Isolated SPI controller that can be extended up to 100m in harsh noise environments

"LTC6820" is an isoSPI isolated communication interface provided by Analog Devices.
The LTC6820 provides bidirectional SPI communication over a single twisted-pair connection between two isolated devices.

By installing this device and placing a general-purpose transformer, it is possible to achieve noise-resistant differential communication and isolated communication at the same time without spending development man-hours. This eliminates the need for isolated power supplies and photocouplers that were required with conventional isolated RS485, etc., reducing costs and making it an ideal product for interfacing industrial measurement equipment.

This article discusses the following features of the LTC6820:

1. Easy isolated differential conversion for SPI interface without software changes
2. With a general-purpose transformer and a general-purpose twisted pair cable, 100m communication is possible even in a high-noise environment.
3. Reduces costs by eliminating the need for isolated power supplies, optocouplers, and format conversion

1. Easy isolated differential conversion without software changes for SPI interface

The LTC6820 encodes the logic state into a signal and transmits that signal across the isolation barrier to another LTC6820.

The receiving LTC6820 decodes the transmitted signal and drives the slave bus to the appropriate logic state. A precision window comparator in the receiver detects differential signals. The drive current and comparator threshold are set with a simple external resistor divider, allowing the system to be optimized for the required cable length and desired signal-to-noise ratio performance.

2. With a general-purpose transformer and a general-purpose twisted pair cable, 100m communication is possible even in a high noise environment.

The isolation barrier can be bridged with a simple pulse transformer to achieve hundreds of volts of isolation.

The LTC6820 drives differential signals with matched sink and source currents, eliminating the need for a transformer center tap and reducing EMI.

3. Cost reduction due to no need for isolated power supply, optocoupler, or format change

Compared to the isolated RS485, the number of cables, isolated power supplies and photocouplers are reduced, simplifying the system configuration and reducing costs.

When the sensor and MCU are mounted on the same board, serial interfaces such as SPI and I2C are often used.

However, when the sensor and control board are sufficiently far apart (1m or more), it is popular to use an interface such as UART/RS485 to transmit data from the sensor board to the MCU on the control board. However, in order to do so, it is necessary to convert the format of the sensor's interface (SPI/I2C) to UART/RS485, etc., using an MCU.

The LT6820 can be used to route SPI data without format conversion to another interface.

"DC1941D" evaluation kit for immediate evaluation

The DC1941D evaluation kit is available for immediate evaluation of the LTC6820.

DC1941D can select master/slave by jumper setting.
SPI communication is possible by connecting two DC1941Ds with an Ethernet cable.

This evaluation board has a 14-pin ribbon cable connector, so it can be connected to another evaluation board for evaluation.

In addition, the DC1907A evaluation kit (isoSPI2-Wire Serial Analyzer) can be used to monitor SPI communication between master and slave.

By connecting the SPI output of the DC1941D to the DC1907A and connecting the DC1907A to a PC via USB, it is possible to analyze SPI communication using the application for the DC1907A.

Application example

• Systems with sensors with SPI interface
• industrial network communication
• Battery monitor system
• remote sensor

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