ALLPCB
Overview
As electrical grids evolve, power line monitoring and relay protection products have been updated and their design approaches have changed. Analog Devices (ADI) has produced multiple generations of ADCs used in these products. Early relay protection designs used analog switches with single-channel 16-bit ADCs such as the AD976 and AD574. Later designs used multi-channel devices like the 16-bit AD7656 and 14-bit AD7865 together with analog switches; AD7656 became a mainstream choice due to features such as bipolar +/-10 V inputs and synchronous multi-channel sampling. AD7656 remains in use in many monitoring and protection devices.
AD7606 Simplifies Power Line Monitoring System Design
The AD7606 family uses a single 5 V supply and supports true +/-10 V and +/-5 V bipolar inputs. Each channel can sample up to 200 kSPS. The multi-channel single-chip integration supports three-phase current, voltage, and neutral line measurements in substation automation equipment. Synchronous sampling preserves phase information and enables bipolar voltage and current sampling across a wide dynamic range.
All eight channels of the AD7606 family can achieve up to 200 kSPS. The device integrates low-noise, high-impedance inputs and signal-conditioning amplifiers and can handle input frequencies up to 22 kHz. AD7606 offers a signal-to-noise ratio (SNR) up to 90 dB. On-chip digital filters can further improve SNR, reduce measurement error, increase effective resolution, and improve anti-aliasing suppression. Conversions and data acquisition are controlled by CONVST signals and an internal oscillator. Two CONVST pins allow simultaneous sampling of all eight inputs or two groups of four inputs, accommodating phase differences between transformers.
AD7606 includes integrated low-noise, high-input-impedance signal conditioning. The equivalent input impedance is independent of sample rate and is fixed at 1 MΩ. The input stage also integrates filters with approximately 40 dB anti-aliasing suppression, simplifying the front-end design and removing the need for external driver and filter circuits. Outputs from instrument transformers can be connected directly to AD7606 without additional op amp buffering.
The device integrates a 2.5 V bandgap reference and buffer circuits. Designers may choose the internal reference or an external reference depending on system accuracy requirements. For multi-ADC systems that require high absolute accuracy, an external reference with low initial error and low temperature coefficient should be used to eliminate mismatch between internal references. An example recommendation is the ADR421B with initial accuracy 0.04% and a temperature coefficient of 3 ppm/°C. If channel matching across multiple AD7606 devices is the priority, one AD7606 may use its internal reference while the others use an external reference supplied from the first device, ensuring matched readings without an external reference for every device.
AD7606 operates with typical power consumption as low as 100 mW during conversion and 25 mW in standby, which is beneficial when multiple multi-channel ADCs are present on a board. Lower power reduces thermal design complexity and can improve overall system reliability.
The AD7606 family includes 8-channel, 6-channel, and 4-channel synchronous-sampling ADC variants. Designers can combine parts (for example, 8+4 or 8+6) to build flexible multi-channel systems. The 4-channel and 6-channel devices are lower cost alternatives to a dual 8-channel configuration. These devices are pin-compatible, allowing the same PCB design to support different channel-count configurations.
Circuit Design Recommendations
Multiple AD7606 devices are typically used to implement multi-channel synchronous sampling in power line monitoring systems. To ensure matched performance between devices, place parts symmetrically. Route the AVCC plane along the right side of devices and the VDRIVE supply along the left. Position the reference voltage chip (for example, ADR421) between two AD7606 devices, routing the reference trace upward to the appropriate pin on the upper device and downward to the corresponding pin on the lower device. Use a solid ground plane. The same symmetry principles apply for systems with more than two AD7606 devices; a north-south placement with the reference in the middle and reference routing along the north-south axis is recommended.
Proper decoupling is important for overall system performance. Appropriate decoupling reduces the AD7606 supply impedance and limits supply transients. Decoupling for REFIN/REFOUT and for REFCAPA/REFCAPB capacitors is critical and these capacitors should be placed as close as possible to the corresponding AD7606 pins. If possible, place these capacitors on the same PCB side as the devices. The recommended decoupling network uses nine low-value ceramic capacitors: two 10 uF, two 1 uF, and five 0.1 uF capacitors. In high-channel-count systems, good channel-to-channel and device-to-device matching can simplify calibration. Symmetrical placement of AD7606 devices, analog input channels, and decoupling capacitors supports better matching across multiple devices.
AD7606 + BF518 Reference Solution
Contemporary relay protection products require both traditional protection functions and substantial data processing and communication capability, along with high requirements for reliability, selectivity, sensitivity, and speed. With increasing adoption of standards such as IEC 61580, processors need greater communication, storage, and intelligent features. ADI and third-party company Yiqi (Beijing) developed a complete power line monitoring reference solution based on AD7606 devices and the Blackfin BF518 processor.
The BF518 Blackfin processor integrates strong processing capability and a convergent architecture that eliminates the need for separate communications processors in protection devices, which can improve system robustness to interference and help meet IEC 61000-4 standards. BF518 integrates a 10/100 MAC with a simplified media-independent interface and includes a hardware PTP_TSYNC engine to support IEEE 1588 timestamping and hardware-assisted clock synchronization, fully compatible with IEEE 1588 version 2. Support for IEEE 1588 helps equipment vendors address networked and intelligent substation requirements and standards such as IEC 61850.
Test results for the combined AD7606 and BF518 solution show strong performance. With AD7606's oversampling, anti-aliasing filtering, and digital filtering features, the solution can achieve effective sampling accuracy up to 19 bits in practice. In high-voltage protection applications, AD7606 provides clear performance advantages and competitive single-channel cost-effectiveness.
