Design Article

Simplifying design of industrial process-control systems with PLC evaluation boards (Part 2 of 2)

Colm Slattery, Derrick Hartmann, and Li Ke, Analog Devices Incorporated

9/3/2009 1:48 PM EDT

Click here for part 1

Evaluation System Software and Evaluation Tools: The evaluation system is very versatile. Communication with the PC is achieved using LabView. The firmware for the microcontroller (ADuC7027) is written in C, which controls the low-level commands to and from the ADC and DAC channels.

Figure 9 shows the main screen interface. Pull-down menus on the left side allow the user to choose active ADC and DAC channels. Under each ADC and DAC menu there is a pull-down range menu, which is used to select the desired input and output ranges to be measured and controlled. The following input and output ranges are available: 4 mA to 20 mA, 0 mA to 20 mA, 0 mA to 24 mA, 0 V to 5 V, 0 V to 10 V, +/-5 V, and +/-10 V. Small signal input ranges can also be accommodated directly on the ADC by using its internal PGA.

Figure 9. Evaluation software main screen controller.
(Click on image to enlarge)

The ADC Configure screen, shown in Figure 10, is used to set the ADC channel, update rate, and PGA gain; to enable or disable excitation currents; and for other general-purpose ADC settings. Each ADC channel is calibrated by connecting the corresponding DAC output channel to the ADC input terminal and adjusting each range. When using this method of calibration, therefore, the offset and gain errors of the AD5422 dictate the offset and gain of each channel. If these provide insufficient accuracy, ultrahigh-precision current and voltage sources can be used for calibration if desired.

Figure 10. ADC Configure screen.
(Click on image to enlarge)
After selecting the ADC's input channel, input range, and update rate, we can now use the ADC Stats screen, shown in Figure 11, to display some measured data. On this screen, the user chooses the number of data points to record; the software generates a histogram of the selected channel, calculates the peak-to-peak and rms noise, and displays the results. In the measurement shown here, the input is connected through the AD8220 to the AD7793: gain = 1, update rate = 16.7 Hz, number of samples = 512, input range = +/-10 V, input voltage = 2.5 V. The peak-to-peak resolution is 18.2 bits.

Figure 11. ADC Stats screen.
(Click on image to enlarge)
In Figure 12, the input is connected directly to the AD7793, bypassing the AD8220. The on-chip 2.5-V reference is connected directly to the AIN+ and AIN" channels of the AD7793, providing a 0-V differential signal to the ADC. The peak-to-peak resolution is 20.0 bits. If the ADC conditions remain the same but the 2.5-V input is connected through the AD8220, the peak-to-peak resolution degrades to 18.9 bits for two reasons: at low gains, the AD8220 contributes some noise to the system; and the scaling resistors that provide the input attenuation result in some range loss to the ADC. The PLC evaluation system allows the user to change the scaling resistors to optimize the ADC's full-scale range, thereby improving the peak-to-peak resolution.

Figure 12. AD7793 performance.
(Click on image to enlarge)
Power Supply Input Protection: The PLC evaluation system uses best practices for electromagnetic compatibility (EMC). A regulated dc supply (18 V to 36 V) is connected to the board through a 2- or 3-wire interface. This supply must be protected against faults and electromagnetic interference (EMI). The following precautions, shown in Figure 13, were taken in the board design to ensure that the PLC evaluation system will survive any interference that may be generated on the power ports.

Figure 13. Power supply input protection.
(Click on image to enlarge)

A piezoresistor, R1, is connected to ground adjacent to the power input ports. During normal operation, the resistance of R1 is very high (megohms), so the leakage current is very low (microamperes). When an electric current surge (caused by lightning, for example) is induced on the port, the piezo-resistor breaks down, and tiny voltage changes produce rapid current changes. Within tens of nanoseconds, the resistance of the piezo resistor drops dramatically. This low-resistance path allows the unwanted energy surge to return to the input, thus protecting the IC circuitry. Three optional piezoresistors (R2, R3, and R4) are also connected in the input path to provide protection in cases when the PLC board is powered using the 3-wire configuration. The piezoresistors typically cost well under one US dollar.
A positive temperature coefficient resistor, PTC1, is connected in series with the power input trace. The PTC1 resistance appears very low during normal operation, with no impact to the rest of the circuit. When the current exceeds the nominal, PTC1's temperature and resistance rapidly increase. This high-resistance mode limits the current and protects the input circuit. The resistance returns to its normal value when the current flow decreases to the nominal limit.
Y capacitors C2, C3, and C4 suppress the common-mode conductive EMI when the PLC board operates with a floating ground. These safety capacitors require low resistance and high voltage endurance. Designers must use Y capacitors that have UL or CAS certification and comply with the regulatory standard for insulation strength.
Inductors L1 and L2 filter out the common-mode conducted interference coming in from the power ports. Diode D1 protects the system from reverse voltages. A general-purpose silicon or Schottky diode specifying a low forward voltage at the working current can be used.

Analog Input Protection: The PLC board can accommodate both voltage and current inputs. Figure 14 shows the input structure. Load resistor R5 is switched in for current mode. Resistors R6 and R7 attenuate the input. Resistor R8 sets the gain of the AD8220.

Figure 14. Analog input protection.
(Click on image to enlarge)
These analog input ports can be subjected to electric surge or electrostatic discharge on the external terminal connections. Transient voltage suppressors (TVSs) provide highly effective protection against such discharges. When a high-energy transient appears on the analog input, the TVS goes from high impedance to low impedance within a few nanoseconds. It can absorb thousands of watts of surge power and clamp the analog input to a preset voltage, thus protecting precision components from being damaged by the surge. Its advantages include fast response time, high transient power absorption, low leakage current, low breakdown voltage error, and small package size.
Instrumentation amplifiers are often used to process the analog input signal. These precision, low-noise components are sensitive to interference, so the current flowing into the analog input should be limited to less than a few milliamperes. External Schottky diodes generally protect the instrumentation amplifier. Even when internal ESD protection diodes are provided, the use of external diodes allows smaller limiting resistors and lower noise and offset errors. Dual series Schottky barrier diodes D4-A and D4-B divert the overcurrent to the power supply or ground.

When connecting external sensors, such as thermocouples (TCs) or resistance temperature devices (RTDs), directly to the ADC, similar protection is needed, as shown in Figure 15.

Figure 15. Analog input protection.

Two quad TVS networks, D5-C and D5-D, are put in after the J2 input pins to suppress transients coming from the port.
C7, C8, C9, R9, and R10 form the RF attenuation filter ahead of the ADC. The filter has three functions: to remove as much RF energy from the input lines as possible, to preserve the ac signal balance between each line and ground, and to maintain a high enough input impedance over the measurement bandwidth to avoid loading the signal source. The "3-dB differential-mode and common-mode bandwidth of this filter are 7.9 kHz and 1.6 MHz, respectively. The RTD input channel to AIN2+ and AIN2" is protected in the same manner.

Analog Output Protection: The PLC evaluation system can be software-configured to output analog voltages or currents in various ranges. The output is provided by the AD5422 precision, low-cost, fully integrated, 16-bit digital-to-analog converter, which offers a programmable current source and programmable voltage output. The AD5422 voltage and current outputs may be directly connected to the external loads, so they are susceptible to voltage surges and EFT pulses.

The output structure is shown in Figure 16.

Figure 16. Analog output protection.
(Click on image to enlarge)

A TVS (D11) is used to filter and suppress any transients coming from port J5.
A nonconductive ceramic ferrite bead (L3) is connected in series with the output path to add isolation and decoupling from high-frequency transient noises. At low frequencies (<100 kHz), ferrites are inductive; thus, they are useful in low-pass LC filters. Above 100 kHz, ferrites become resistive, an important characteristic in high-frequency filter designs. The ferrite bead provides three functions: localizing the noise in the system, preventing external high frequency noise from reaching the AD5422, and keeping internally generated noise from propagating to the rest of the system. When ferrites saturate, they becomes nonlinear and lose their filtering properties. Thus, the dc saturation current of the ferrites must not go over their limit, especially when producing high currents.
Dual series Schottky barrier diodes D9-A and D9-B divert any overcurrent to the positive or the negative power supply. C22 provides the voltage output buffer and the phase compensation when the AD5422 drives capacitive loads up to 1 uF.
The protection circuitry on the current output channel is quite similar to that on the voltage output channel except that a 10-ohm resistor (R17) replaces the ferrite bead. The current output from the AD5422 is boosted by the external discrete NPN transistor Q1. The addition of the external boost transistor will reduce the power dissipated in the AD5422 by reducing the current flowing in the on-chip output transistor. The breakdown voltage BVCEO of Q1 should be greater than 60 V. The external boost capability is useful in applications where the AD5422 is used at the extremes of the supply voltage, load current, and temperature range. The boost transistor can also be used to reduce the amount of temperature-induced drift, thus minimizing the drift of the on-chip voltage reference and improving the device's drift and linearity.
A 15-kohm, precision, low-drift current-setting resistor (R15) is connected to RSET to improve stability of the current output over temperature.
The PLC demo system can be configured to provide a voltage output higher than 15 V when the AD5422 is powered by an external voltage. A TVS is used to protect the power input port. Diodes D6 and D7 provide protection from reverse biasing. All the supplies are decoupled by 10-uF solid tantalum electrolytic and 0.1-uF ceramic capacitors.

IEC Tests and Results: The results in Table 3 show the deviations of the DAC output that occurred during the testing. The output recovered to the original values after the tests were completed. This is generally referred to as Class B. Class A means that the deviation was within the allowed system accuracy during the test. Typical industrial control system accuracies are approximately 0.05%.

Table 3. IEC test results.
(Click on image to enlarge)

Figure 17. DAC channel dc voltage output. Radiated immunity 80 MHz to 1 GHz at 10 V/mH.

Figure 18. DAC channel 1 dc voltage output. Radiated immunity 1.4 GHz to 2 GHz at 3 V/mH.
Typical System Configuration: Figure 19 shows a photo of the evaluation system and how a typical system might be configured. The input channels can readily accept both loop-powered and nonloop-powered sensor inputs, as well as the standard industrial current and voltage inputs. The complete design uses Analog Devices converters, isolation technology, processors, and power-management products, allowing customers to easily evaluate the whole signal chain.

Figure 19. Industrial control evaluation system.
(Click on image to enlarge)
References

Authors
Colm Slattery is an applications engineer for Analog Devices Precision Converters group in Limerick, Ireland. He graduated from the University of Limerick with a bachelor's degree in engineering. He joined Analog Devices Digital-to-Analog Converter group as a test engineer and has spent three years working for Analog Devices in China. He can be reached via email at colm.slattery@analog.com.

Derrick Hartmann is an applications engineer in the Digital-to-Analog Converter group at Analog Devices in Limerick, Ireland. Derrick joined ADI in 2008 after graduating with a bachelor's degree in engineering from the University of Limerick. He can be reached via email at derrick.hartmann@analog.com.

Li Ke joined Analog Devices in 2007 and is an applications engineer with the Precision Converters product line, located in Shanghai, China. Previously, he spent four years as an R&D engineer with the Chemical Analysis group at Agilent Technologies. Li received a master's degree in biomedical engineering in 2003 and a bachelor's degree in electric engineering in 1999, both from Xi'an Jiaotong University. He has been a professional member of the Chinese Institute of Electronics since 2005. He can be reached at li.ke@analog.com.

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