For low-cost, high-channel-count applications requiring wide dynamic range, the AD7607 , an 8-channel integrated data acquisition system (DAS) with an on-chip 14-bit SAR ADC, can be used to effectively achieve dynamic range in excess of 80 dB.

Typical applications of DAS are power line measurement and protection equipment, where a large number of current and voltage channels of a multiphase transmission and distribution network must be sampled simultaneously.

Many low voltage power line measurement and protection systems do not require the full 16-bit ADC resolution (such as that provided by the AD7606 DAS), but still require more than 80 dB of dynamic range in order to capture undervoltage/undercurrent and overvoltage/overcurrent condition. Synchronous sampling capabilities are also required to maintain phase information between the current and voltage channels of multiphase power lines.

The AD7607 is an 8-channel DAS integrating a 14-bit, bipolar input, simultaneous sampling SAR ADC with an 84 dB signal-to-noise ratio (SNR) to meet the requirements of this type of low-voltage protection and measurement system. The circuit shown in Figure 1 also uses an external precision, low-temperature drift, low-noise voltage reference, the ADR421 , to support high-channel-count applications that require absolute accuracy performance.

The AD7607 is an integrated data acquisition system that integrates an input amplifier, overvoltage protection circuit, analog anti-aliasing filter, 14-bit SAR ADC and digital filter on-chip. This circuit consists of AD7607 and 2.5 V reference voltage source ADR421. To achieve good channel-to-channel matching, a symmetrical layout around the analog input channels and device decoupling is important.

The following outlines the recommended layout for the AD7607 and ADR421 to achieve excellent channel-to-channel matching and 84 dB SNR performance.

AD7607 Evaluation Board Layout

To ensure good channel-to-channel matching, it is important to have a symmetrical layout of the analog input channels. In a system containing multiple AD7607 devices, the devices must be laid out symmetrically to ensure good performance matching between the devices.

Figure 2 shows the optimal board layout for the AD7607 and ADR421. The AVCC voltage plane is routed along the right side of the AD7607, and the VDRIVE power trace is routed along the left side of the AD7607. The reference voltage source ADR421 is located south of the AD7607. A solid ground plane is used here.

These symmetrical layout principles also apply to systems containing multiple AD7607 devices. The AD7607 devices should be placed in a north-south direction with the reference voltage in the middle of the devices and the reference voltage traces routed in a north-south direction, similar to Figure 2. For system details using multiple AD7606 (16-bit 8-channel DAS), see Circuit Note CN-0148 .

Good decoupling is also important in order to lower the supply impedance of the AD7607 and reduce the amplitude of the supply spikes. Decoupling capacitors should be placed as close as possible to the DUT power pins and their corresponding ground pins. The decoupling capacitors of the REFIN/REFOUT pin and the REFCAPA and REFCAPB pins are also important decoupling capacitors related to performance and should be as close as possible to the corresponding AD7607 pins. If possible, these capacitors should be placed on the same side of the board as the AD7607 device. Figure 3 shows the recommended decoupling layout for the top layer of the AD7607 circuit board.

The four ceramic capacitors on the top layer of the board are decoupling capacitors for the REFIN/REFOUT pin, REFCAPA pin, and REFCAPB pin. These capacitors are placed in a north-south direction so as to be as close as possible to the corresponding pins. Figure 4 shows the low-level decoupling layout used for decoupling the four AV _CC pins and the V _DRIVE pin. Multiple vias are used here to connect the pins to their corresponding decoupling capacitors. The symmetrical layout of decoupling capacitors around the AD7607 device facilitates performance matching between devices. Multiple vias are used to connect capacitor pads and pin pads to ground and to voltage planes and reference voltage traces.

Matching between channels

In high-channel-count systems, good channel-to-channel and device-to-device performance matching can greatly simplify the calibration procedure. The symmetrical layout of the AD7607 device, analog input channels, and decoupling capacitors helps match performance between multiple devices. Using a common system reference voltage can further enhance the matching performance of the system. Figure 5 shows the measured matching performance of eight channels on the board with all inputs tied to ground. The distribution histograms of up to three codes are shown here. The center of each channel histogram is code 1.

AC performance

In this circuit, the AD7607 is configured to operate in external reference mode. The ADR421 provides the 2.5 V reference voltage to the REFIN/REFOUT pin of the AD7607. Apply a 1 kHz signal to channel 1 of the AD7607. The input range of the AD7607 is configured to ±5 V. The AD7607 achieves an SNR of 84.12 dB when sampling at 200 kSPS on all eight channels. This performance is equivalent to approximately 13.7 effective number of bits (ENOB), where ENOB = (SNR − 1.76 dB)/6.02.

To further improve SNR performance and the ENOB of the system, the AD7607 can be configured to operate in 8x oversampling mode. In this mode, the SNR is increased to 85.25 dB, thus the effective number of bits is increased to 13.9 bits. When the AD7607 is used in 8x oversampling mode, the throughput rate drops to 25 kSPS per channel.

The above recommended layout can ensure that one AD7607 achieves good channel-to-channel matching performance, and multiple AD7607s on the same PCB board also have good device-to-device matching performance. The AD7607 and ADR421 are capable of achieving an SNR of 84 dB, meeting the dynamic range requirements of over 80 dB for low-voltage protection and measurement applications in substation automation equipment.