High-Density Industrial Data Acquisition System

The maximum aggregate sampling rate depends on the number of AD7606 devices and interface mode: With 4 AD7606 devices (32 channels), each sampling at 200kSPS, the total aggregate throughput is 6.4MSPS (32 channels x 200kSPS). In parallel mode, this requires a 16-bit data bus running at 6.4MHz (with CS/RD control). In serial mode, each AD7606 uses two SPI data lines (DOUTA and DOUTB) at 12.8MHz clock rate. The practical limit is often the host processor's ability to read and process the data. For continuous streaming to a PC, USB 2.0 high-speed (480Mbps) can sustain 6.4MSPS x 16 bits = 102.4Mbps with margin. For PLC interfaces, use the RS-485 mode with Modbus protocol at 115.2kbps - this limits sampling rate to approximately 3.6kSPS aggregate for all channels.

Use parallel mode for maximum throughput with FPGAs; use serial mode for MCUs; use RS-485 for PLC integration with lower rates.

Multiple AD7606 devices can be synchronized for simultaneous sampling across all channels: 1) Connect the CONVST (conversion start) pins of all AD7606s together to a common GPIO from the host processor

2) Use a single chip select (CS) if sharing the data bus, or individual CS lines if each device has dedicated data lines

3) Drive CONVST high simultaneously to start conversion on all devices at the same time

4) Wait for BUSY signals to go low (conversion complete)

5) Read data from each device sequentially or in parallel depending on interface mode. The CONVST pulse width must be at least 25ns. For precise timing, use a FPGA to generate CONVST with <10ns skew between devices. The maximum channel-to-channel skew is typically <5ns when using common CONVST.

Connect all CONVST pins together; use common conversion start signal from FPGA for <10ns skew.

The reference design includes 2500Vrms digital isolation using ADuM140E quad-channel isolators on the SPI interface. For higher isolation requirements: 1) Use ADuM5401 isolated DC-DC converter to generate isolated 5V for the analog side

2) For channel-to-channel isolation, use ADG1414 switch with 100V isolation per channel

3) For complete analog input isolation, use isolated amplifiers (ADuM3190) before the ADC - this adds cost but provides highest noise immunity

4) For RS-485 communication to PLCs, use ADM2587E with integrated isolated power. The isolation barrier should be designed with appropriate creepage and clearance distances per IEC 61010-1 for the target voltage rating. Always include TVS diodes on field-side inputs for surge protection.

Use ADuM140E for digital isolation; use isolated amplifiers for channel-to-channel isolation in high-noise environments.

The DAQ system supports multiple PLC interface options: 1) Modbus RTU over RS-485 - use ADM2587E isolated RS-485 transceiver, implement Modbus slave firmware with register mapping for channel data

2) Modbus TCP over Ethernet - add W5500 Ethernet controller, implement Modbus TCP stack

3) Analog output - use AD5761 DACs to provide 4-20mA or 0-10V outputs for PLC analog inputs

4) Digital I/O - use ADM1270 for digital status outputs to PLC. For most PLCs, Modbus RTU is the simplest integration - configure the PLC as Modbus master polling the DAQ system for channel data. Typical register map: Holding registers 40001-40032 for channel data, coils for alarm/status. Update rates of 10-100Hz are typical for PLC process control applications.

Use Modbus RTU over RS-485 for simplest PLC integration; use Modbus TCP for Ethernet-based systems.

The AD7606 includes an internal anti-aliasing filter with approximately 23kHz cutoff (-3dB) when sampling at 200kSPS. This provides approximately 80dB attenuation at the Nyquist frequency (100kHz). For most industrial applications, no external filtering is required. For applications with high-frequency noise or interference: 1) Add a passive RC filter (1kΩ + 10nF) before each input for additional 16kHz cutoff

2) For 50/60Hz power line monitoring, the AD7606's oversampling feature provides excellent rejection (80dB at 50/60Hz when using OS x64)

3) For severe EMI environments, add ferrite beads on all input lines. The AD7606's internal filter is optimized for the ADC sampling rate and provides good phase matching between channels. External filters should be designed to maintain this phase matching if critical for your application.

Use internal filter for most apps; add external RC filter for high-noise environments; use oversampling for 50/60Hz rejection.