range. (The 3B47 linearized thermocouple input module can not be user ranged with an AC1310 since approximately 15 components are required for linearization. A factory configured 3B47-CUSTOM can provide the desired input range.)