CN0325: PLC/DCS Universal Analog Input Using Either 4 or 6 Pin Terminal Block

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OVERVIEW

Benefits & Features
  • PLC/DCS universal input module
  • 4-pin or 6-pin terminal block
  • Fully isolated
  • 16-bit sigma-delta ADC
    Applications: 
  • Programmable Logic Controllers/ Distributed Control Systems
Design Resources
Design & Integration Files
  • Schematics
  • Bill of Materials
  • Gerber Files
  • Allegro Files
  • Assembly Drawing
Download Design Files (1515 kB)
Evaluation Hardware
Part numbers with "Z" indicate RoHS Compliance.
Boards checked are needed to evaluate this circuit.
  • EVAL-CN0325-SDPZ ($150.00) PLC/DCS Universal Analog Input Using Either 4 or 6 Pin Terminal Block
  • EVAL-SDP-CB1Z ($99.00) Eval Control Board
Check Inventory & Purchase
Device Drivers
Software, such as C code and/or FPGA code, used to communicate with a component's digital interface.
FPGA HDL
Connectivity Options
This circuit supports 3rd party connectivity.

CIRCUIT FUNCTION AND BENEFITS

The circuit shown in Figure1 provides two, 16-bit, fully isolated, universal analog input channels suitable for programmable logic controllers (PLCs) and distributed control system (DCS) modules. Both channels are software programmable and support a number of voltage, current ranges and thermocouple, RTD types as specified in Figure 1. The inputs are protected for dc overvoltage conditions of ±30 V.

Figure 1. Universal Analog Input Overview

The demonstration board contains two different fully isolated universal input channels, one with a 4-pin terminal block (CH2), and one with a 6-pin terminal block (CH1). For the 4 terminal block channel (CH2), the voltage, current, thermocouple and RTD inputs all share the same 4 terminals, thus minimizing the number of terminal pins required. For the 6-pin terminal block channel (CH1) the voltage and current inputs share a set of 3 terminal, and the thermocouple and RTD inputs share another set of 3 terminals; this requires more terminals but has a lower part countand component cost. Figure 2 shows a photo of the PCB, and Figure 3 shows a more detailed schematic of the circuit.

Figure 2. Universal Analog Input Board
Figure 3. Functional Block Diagram (Simplified Schematic: All Connections and Decoupling Not Shown)

CIRCUIT DESCRIPTION

The AD7795 low noise, 16-bit, sigma delta ADC, with on-chip in-amp and reference is used for the data conversion. The on-chip in-amp and current sources provide a complete solution for RTD and thermocouple measurement. For the voltage and current inputs, the AD8226 instrumentation amplifier with >90 dB CMR is used to provide a high input impedance and reject any common-mode interference. The voltage and current signals are scaled to the range of the ADC using a precision resistor divider.

The ADR441, an ultralow noise, low dropout XFET® 2.5 V voltage reference is used as the reference for the ADC.

For the 4-pin terminal block channel (CH2), the ADG442, low RON, latch-up proof switch is used to switch between voltage, current, thermocouple, and RT D input modes.

Digital and power isolation is achieved using ADuM3471, a PWM controller and transformer driver with quad-channel isolator which is used to generate an isolated ±15 V supply using an external transformer. The ADuM1311, triple-channel digital isolator is also used in the 4-pin terminal block circuit to isolate the control lines for the ADG442 switches.

The ADP2441, 36 V step-down dc-to-dc regulator has a wide tolerance on its input supply making it ideal for accepting a 24 V industrial supply. It accepts up to 36 V, thereby making reliable transient protection of the supply input more easily achievable. It steps the input voltage down to 5 V to power the ADuM3471 as well as all other controller-side circuitry. The circuit also includes standard external protection on the 24 V supply terminals.

The ADP2441 also features a number of other safety and reliability functions, such as undervoltage lockout (UVLO), a precision enable, a power good pin, and overcurrent-limit protection. It also can achieve up to 90% efficiency in the 24 V input, 5 V output configuration.

HARDWARE


Figure 2 shows the location of the channel containing a 4-pin terminal block and the channel with a 6-pin terminal block. It also shows the location of the 24 V supply input.

Figure 4. Channel Locations

Channel Selection

Jumpers need to be inserted and switched to configure both supply and SPI signals between CH1 and CH2 as shown in Table 1.

Power Configurations


A 24 V supply is used to power the controller side of the board. Alternately a 5 V supply can be used, bypassing the ADP2441 circuitry. This 5 V input does not have any overvoltage protection and should not exceed 6 V. The supply used should be configured using the J4 link option as described in Table 2.

For the analog input side of the isolation barrier, there are two options for powering a regulated 5 V for the analog circuitry. Either the ADP1720 linear regulator can be used to step the 15 V down to 5 V, or else the ADuM3471’s internal 5 V regulator can be used. The link configurations for each is shown in Table 3.

CH2: 4-PIN TERMINAL BLOCK CHANNEL

Input Connectors

Figure 5. CH2 input connectors

Voltage and Current


The P12 connector is used for voltage and current input connections. Figure 11 and Figure 12 show simplified schematics for this input connection and configuration. This allows differential inputs in the ranges: 0 V to 5V, 0 V to 10V, ±5V, ±10 V, 0 mA to 20 mA, 4 mA to 20 mA and ±20 mA. Connect voltage or current inputs between V1+ and V1–, because current inputs also short V1+ and I1 pins together. This connects a 249 Ω precision current sensing resistor with 0.1% accuracy and 0.25 W rating.

Thermocouple

The P12 connector is also for thermocouple inputs. Various thermocouple types can be connected including J, K, T, and S. The thermocouple is connected between the V1+ and V1- inputs (see Figure 5). Figure 6 below shows how to connect a thermocouple (Type T, for example) to the universal analog input board. See Figure 13 for a simplified schematic of the thermocouple input.

Figure 6. CH2 Thermocouple connector

RTD


The P12, P13 connectors are used for RTD inputs. The hardware can support both 1000 Ω and 100 Ω platinum RTD inputs. For 3-wire mode, the two common wires are connected to V1+ and V1– and the return is connected to Vm (see Figure 5). Figure 7 below shows how to connect a 3-wire RTD sensor to the universal analog input board. See Figure 14 for a simplified schematic of the RTD input.

Figure 7. CH2 RTD connector

CH1: 6-PIN TERMINAL BLOCK CHANNEL

Input Connectors

Figure 8. CH1 channel input connectors. See Figure 13 for a simplified input diagram.

Voltage and Current


The P10 connector is used for voltage and current input connections. This allows differential inputs in the ranges: 0 V to 5 V, 0 V to 10V, ±5V, ±10V, 0 mA to20 mA, 4 mA to 20mA, and ±20 mA. Connect voltage or current inputs between V1+ and V1– (see Figure 13), though for current inputs also short V1+ and I1 pins together, this connects a 249 Ω precision current sensing resistor with 0.1% accuracy and 0.25 W rating.

Thermocouple


The P11 connector is for thermocouple inputs. Various thermocouple types can be connected including J, K, T and S. The thermocouple is connected between the V+ and V- inputs (see Figure 8). Figure 9 below shows how to connect a thermocouple (Type T in this example) to the universal analog input board.

Figure 9. CH1 Thermocouple Connector

RTD


The P11 connector is also for RTD input. The hardware can support both 1000 Ω and 100 Ω platinum RTD inputs. For 3-wire mode, the two common wires are connected to V+ and V–, and the return is connected to Vm (see Figure 8). Figure 10 below shows how to connect a 3-wire RTD sensor to the universal analog input board.

Figure 10. CH1 RTD Connector

SIMPLIFIED INPUT CIRCUIT DIAGRAMS

Figure 11. CH2 Simplified Voltage Input diagram
Figure 12. CH2 Simplified Current Input diagram
Figure 13. Ch2 Simplified Thermocouple Input diagram
Figure 14. Ch2 Simplified RTD Input diagram
Figure 15. CH1, simplified Input diagram

For a description of the software operation, please refer to the PDF file for CN0325.

SAMPLE PRODUCTS USED IN THIS CIRCUIT

Product Description Available Product Models to Sample
AD7795 6-Channel, Low Noise, Low Power, 16-Bit Sigma Delta ADC with On-Chip In-Amp and Reference AD7795BRUZ
AD8226 Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier AD8226ARZ AD8226ARMZ
ADG442 LC2MOS Quad SPST Switch ADG442BRZ
ADP1720 50 mA, High Voltage, Micropower Linear Regulator ADP1720ARMZ-3.3-R7 ADP1720ARMZ-5-R7 ADP1720ARMZ-R7
ADP2441 36 V,1 A, Synchronous, Step-Down DC-DC Regulator ADP2441ACPZ-R7
ADR441 Ultralow Noise, LDO XFET® 2.5V Voltage Reference w/Current Sink and Source ADR441ARMZ ADR441ARZ
ADT7310 ±0.5°C Accurate, 16-Bit Digital SPI Temperature Sensor ADT7310TRZ
ADUM1311 Triple-Channel Digital Isolators ADUM1311ARWZ ADUM1311BRWZ
ADUM3471 Isolated Switching Regulators (3/1 Channel Directionality) ADUM3471WARSZ ADUM3471ARSZ ADUM3471WCRSZ ADUM3471CRSZ
Evaluation Hardware
Boards checked are needed to evaluate this circuit.
Model Description Price RoHS Check Inventory/
Purchase/Sample
EVAL-CN0325-SDPZ PLC/DCS Universal Analog Input Using Either 4 or 6 Pin Terminal Block $150.00 Yes
EVAL-SDP-CB1Z Eval Control Board $99.00 Yes
Pricing displayed is based on 1-piece. The USA list pricing shown is for budgetary use only, shown in United States dollars (FOB USA per unit), and is subject to change. International prices may vary due to local duties, taxes, fees and exchange rates.
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