AN-2569: PLC and DCS Universal Analog Input Using Either 4-Pin or 6-Pin Terminal Block

Circuit Function and Benefits

The circuit shown in Figure 1 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 configurable and support a number of voltage, current ranges, thermocouple, and RTD types, as shown in Figure 2.

When using Channel 2 (CH2), the voltage, current, thermocouple, and RTD inputs all share the same 4 terminals, thus minimizing the number of terminal pins required. When using Channel 1 (CH1), the voltage and current inputs share a set of 3 terminals, and the thermocouple and RTD inputs share another set of 3 terminals; this configuration requires more terminals but has a lower part count and component cost.

Circuit Description

The AD7795 low noise, 16-bit, Σ-Δ 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 CMRR 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 CH2, the ADG442, low RON, latch-up proof switch is used to switch between voltage, current, thermocouple, and RTD 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 3 shows the location of the channel containing the 4-pin terminal block and the channel with the 6-pin terminal block. It also shows the location of the 24 V supply input.

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 powers the controller side of the board. Alternately, a 5 V supply can be used, bypassing the ADP2441 circuitry. This 5 V input has no overvoltage protection and must not exceed 6 V. The supply used must 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 internal 5 V regulator of the ADuM3471 can be used. The link configurations for each is shown in Table 3.

CH2: 4-Pin Terminal Block Channel

Input Connectors

Voltage and Current

The P12 connector is used for voltage and current input connections. Figure 10 and Figure 11 show simplified schematics for this input connection and configuration. This configuration allows differential inputs in the ranges of 0 V to 5 V, 0 V to 10 V, ±5 V, ±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 the V1+ and I1 pins together. Shorting V1+ to I1 allows the 249 Ω, 0.1%, 0.25 W to be used as a current sensing resistor.

Thermocouple

The P12 connector is also used for thermocouple inputs. Various thermocouple types can be connected including J, K, T, and S types. The thermocouple is connected between the V1+ and V1− inputs (see Figure 4). See Figure 5 for a simplified schematic of the thermocouple input.

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 4). See Figure 6 for a simplified schematic of the RTD input.

CH1: 6-Pin Terminal Block Channel

Input Connectors

Voltage and Current

The P10 connector is used for voltage and current input connections. This allows differential inputs in the ranges of 0 V to 5 V, 0 V to 10 V, ±5 V, ±10 V, 0 mA to20 mA, 4 mA to 20 mA, and ±20 mA. Connect voltage or current inputs between V1+ and V1− (see Figure 10 and Figure 11). For current inputs, also short V1+ and I1 pins together, thereby connecting a 249 Ω precision current sensing resistor with 0.1% accuracy and 0.25 W rating.

Thermocouple

The P11 connector is used for thermocouple inputs. Various thermocouple types can be connected including J, K, T, and S types. The thermocouple is connected between the V+ and V− inputs (see Figure 7). Figure 8 shows how to connect a thermocouple (Type T in this example) to the universal analog input board.

RTD

The P11 connector is also used 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 7). Figure 9 shows how to connect a 3-wire RTD sensor to the universal analog input board.

System Resolution for 4-Pin Terminal Block Channel

With chop enable or disable selected and the data update rate selected, Table 4 shows the 4-pin terminal block channel system resolution measured with effective resolution and peak-to-peak resolutions for each input type. Note that all resolution measurements are based on the full-scale ranges as follows:

• ±10 V: full-scale range referred to 20 V
• 0 V to 5 V: full-scale range referred to 5 V
• Type K: full-scale range referred to 1520°C
• Type J : full-scale range referred to 900°C
• Type T: full-scale range referred to 550°C
• Type S: full-scale range referred to 1765°C
• PT100: full-scale range referred to 850°C
• PT1000: full-scale range referred to 850°C

System Resolution For 6-Pin Terminal Block Channel

With chop enable or disable selected and the data update rate selected, Table 5 shows the 6-pin terminal block channel system resolution measured with effective resolution and peak-to-peak resolutions for each input type. Note that all resolution measurements are based on the full scale ranges as follows:

• ±10 V: full-scale range referred to 20 V
• 0 V to 5 V: full-scale range referred to 5 V
• Type K: full-scale range referred to 1520°C
• Type J: full-scale range referred to 900°C
• Type T: full-scale range referred to 550°C
• Type S: full-scale range referred to 1765°C
• PT100: full-scale range referred to 850°C
• PT1000: full-scale range referred to 850°C

Simplified Input Circuit Diagrams