CN0175

The AD7607 is an integrated data acquisition system with input amplifiers, overvoltage protection, analog antialiasing filter, 14-bit SAR ADC, and a digital filter all included on chip. This circuit consists of the AD7607 in conjunction with the ADR421 2.5 V reference. Symmetrical layout around the analog input channels and device decoupling is critical for good channel-to-channel matching.

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

AD7607 Evaluation Board Layout

To ensure good channel-to-channel matching, a symmetrical layout of the analog input channels is important. In a system that contains multiple AD7607 devices, to ensure good device-to-device performance matching, a symmetrical layout between the AD7607 devices is important.

Figure 2 shows an optimum board layout of the AD7607 and the ADR421. The AVCC supply plane runs to the right of the AD7607. The VDRIVE supply track runs to the left of the AD7607. The ADR421 reference is positioned to the south of the AD7607. A solid ground plane layer is used.

These symmetrical layout principles should also be applied to a system that contains multiple AD7607 devices. The AD7607 devices should be placed in a north-south direction with the reference voltage located midway between the AD7607 devices, and running in the north-south direction similar to Figure 2. Details of a system using multiple AD7606 (16-bit 8-channel DAS) devices can be found in Circuit Note CN-0148.

Good decoupling is also important to lower the supply impedance presented to the AD7607 and to reduce the magnitude of the supply spikes. The decoupling capacitors should be placed as close as possible to the DUT power pins and their corresponding ground pins. The decoupling capacitors for the REFIN/REFOUT pin and the REFCAPA and REFCAPB pins are also critical decoupling capacitors and should be placed as close as possible to their respective AD7607 pins and, where possible, they should be placed on the same side of the board as the AD7607 device. Figure 3 shows the recommended decoupling on the top layer of the AD7607 board.

The four ceramic capacitors on the top layer of the board are the decoupling caps for REFIN/REFOUT pin and the REFCAPA and REFCAPB pins. These capacitors are placed in a north-south direction to get as close to their respective pins as possible. Figure 4 shows the bottom layer decoupling which is used for the four AV_CC pins and the V_DRIVE pin. Multiple vias are used to connect the pins to their respective decoupling capacitors. Symmetrical layout of the decoupling capacitors around the AD7607 devices helps with part-to-part performance matching. Multiple vias are used to connect capacitor pads and pin pads to ground and supply planes and the reference track.

Channel-to-Channel Matching

In a high channel count system, good channel-to-channel and part-to-part performance matching greatly simplifies calibration routines. Symmetrical layout of the AD7607 devices, the analog input channels, and the decoupling capacitors aids performance matching between multiple devices. The use of a common system reference further enhances matching performance in the system. Figure 5 shows the matching measured for the eight channels on the board when all inputs are grounded. There is a maximum histogram spread of three codes, with each channel histogram centered on Code 1. 

AC Performance

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

To further increase the SNR performance and increase the ENOB of the system, the AD7607 is configured to operate in an 8× oversampling mode. In this mode, the SNR increases to 85.25 dB, therefore increasing the effective number of bits to 13.9 bits. When using the AD7607 in an 8× oversampling mode, the throughput rate on each channel is reduced to 25 kSPS.

The layout suggested above ensures good channel-to-channel matching on a single AD7607 and also good part-to-part matching between multiple AD7607s on the same PCB board. The AD7607 and ADR421 achieve 84 dB SNR, satisfying the need for greater than 80 dB dynamic range for low voltage protection and measurement applications in substation automation equipment.