Improve system-level performance and reliability of power line monitoring applications

“For many applications, monitoring power lines means using current transformers and resistor divider networks to sense three-phase and zero-sequence current and voltage, as shown in Figure 1. AD7606B has a high input impedance, can be directly connected to the sensor, and it provides all the required built-in input modules, thereby simplifying the design of the data acquisition system.

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Author: Lluis Beltran Gil | ADI Application Engineer

background knowledge

For many applications, monitoring power lines means using current transformers and resistor divider networks to sense three-phase and zero-sequence current and voltage, as shown in Figure 1. AD7606B has a high input impedance, can be directly connected to the sensor, and it provides all the required built-in input modules, thereby simplifying the design of the data acquisition system.

Figure 1. AD7606B in a typical power line monitoring application

AD7606B integrates 8 independent signal chains on-chip, even if it is powered by a single 5 V power supply (not counting the digital interface voltage Vdrive), it can still accept true bipolar analog input signals of ±10 V or ±5 V. Therefore, there is no need to use an external driving operational amplifier and an external bipolar power supply.

Each channel consists of a 21V analog input clamp protection circuit, a resistance programmable gain amplifier with 5MΩ input impedance, a first-order anti-aliasing filter and a 16-bit SARADC. In addition, it can also include a digital averaging filter with an oversampling rate of up to 256, and a low-temperature drift 2.5 V reference voltage source to help build a complete power line data acquisition system.

In addition to providing a complete analog signal chain, AD7606B also provides many calibration and diagnostic functions to improve system-level performance and reliability.

Direct sensor interface

Unlike the AD7606, the input impedance of the AD7606B has been increased to 5 MΩ, allowing it to directly interface with a variety of sensors, thereby obtaining two benefits:

・Reduce the gain error caused by external series resistance (for example, filter or resistor divider network).

・ When the sensor is disconnected, the amount of offset seen will be reduced, and the sensor disconnection detection function can be easily realized.

Gain error caused by external resistance

During factory refurbishment, the R of PGA will be strictly controlled_FBAnd R_IN N (usually 5MΩ), to ensure that the gain of AD7606B is set accurately. However, as shown in Figure 1, if an external resistor is placed on the front end, the actual gain and the ideal trim R_FB/R_INThere will be deviations between the values.

R_FILTERThe higher the gain, the greater the gain error, which needs to be compensated from the controller side. However, R_INHigher, same R_FILTER The smaller the impact. Unlike AD7606 which has 1 MΩ input impedance, AD7606B has 5 MΩ impedance, which means that without any calibration, the same series resistance (R_FILTER) Gain error will be reduced to about 1/5, as shown in Figure 2.

Figure 2. Gain error caused by series resistance

However, by using AD7606B in software mode, the system gain error can be automatically compensated on-chip based on each channel, so there is no need to implement any gain calibration calculations on the controller side.

Sensor disconnection detection

Traditionally, the pull-down resistor (R_PD) Is connected in parallel with the sensor (the current transformer shown in Figure 1), and the user can detect when the sensor is disconnected by monitoring whether the ADC output code of multiple samples (N) is repeated less than 20 LSBs.

It is recommended to use R much larger than the source impedance of the sensor_PD, To minimize the error that the parallel resistance may produce. However, R_PDThe larger the value, the larger the ADC output code generated when the sensor is disconnected, which is not the result we expect. As the R of AD7606B_INBigger than AD7606, for a given R_PDIf the sensor is disconnected, the ADC output code will be reduced (as shown in Figure 3), thereby reducing the risk of false alarms.

Figure 3. Offset error when the sensor is disconnected from the analog input of the ADC

When entering the software mode of AD7606B, the open circuit detection function can be used, so that back-end software is not required to detect the sensor disconnection. After programming the number of samples N (in the example of Figure 4, N = 3), if the analog input maintains a small DC value reported by several samples, the algorithm will run automatically, and it will be judged as an open circuit when the analog input signal is disconnected When, set a flag bit.

Figure 4. Sensor disconnection detection

System level performance system offset calibration

When using a pair of external resistors, as shown in Figure 1, any mismatch between them will cause an offset. When the sensor is shorted to ground, this offset can be measured as the ADC output code. Then, the corresponding channel offset register can be programmed to add or subtract the offset of C128 LSBs to +127 LSBs from the conversion result to compensate for the system offset.

System phase calibration

The CONVST pin is used to manage the start of analog-to-digital conversion so that the process can be triggered on all channels at the same time. However, for applications where the current is measured by a current transformer (CT) and the voltage is measured by scaling down through a voltage divider resistor, there is a phase mismatch between the current and voltage channels. In order to compensate for this mismatch, AD7606B can delay the sampling moment on any channel (the channel with a relatively large delay) in order to adjust the output signal to the same phase, as shown in Figure 5.

Figure 5. Phase adjustment

System reliability

In order to improve the reliability of the system, several diagnostic functions have been added to the chip, including:

・ Overvoltage/undervoltage comparators on each channel.

・ An interface check that outputs fixed data on each channel to verify the communication status.

・ If you try to write or read an invalid register, an SPI invalid read/write alarm will be issued.

・ After the conversion starts, if the BUSY line lasts longer than normal, a BUSY STUCK HIGH alarm will be issued.

・ If a full, partial, or power-on reset of the internal LDO regulator is detected, a reset detection alarm will be issued.

・ CRC check can be implemented for memory map, ROM and each interface communication to ensure correct initialization and/or operation.

Summarize

AD7606B brings a complete chip data acquisition system to the market. All built-in analog front-end modules can be realized. It provides a complete set of advanced diagnostic functions, as well as gain, offset and phase calibration. Therefore, the AD7606B reduces the cost of components and the complexity of system design, thereby simplifying the design of power line monitoring applications.

author

Lluis Beltran Gil

Lluis Beltran Gil graduated from the Polytechnic University of Valencia with a bachelor’s degree in Electronic engineering in 2009 and a bachelor’s degree in industrial engineering in 2012. After graduation, Lluis joined ADI in 2013 as an application engineer in the Limerick Precision Converter Division, supporting temperature sensor development. Currently, Lluis works in the SAR ADC application team of ADI’s Precision Converter Division, based in Valencia, Spain.