Design of a low-cost, 8-channel, synchronous sampling data acquisition system

Circuit Function and Advantages

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

A typical application of DAS is 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 full 16-bit ADC resolution (such as that provided by the AD7606 DAS), but still require more than 80 dB of dynamic range to capture undervoltage/undercurrent and overvoltage/overcurrent conditions. Synchronous sampling capability is also required to maintain phase information between current and voltage channels of multiphase power lines.

The AD7607 is an 8-channel DAS with an integrated 14-bit, bipolar input, simultaneous sampling SAR ADC that has 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 reference ADR421 to support high channel count applications that require absolute accuracy performance.

Figure 1. Low-Cost, 8-Channel, Simultaneous Sampling Data Acquisition System (Simplified Electrical Schematic: Decoupling and All Connections Not Shown)

Circuit Description

The AD7607 is an integrated data acquisition system that integrates input amplifiers, overvoltage protection circuits, analog antialiasing filters, 14-bit SAR ADCs, and digital filters. This circuit consists of the AD7607 and the 2.5 V reference voltage source ADR421. Symmetrical layout around the analog input channels and device decoupling is very important to achieve good channel-to-channel matching.

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

AD7607* Evaluation Board Layout

Symmetrical layout of the analog input channels is very important to ensure good channel-to-channel matching. In a system with multiple AD7607 devices, they 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 supply trace is routed along the left side of the AD7607. The ADR421 voltage reference is located to the south of the AD7607. A solid ground plane is used here.

Figure 2. PCB layout showing the AD7607 DAS and ADR421 voltage reference

These symmetrical layout principles also apply to systems that contain multiple AD7607 devices. The AD7607 devices should be placed in a north-south direction with the reference voltage located in the middle of the devices and the reference voltage traces running in a north-south direction, similar to Figure 2. For more detailed information on systems using multiple AD7606s (16-bit, 8-channel DAS), see Circuit Note CN-0148.

Good decoupling is also important to reduce the power supply impedance of the AD7607 and reduce the amplitude of power 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 pins and the REFCAPA, REFCAPB pins are also important decoupling capacitors that are critical to performance and should be placed 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 on the top layer of the AD7607 board.

Figure 3. Top-level decoupling

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 bottom layer decoupling layout, which is used for decoupling the four AVCC pins and the VDRIVE pin. Multiple vias are used here to connect the pins to their corresponding decoupling capacitors. The symmetrical layout of the decoupling capacitors around the AD7607 device facilitates performance matching between the devices. Multiple vias are used to connect the capacitor pads and the pin pads to ground and to the voltage plane and reference voltage traces.

Figure 4. Bottom-layer decoupling

Channel-to-channel matching

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

Figure 5. Histogram of 8 channels with input grounded.

AC Performance

In this circuit, the AD7607 is configured to operate in external reference mode. The ADR421 provides a 2.5 V reference voltage to the REFIN/REFOUT pin of the AD7607. A 1 kHz signal is applied to Channel 1 of the AD7607. The input range of the AD7607 is configured for ±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 an effective number of bits (ENOB) of approximately 13.7 bits, where ENOB = (SNR - 1.76 dB)/6.02.

Figure 6. FFT shows SNR of 84.12 dB (input = 1 kHz, fs = 200 kSPS)

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

Figure 7. FFT shows SNR of 85.26 dB (8x oversampling, fs = 25 kSPS)

The above recommended layout can ensure good channel-to-channel matching performance for one AD7607, and good device-to-device matching performance between multiple AD7607s on the same PCB board. The AD7607 and ADR421 can achieve an SNR of 84 dB, meeting the dynamic range requirement of more than 80 dB for low voltage protection and measurement applications in substation automation equipment.

Common changes

The AD7607 is an 8-channel, 14-bit DAS. ADI also offers the AD7606-6 (6-channel, 16-bit DAS) and the AD760* (4-channel, 16-bit DAS). The AD7608 is an 8-channel, 18-bit DAS. The appropriate reference can be selected using the Reference Selection and Estimation Tool.