Complete sensor data acquisition solution simplifies industrial data acquisition system design

Introduction

　　Programmable logic controllers (PLCs) are at the heart of many industrial automation and process control systems, monitoring and controlling complex system variables. PLC-based systems use multiple sensors and actuators to measure and control analog process variables such as pressure, temperature, and flow. PLCs are used in a wide range of applications, such as factories, refineries, medical devices, and aerospace systems, which require high accuracy and stable operation over long periods of time. In addition, the fierce market competition requires cost reduction and shortened design time. As a result, designers of industrial equipment and critical infrastructure face severe challenges in meeting customers' stringent requirements for accuracy, noise, drift, speed, and safety. This article uses a PLC application as an example to illustrate how the versatile, low-cost, highly integrated ADAS3022 can reduce complexity and solve many of the challenges encountered in the design of multi-channel data acquisition systems by replacing the analog front-end (AFE) stage. This high-performance device has multiple input ranges and is ideal for high-precision industrial, instrumentation, power line, and medical data acquisition card applications, which can reduce costs and speed time to market while taking up a small space, being easy to use, and providing true 16-bit accuracy at 1 MSPS.

　　PLC Application Examples

　　Figure 1 shows a simplified signal chain for using a PLC in industrial automation and process control systems. A PLC typically includes analog and digital input/output (I/O) modules, a central processing unit (CPU), and power management circuits.

　　In industrial applications, analog input modules can acquire and monitor remote sensor signals in harsh environments, such as those with extreme temperature and humidity, vibration, and explosive chemicals. Typical signals include single-ended or differential voltages with full-scale ranges of 5 V, 10 V, ±5 V, and ±10 V, or loop currents with ranges of 0 mA to 20 mA, 4 mA to 20 mA, and ±20 mA. Current loops are often used when encountering long cables with severe electromagnetic interference (EMI) because they inherently have good noise immunity.

　　Analog output modules usually control actuators such as relays, solenoids, and valves to form a complete automation control system. They usually provide output voltages with 5 V, 10 V, ±5 V, and ±10 V full-scale ranges, as well as 4 mA to 20 mA loop current outputs.

　　Typical analog I/O modules include 2, 4, 8, or 16 channels. To meet stringent industry standards, these modules need to provide overvoltage, overcurrent, and EMI surge protection. Most PLCs include digital isolation between the ADC and the CPU, and between the CPU and the DAC. High-end PLCs may also have channel-to-channel isolation as specified by the International Electrotechnical Commission (IEC) standard. Many I/O modules can be software-programmed for each channel for single-ended or differential input range, bandwidth, and throughput.

　　In modern PLCs, the CPU automatically performs multiple control tasks and makes intelligent decisions using real-time information access. The CPU may contain advanced software and algorithms as well as web connectivity for error-checking diagnostics and fault detection. Common communication interfaces include RS-232, RS-485, Industrial Ethernet, SPI, and UART.

　　Figure 1. Typical PLC signal chain.

　　Discrete Data Acquisition System Solution

　　Industrial designers can use discrete high-performance components to build analog blocks for PLC or similar data acquisition systems, as shown in Figure 2. Key design considerations include input signal configuration, overall system speed, accuracy, and precision. The signal chain shown here uses the ADG1208/ADG1209 low leakage multiplexer, the AD8251 fast settling programmable gain instrumentation amplifier (PGIA), the AD8475 high speed funnel amplifier, the AD7982 differential input 18-bit PulSAR® ADC, and the ADR4550 ultralow noise reference. This solution provides four different gain ranges, but with a maximum input signal of ±10 V, designers must worry about the switching and settling time of the multiplexer, as well as other analog signal conditioning issues. In addition, achieving true 16-bit performance at 1 MSPS can be a serious challenge, even when using these high-performance devices.

　　The AD7982 has a 290 ns transient response for a full-scale step. Therefore, to maintain the specified performance while converting at 1 MSPS, the PGIA and funnel amplifier must settle within 710 ns. However, the AD8251 settles to 16 bits (0.001%) for a 10 V step in 785 ns, so the guaranteed maximum throughput of this signal chain will be less than 1 MSPS.

　　Figure 2. Analog input signal chain using discrete components.

　　Integrated Solutions Simplify Data Acquisition System Design

　　The 16-bit 1 MSPS ADAS3022 data acquisition system IC is manufactured using the proprietary high-voltage industrial process technology iCMOS® and integrates an 8-channel, low-leakage multiplexer; a high-impedance PGIA (with high common-mode rejection); a high-precision, low-drift 4.096 V reference voltage source and buffer; and a 16-bit successive approximation ADC, as shown in Figure 3.

　　Figure 3. ADAS3022 functional block diagram.

　　This complete sensor data acquisition solution occupies only one-third of the board space of discrete solutions, helping engineers simplify designs while reducing the size of advanced industrial data acquisition systems, shortening time to market, and saving costs. It eliminates the need to buffer, level-shift, amplify, attenuate, or otherwise condition the input signal, and eliminates concerns about common-mode rejection, noise, and settling time, and solves many of the challenges associated with designing high-precision 16-bit 1 MSPS data acquisition systems. It provides best-in-class 16-bit accuracy (typical INL of ±0.6 LSB), low offset voltage, low temperature drift, and optimized noise performance at 1 MSPS (typical SNR of 91 dB), as shown in Figure 4. The device is rated for the industrial temperature range of –40°C to +85°C.

　　Figure 4. INL and FFT performance of the ADAS3022.

　　The PGIA has a large common-mode input range, true high-impedance inputs (>500 MΩ), and wide dynamic range, which enables it to process 4 mA to 20 mA loop currents, accurately measure small sensor signals, and reject interference from AC power lines, motors, and other sources (90 dB minimum CMR).

　　The auxiliary differential input channel can handle ±4.096 V input signals. It bypasses the multiplexer and PGIA stages, allowing direct interface with a 16-bit SAR ADC. An on-chip temperature sensor can monitor the local temperature.

　　This high level of integration saves board space and reduces overall part cost, making the ADAS3022 ideal for space-constrained applications such as automatic test equipment, power line monitoring, industrial automation, process control, patient monitoring, and other industrial and instrumentation systems that operate with ±10 V industrial signal levels.

　　Figure 5. Complete 5 V, single-supply, 8-channel data acquisition solution with integrated PGA.

　　Figure 5 shows a complete 8-channel data acquisition system (DAS). The ADAS3022 uses ±15 V and +5 V analog and digital supplies, as well as 1.8 V to 5 V logic I/O supplies. The ADP1613, a high efficiency, low ripple dc-to-dc step-up converter, enables the DAS to operate from a single 5 V supply. Configured in a single-ended primary inductor (SEPIC) topology using the ADIsimPower™ design tool, the ADP1613 provides the ±15 V bipolar supplies required by the multiplexer and PGIA without sacrificing performance.

　　Table 1 compares the noise performance of the ADAS3022 and a discrete signal chain and calculates the total noise of the entire signal chain using the input signal amplitude, gain, equivalent noise bandwidth (ENBW), and input-referred (RTI) noise of each component.

　　Table 1. Noise Performance of the ADAS3022 and Discrete Signal Chain

　　A single-pole low-pass filter (LPF) between the AD8475 and the AD7982 (Figure 2) attenuates the kickback from the switched capacitor input of the AD7982, limiting the amount of high frequency noise. The LPF has a –3 dB bandwidth (f–3dB) of 6.1 MHz (R = 20 Ω, C = 1.3 nF), which allows for fast settling of the input signal when converting at 1 MSPS. The ENBW of the LPF is calculated as:

　　ENBW = π/2 × f _–3dB  = 9.6 MHz

　　Note that this calculation ignores the noise from the reference and LPF, as it does not contribute much to the total noise which is primarily determined by the PGIA.

　　For example, let's use a ±5 V input range. In this case, the AD8251 is set to a gain of 2. The funnel amplifier is set to a fixed gain of 0.4 for all four input ranges. The AD7982 is therefore processing a 0.5 V to 4.5 V differential signal (4 V pp). The RTI noise of the ADG1208 is derived from the Johnson/Nyquist noise equation: en ²  = 4K _B TR _ON , where K _B  = 1.38 × 10 ²³ J/K, T = 300K , and R _ON = 270 Ω.    

　　The RTI noise for the AD8251 is derived from the data sheet noise density of 27 nV/√Hz at a gain of 2. Similarly, the RTI noise for the AD8475 is derived from the 10 nV/√Hz noise density using a gain of 0.8 (2 × 0.4). In these calculations, ENBW = 9.6 MHz. The RTI noise for the AD7982 is calculated from the data sheet SNR of 95.5 dB at a gain of 0.8. The total RTI noise for the entire signal chain is calculated from the root sum square (rss) of the RTI noise of the discrete components. The total SNR of 89.5 dB is calculated using the equation SNR = 20 log (VINrms/RTITotal).

　　Although the theoretical noise estimate (SNR) and overall performance of the discrete signal chain are comparable to the ADAS3022, especially at low gains (G = 1 and G = 2) and low throughput (well below 1 MSPS), it is not an ideal solution. The ADAS3022 can save about 50% of the cost and about 67% of the board space compared to the discrete solution, and it can also accept three other input ranges (±0.64 V, ±20.48 V, ±24.576 V), which the discrete solution cannot provide.

　　in conclusion

　　The next generation of industrial PLC modules requires high accuracy, reliable operation, and functional flexibility, all of which must be provided by low-cost products with a small form factor. The ADAS3022 has industry-leading integration and performance, supporting a wide range of voltage and current inputs to process a variety of sensor signals for industrial automation and process control. The ADAS3022 is ideal for PLC analog input modules and other data acquisition cards, enabling industrial manufacturers to differentiate their systems while meeting more stringent user requirements.

　　References

　　Kessler, Matthew. Synchronous Inverse SEPIC Topology Provides High Efficiency for Noninverting Buck/Boost Voltage Converters, Analog Dialogue, Vol. 44, No. 2, 2010

　　Slattery, Colm, Derrick Hartmann, and Li Ke. PLC Evaluation Board Simplifies Design of Industrial Process Control Systems, Analog Dialogue, Vol. 43, No. 2, 2009

　　Circuit Note CN0201. Complete 5 V, Single-Supply, 8-Channel Multiplexed Data Acquisition System with PGIA for Industrial Signal Levels

　　MT-048 Tutorial. Op Amp Noise Relationships; 1/f Noise, RMS Noise, and Equivalent Noise Bandwidth