CN0325

PLC/DCS Universal Analog Input Using Either 4 or 6 Pin Terminal Block
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Circuit Function & Benefits

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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 count and 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

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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.

 Table 1. Channel selection configuration settings


Link No. Digital Function Link position to select CH1,
6 pin terminal block
Link position to select CH2,
4 pin terminal block
 JK0  5V supply  CH1  CH2
 JK1  SCLK  CH1
 CH2
 JK2  /CS  CH1
 CH2
 JK3  Din  CH1
 CH2
 JK4  Dout  CH1
 CH2
 JK11  TEMP_CS  Not Inserted  Inserted


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.

   Table 2. External Power Supply configuration settings


Link position to select 24V
Input (default)
Link position to select 5V
Input (default)
J4  VCC1 VCC2


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.

  Table 3. Field 5V supply configuration settings

  Link position for ADP1720,
5V regulator (default)
Link position for
ADuM3471's 5V regulator
J3 Vreg  Vaum 
J9 Vreg  Vaum 


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

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Samples

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

ADR441 Ultralow Noise, LDO XFET® 2.5V Voltage Reference w/Current Sink and Source

ADR441ARMZ

ADR441ARZ

ADUM1311 Triple-Channel Digital Isolators

ADUM1311ARWZ

ADUM1311BRWZ

ADG442 LC2MOS Quad SPST Switch

ADG442BRZ

AD8226 Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier

AD8226ARMZ

AD8226ARZ

ADT7310 ±0.5°C Accurate, 16-Bit Digital SPI Temperature Sensor

ADT7310TRZ

ADP2441 36 V,1 A, Synchronous, Step-Down DC-DC Regulator

ADP2441ACPZ-R7

ADUM3471 Isolated Switching Regulators (3/1 Channel Directionality)

ADUM3471ARSZ

ADUM3471CRSZ

ADUM3471WARSZ

ADUM3471WCRSZ

ADP1720 50 mA, High Voltage, Micropower Linear Regulator

ADP1720ARMZ-3.3-R7

ADP1720ARMZ-5-R7

ADP1720ARMZ-R7

ADP1720TRMZ-EP

ADP1720TRMZ-EP-R7

ADP1720TRMZ3.3EPR7

Evaluation Boards Pricing displayed is based on 1-piece.
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.