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Complete Sensor-to-Bits Solution Simplifies Industrial Data-Acquisition System Design
PLC Application Example
In industrial applications, analog input modules acquire and monitor signals from remote sensors located in harsh environments characterized by extreme temperature and humidity, vibration, and explosive chemicals. Typical signals include single-ended or differential voltages with 5 V, 10 V, ±5 V, and ±10 V full-scale ranges, or current loops with 0 mA to 20 mA, 4 mA to 20 mA, and ±20 mA ranges. When long cables with substantial electromagnetic interference (EMI) are encountered, current loops are often used due to their inherently high noise immunity.
Analog output modules typically control actuators, such as relays, solenoids, and valves, to complete the automated-control system. They typically provide output voltages with 5 V, 10 V, ±5 V, and ±10 V full-scale ranges and 4 mA to 20 mA current-loop outputs.
Typical analog I/O modules include 2, 4, 8, or 16 channels. To meet stringent industry standards, these modules require protection against overvoltage, overcurrent, and EMI surges. Most PLCs include digital isolation between the ADC and the CPU and between the CPU and the DAC. High-end PLCs may also incorporate channel-to-channel isolation, as specified by the International Electrotechnical Commission (IEC) standards. Many I/O modules include per-channel software programmable single-ended or differential input ranges, bandwidth, and throughput rate.
In modern PLCs, the CPU performs numerous control tasks in an automated manner, employing real-time access to information to make intelligent decisions. The CPU may embody advanced software and algorithms, and web connectivity for diagnostic error checking and fault detection. Commonly used communication interfaces include RS-232, RS-485, industrial Ethernet, SPI, and UART.
Figure 1. Typical PLC signal chain.
Discrete Implementation of Data-Acquisition System
The AD7982 specifies a 290-ns transient response from a full-scale step. Thus, to guarantee the specified performance while converting at 1 MSPS, the PGIA and funnel amp must settle in less than 710 ns. The AD8251 specifies
Figure 2. Analog input signal chain using discrete components.
Integrated Solution Simplifies Data-Acquisition System Design
Figure 3. Functional block diagram of ADAS3022.
This complete sensor-to-bits solution utilizes only one-third of the board space of discrete implementations, helping engineers to simplify their designs while reducing the size, time to market, and cost of advanced industrial data-acquisition systems. Eliminating the necessity to buffer, level shift, amplify, attenuate, or otherwise condition the input signal, and the concerns regarding common-mode rejection, noise, and settling time, it alleviates many of the challenges associated with designing a precision 16-bit, 1-MSPS data-acquisition system. It delivers the best-in-class 16-bit accuracy (±0.6-LSB typical INL), low offset voltage, low drift overtemperature, and optimized noise performance at 1 MSPS (91-dB typical SNR), as shown in Figure 4. The device is specified over the –40°C to +85°C industrial temperature range.
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 a wide dynamic range, allowing it to accommodate 4-mA to 20-mA current loops, accurately measure small sensor signals, and reject interference from ac power lines, electric motors, and other sources (90-dB minimum CMR).
An auxiliary differential input channel can accommodate ±4.096 V input signals. It bypasses the multiplexer and PGIA stages, allowing direct interface to the 16-bit SAR ADC. An on-chip temperature sensor can monitor the local temperature.
This high level of integration saves board space and lowers the overall parts’ 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 operates with ±15-V and +5-V analog and digital supplies, and a 1.8-V to 5-V logic I/O supply. The ADP1613 high-efficiency, low-ripple dc-to-dc boost converter allows the DAS to operate with a single 5-V supply. Configured as a single-ended, primary inductance Ćuk (SEPIC) topology using the ADIsimPower™ design tool, the ADP1613 furnishes the ±15-V bipolar supplies required for the multiplexer and PGIA without compromising performance.
The noise performance of the ADAS3022 and the discrete signal chain are compared in Table 1, which uses the input signal amplitude, gain, equivalent noise bandwidth (ENBW), and input-referred (RTI) noise of each component to calculate the total noise of the complete signal chain.
Table 1. Noise Performance for the ADAS3022 and the Discrete Signal Chain
The single-pole low-pass filter (LPF) between the AD8475 and AD7982 (Figure 2) attenuates the kick coming from the switched-capacitor input of the AD7982 and limits the amount of high-frequency noise. The –3-dB bandwidth (f–3dB) of the LPF is
ENBW = π/2 × f–3dB = 9.6 MHz.
Note that this calculation ignores the noise from the voltage reference and LPF as it does not significantly affect the total noise, which is dominated by the PGIA.
Consider an example using the ±5-V input range. In this case, the AD8251 is set for a gain of 2. The funnel amplifier is set to a fixed gain of 0.4 for all four input ranges, so a 0.5-V to 4.5-V differential signal (4 V p-p) will be applied to the AD7982. The RTI noise of the ADG1208 is derived from the Johnson/Nyquist noise equation en2 = 4KBTRON,
where KB = 1.38 × 10 23 J/K,
The RTI noise of the AD8251 is derived from its 27-nV/√Hz noise density as specified in the data sheet for a gain of 2. Similarly, the RTI noise of the AD8475 is derived from its 10-nV/√Hz noise density using a gain of 0.8
Although the theoretical noise estimate (SNR) and the overall performance of the discrete signal chain is comparable to that of the ADAS3022, especially at lower gains (G = 1 and G = 2) and lower throughput rates (much less than 1 MSPS), it’s not an ideal solution. The ADAS3022 can reduce cost by about 50% and board space by about 67%, as compared to the discrete solution, and it can also accept three additional input ranges (±0.64 V, ±20.48 V, and ±24.576 V) that the discrete solution cannot offer.
Slattery, Colm, Derrick Hartmann, and Li Ke. PLC Evaluation Board Simplifies Design of Industrial Process Control Systems, Analog Dialogue, Vol. 43, No. 2, 2009.
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