PLC/DCS Universal Analog Input Using Either 4 or 6 Pin Terminal Block
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)
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.
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
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
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
|Link position to select 5V
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
CH2: 4-PIN TERMINAL BLOCK CHANNEL
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.
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
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
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.
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
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.
|AD7795||6-Channel, Low Noise, Low Power, 16-Bit Sigma Delta ADC with On-Chip In-Amp and Reference||
|ADR441||Ultralow Noise, LDO XFET® 2.5V Voltage Reference w/Current Sink and Source||
|ADUM1311||Triple-Channel Digital Isolators||
|ADG442||LC2MOS Quad SPST Switch||
|AD8226||Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier||
|ADT7310||±0.5°C Accurate, 16-Bit Digital SPI Temperature Sensor||
|ADP2441||36 V,1 A, Synchronous, Step-Down DC-DC Regulator||
|ADUM3471||Isolated Switching Regulators (3/1 Channel Directionality)||
|ADP1720||50 mA, High Voltage, Micropower Linear Regulator||