Reference Circuits for Process Control

Standard single-ended industrial signal levels of ±5 V, ±10 V, and 0 V to +10 V are not directly compatible with the differential input ranges of modern high performance 16-bit or 18-bit single-supply SAR ADCs. A suitable interface drive circuit is required to attenuate, level shift, and convert the industrial signal into a differential one with the correct amplitude and common-mode voltage that match the input requirements of the ADC.

Whereas suitable interface circuits can be designed using resistor networks and dual op amps, errors in the ratio matching of the resistors, and between the amplifiers, produce errors at the final output. Achieving the required output phase matching and settling time can be a challenge, especially at low power levels.

The circuit shown in Figure 1 uses the AD8475 differential funnel amplifier to perform attenuation, level shifting, and conversion to differential without the need for any external components. The ac and dc performances are compatible with those of the 18-bit, 1 MSPS AD7982 PulSAR^® ADC and other 16- and 18-bit members of the family, which have sampling rates up to 4 MSPS.

The AD8475 is a fully differential attenuating amplifier with integrated precision thin film gain setting resistors. It provides precision attenuation (by 0.4× or 0.8×), common-mode level shifting, and single-ended-to-differential conversion along with input overvoltage protection. Power dissipation on a single 5 V supply is only 15 mW. The 18-bit, 1 MSPS AD7982 consumes only 7 mW, which is 30× lower than competitive ADCs. The total power dissipated by the combination is only 22 mW.

The circuit, shown in Figure 1, provides a full function, high voltage (up to 44 V), flexible, programmable analog output solution that meets most requirements for programmable logic controller (PLC) and distributed control system (DCS) applications.

The AD5662 low power (0.75 mW typical @ 5 V), rail-to-rail output, 16-bit nanoDAC^® device and the AD5751 industrial current/voltage output driver are well matched with respect to input and output voltage ranges, as well as reference voltage requirements.

The ADR444, with low drift (3 ppm/℃ maximum for B grade), high initial accuracy (0.04% maximum for B grade), and low noise (1.8 μV p-p typical, 0.1 Hz to 10 Hz), provides the reference voltage for both the AD5751 and AD5662 and guarantees ultralow noise, high accuracy, and low temperature drift for the circuit. This circuit provides all the typical voltage and current output ranges with 16-bit resolution and no missing codes, 0.05% linearity, and less than 0.2% total output error.

The ADuM1301 and ADuM5401 provide all the necessary signal isolation between the microcontroller and the analog signal chain. The ADuM5401 also provides isolated 5 V power. The circuit also contains key features for industrial applications, such as on-chip output fault detection, CRC checking to prevent packet error (PEC), and flexible power-up options, making it an ideal choice for robust industrial control systems. No external precision resistors or calibration routines are needed to maintain consistent performance in mass production, thereby making it ideal for PLC or DCS modules.

The circuit shown in Figure 1 provides a high dynamic range four channel simultaneous sampling system with high crosstalk isolation, flexible sampling rates, minimal external components, and simple interface connections to a DSP or FPGA. The circuit uses four AD7765 24-bit sigma-delta ADCs in a daisy chain configuration to minimize the number of connections to the digital host. The AD7765’s fully integrated differential input/output amplifier, and reference buffer significantly reduce the external component requirement.

Using the AD7765 configured in a simultaneous sampling configuration provides

• Channel-to-channel crosstalk isolation superior to that of solutions that integrate multiple 24-bit ADCs on a single chip.
• 112 dB dynamic range at 156 kSPS.
• Adaptable to greater or smaller channel counts.
• Allows multiple SYNC control (can be phase shifted with respect to each other).
• Dual decimation rates (128 and 256) and flexible sampling clock handles wide range of input bandwidths.

The circuit shown in Figure 1 provides a full function, flexible, programmable analog output solution that meets most requirements for programmable logic controller (PLC) and distributed control system (DCS) applications. The AD5662 low power (0.75 mW @ 5 V), rail-to-rail output, 16-bit nanoDAC^® converter and the AD5750 industrial current/voltage output driver are well matched with respect to input and output voltage ranges, as well as reference voltage requirements. The ADR444 low drift (3 ppm/°C maximum for B-grade ), high initial accuracy (0.04% maximum for B-grade), and low noise (1.8 μV p-p typical, 0.1 Hz to 10 Hz) provides the reference voltage for both the AD5750 and AD5662 and guarantees ultralow noise, high accuracy, and low temperature drift for the circuit. This circuit provides all the typical voltage and current output ranges with 16-bit resolution and no missing codes, 0.05% linearity, and less than 0.2% total output error.

The circuit also contains key features for industrial applications, such as on-chip output fault detection and protection (short circuit, undervoltage output, open circuit current output, and overtemperature), CRC checking to prevent packet error (PEC), and flexible power-up options, making it an ideal choice for robust industrial control systems. No external precision resistors or calibration routines are needed to maintain consistent performance in mass production, thereby making it ideal for PLC or DCS modules.

The circuit shown in Figure 1 is a full-featured, flexible, and programmable analog output solution that uses only two analog devices. It meets programmable logic controller (PLC) and distributed control system (DCS) applications Most of the requirements. AD5660-1 is a low power consumption (2.8 mW @ 5 V), rail-to-rail output, 16-bit nanoDAC^® , AD5750-1 is an industrial current/voltage output driver. The combination of the two can provide all typical current and Voltage output range, 16-bit resolution and no missing codes, 0.05% linearity and less than 0.1% output error. The circuit also has some important features that support industrial applications, such as on-chip output fault detection, CRC check to prevent packet errors (PEC), and flexible power-on options, etc., making it very suitable for building robust industrial control systems. In mass production, it can maintain consistent performance without external precision resistors or calibration procedures, so it is an ideal choice for PLC or DCS modules.

The circuit, shown in Figure 1, is a complete analog front end for digitizing ±10 V industrial level signals with a 16-bit differential input PulSAR^® ADC. The circuit provides a high impedance instrumentation amplifier input with high CMR, level shifting, attenuation, and differential conversion, with only two analog components. Because of the high level of integration, the circuit saves printed circuit board space and offers a cost effective solution for a popular industrial application.

Signal levels of up to ±10 V are typical in process control and industrial automation systems. With smaller signal inputs from sensors such as thermocouples and load cells, large common-mode voltage swings are often encountered. This requires a flexible analog input that handles both large and small differential signals with high common-mode rejection and also has a high impedance input.

Attenuation and level shifting are necessary to process industrial level signals with modern low voltage ADCs. In addition, fully differential input ADCs offer the advantages of good common-mode rejection, reduction in second-order distortion products, and simplified dc trim algorithms. Industrial signals, therefore, need further conditioning to properly interface with differential input ADCs.

The circuit in Figure 1 is a complete and highly integrated analog front end industrial level signal conditioner that uses only two active components to drive an AD7687 differential input 16-bit PulSAR ADC: the AD8295 precision in-amp (with two on-chip auxiliary op amps) and the AD8275 level translator/ADC driver. An ADR431 low noise 2.5V XFET^® reference supplies the voltage reference for the ADC.

The AD8295 is a precision instrumentation amplifier with two uncommitted on-chip signal processing amplifiers and two precisely matched 20 kΩ resistors in a small 4 mm × 4 mm package.

The AD8275 is a G = 0.2 difference amplifier that can be used to attenuate ±10 V industrial signals, and the attenuated signal can be easily interfaced to a single supply low voltage ADC. The AD8275 performs the attenuation and level shifting function in the circuit, maintaining good CMR without any need for external components.

The AD7687 is a 16-bit, successive approximation ADC that operates from a single power supply between 2.3 V and 5.5 V. It has a differential input for good CMR and also offers the ease of use associated with SAR ADCs.

The circuit shown in Figure 1 provides a programmable 20-bit voltage with an output range −10 V to +10 V, ±1 LSB integral nonlinearity, ±1 LSB differential nonlinearity, and low noise.

The digital input to the circuit is serial and is compatible with standard SPI, QSPI^™, MICROWIRE^®, and DSP interface standards. For high accuracy applications, the circuit offers high precision, as well as low noise, and this is ensured by the combination of the AD5791, AD8675, and AD8676 precision components.

The reference buffer is critical to the design because the input impedance at the DAC reference input is heavily code dependent and will lead to linearity errors if the DAC reference is not adequately buffered. With a high open-loop gain of 120 dB, the AD8676 has been proven and tested to meet the settling time, offset voltage, and low impedance drive capability required by this circuit application. The AD5791 is characterized and factory calibrated using the AD8676 dual op amp to buffer its voltage reference inputs, further enhancing confidence in partnering the components.

This combination of parts provides industry-leading 20-bit integral nonlinearity (INL) of ±1 LSB and differential nonlinearity (DNL) of ±1 LSB, with guaranteed monotonicity, as well as low power, small PCB area, and cost effectiveness.

This circuit uses the ADuC7060/ADuC7061 precision analog microcontroller in an accurate thermocouple temperature monitoring application. The ADuC7060/ADuC7061 integrates dual 24-bit Σ-Δ ADCs, dual programmable current sources, a 14-bit DAC, and a 1.2 V internal reference, as well as an ARM7 core, 32 kB flash, 4 kB SRAM, and various digital peripherals such as UART, timers, SPI, and I²C interfaces.

In the circuit, the ADuC7060/ADuC7061 are connected to a thermocouple and a 100 Ω Pt RTD. The RTD is used for cold junction compensation.

In the source code, an ADC sampling rate of 4 Hz is chosen. When the ADC input PGA is configured for a gain of 32, the noise free code resolution of the ADuC7060/ADuC7061 is greater than 18 bits.