With the development trend of intelligent power grid management, the design of power line monitoring and protection products is facing more and more challenges. Designers of multi-channel current and voltage monitoring systems And a series of complex design challenges such as the high cost caused by the use of expensive discrete components. As a key solution provider for power secondary equipment manufacturers, ADI has a deep understanding of the technical needs of global power equipment companies. Based on the successful application experience of AD7656, ADI successfully launched the 16-bit 8-channel synchronous sampling AD7606 series to help customers improve To deal with the technical challenges faced by the development of secondary equipment in the era of smart grid.

Figure 1: ADI’s high-performance ADC product development roadmap.

AD7606 Simplifies Power Line Monitoring System Design

AD7606 series devices use a single 5V power supply, and support true ±10V and ±5V bipolar signal input, the sampling rate of each channel can reach 200ksps. The integration of multiple channels in a single chip supports the measurement of three-phase current, voltage and neutral in substation automation equipment. Simultaneous sampling allows the preservation of phase information while simultaneously sampling bipolar voltages and currents over a wide dynamic range.

All eight channels of the AD7606 family can achieve sampling rates up to 200 kSPS. It has built-in low noise, high impedance input and signal conditioning amplifiers that can handle input frequencies up to 22 kHz. The AD7606’s signal-to-noise ratio (SNR) is as high as 90dB, and the use of an on-chip digital filter can further improve SNR performance, reduce bit errors, spread spectrum, and improve anti-aliasing rejection. The conversion process and data acquisition are controlled using the CONVST signal and the internal oscillator. With two CONVST pins, all eight analog inputs or two sets of analog input channels (four analog inputs form a group) can be sampled simultaneously to account for phase differences between transformers.

Figure 2: AD7606 further simplifies circuit design (the upper and lower figures are circuit diagrams using AD7656 and AD7606 respectively)

The signal conditioning circuit inside the AD7606 already includes a signal conditioning circuit with low noise and high input impedance, and its equivalent input impedance is completely independent of the sampling rate and fixed at 1MΩ. At the same time, the input end integrates a filter with 40 dB anti-aliasing suppression characteristics, which simplifies the front-end design and eliminates the need for external drive and filter circuits. Therefore, the signal output by the secondary transformer can be directly connected to the AD7606 without being buffered by an operational amplifier. AD7606 integrates a 2.5V bandgap voltage reference and reference buffer circuit. In the design and application, the built-in reference or external reference can be selected according to the system requirements. In the design of multi-chip ADCs, if high absolute accuracy is required, high initial accuracy and External reference with low temperature coefficient to eliminate errors caused by differences between internal references in different devices. The ADR421B with an initial accuracy of 0.04% and a temperature coefficient of 3ppm/°C is recommended. If the value matching between multiple ADC channels is required, the first AD7606 can be set to work in the built-in reference mode, and the other AD7606s are in the external reference mode, and then the built-in reference output of the first AD7606 can be supplied to the remaining AD7606s. Therefore, the data matching between multiple AD7606 channels can be guaranteed without adding an external reference.

Figure 3: The outstanding performance benefits of the AD7606 make your system design easier

The AD7606’s low 100mW operating power consumption and only 25mW standby power consumption maintains ADI’s advantages in ADC low-power technology, especially when there are several multi-channel ADCs on one board (some systems require a circuit There are as many as hundreds of ADC channels on the board), power consumption is an important consideration, and this low power consumption feature is one of the key factors to simplify system thermal design and improve system reliability.

The AD7606 series includes three synchronous sampling ADC devices, 8-channel, 6-channel and 4-channel. For applications requiring multi-channel systems, designers can use flexible design combinations of 8+4 and 8+6 channels respectively. The high performance of the series products ensures the low cost of the solution (the four-channel and six-channel AD7606-4 and AD7606-6 cost 19% and 34% lower than the dual 8-channel AD7606, respectively). Moreover, these devices are pin-compatible, and the same circuit design can be applied to a variety of system configurations with different channel counts.

Circuit Design Recommendations

Most power line monitoring systems employ multiple AD7606 devices for simultaneous sampling of multiple channels. To ensure good performance matching between devices, these devices must have a symmetrical layout. The AVCC voltage plane is routed along the right side of both devices, and the VDRIVE power supply traces are routed along the left side of both AD7606 devices. The reference voltage chip ADR421 is located between the two AD7606 devices, and the reference voltage trace is routed up to pin 42 of U2 and down to pin 42 of U1. Use a solid ground plane. These symmetrical layout principles apply to systems with more than two AD7606 devices. The AD7606 device can be placed in a north-south direction with the reference voltage in the middle of the device and the reference voltage traces in a north-south direction.

Good decoupling is very important to the overall performance of the power line monitoring system. Proper decoupling configuration can reduce the power supply impedance of the AD7606 and its power supply peak amplitude. The decoupling capacitors for the REFIN/REFOUT pins and the REFCAPA and REFCAPB pins are performance critical and should be placed as close as possible to the corresponding AD7606 pins. When possible, these capacitors should be placed on the same side of the board as the AD7606 device. The decoupling design of the AD7606 is very simple, requiring only 9 low-value ceramic capacitors, including 2 10uF, 2 1uF, and 5 0.1uF. In high-channel systems, good channel-to-channel and device-to-device performance matching can greatly simplify the calibration process. The symmetrical layout of the AD7606 device, analog input channels, and decoupling capacitors facilitates performance matching between multiple devices.

Figure 4: Recommended circuit layout in a multi-channel application

(For more detailed design recommendations, see “Layout Considerations for a Scalable Multi-Channel Simultaneous Sampling Data Acquisition System Based on the 16-Bit 8-Channel DAS AD7606”)

Complete solution of AD7606+BF518

In order to adapt to the rapid development of the global power industry and the complexity of the power system and the improvement of protection requirements, the current relay protection products have powerful data processing capabilities and communication capabilities in addition to the basic testing functions of traditional relay protection equipment. , and have high requirements for reliability, selectivity, sensitivity and rapidity. Moreover, with the gradual implementation of the IEC61580 specification, higher requirements will be placed on the processing capability of the processor, including higher communication capabilities, storage capabilities, and intelligent features. ADI and the third party Beijing Yiqi jointly developed a complete power line monitoring solution based on AD7606 and Blackfin processor BF518.

Figure 5: Demo of 16-channel power line monitoring system implemented by BF518 and dual AD7606

Blackfin processor is ADI’s core flagship product. The powerful processing capability and converged architecture make it unnecessary to add a separate communication processor in the design of relay protection products, which can help customers realize the solution of the bus without the chip, and greatly improve the system. Anti-interference ability, easily meet the international IEC-6100-4 standard requirements. BF518 is a converged processor that has been widely used in new power equipment products around the world. The processor has built-in Ethernet 10/100 MAC with media independent interface and simplified media independent interface. It is the first time in the Blackfin processor. Introduced in the built-in PTP_TSYNC engine, to achieve the hardware-supported IEEE1588 (Precision Clock Synchronization Protocol for Network Measurement and Control System) clock synchronization, fully compatible with the IEEE1588 version2 standard. The networking and intelligence of the power industry has given birth to the networking of relay protection devices, and the introduction of the IEC61850 protocol, an international standard for substation communication networks and systems, reflects this development trend. The BF518 processor supporting the IEEE1588 protocol will certainly help power equipment companies. Take a more active position in this development trend.

The scheme showed excellent performance in the test. Because AD7606 provides functional characteristics such as oversampling, anti-aliasing filter and digital filtering, the sampling accuracy of this scheme can reach up to 19 bits in practical applications. In the application of high voltage protection, AD7606 has more obvious performance advantages, and has a higher single-channel cost performance.

Figure 6: Functional block diagram of a complete solution based on the Blackfin processor BF518 and AD7606