An insulated type SPI controller that is extendable up to 100m in a harsh noise environment

"LTC 6820" is an isoSPI insulated communication interface provided by Analog Devices.
LTC 6820 provides bi-directional SPI communication between 2 isolated devices over 1 twisted-pair connections.

By using this device, it is possible to realize both noise-resistant differential communication and insulated communication without the need for development man-hours by simply placing a general-purpose transformer. As a result, the insulation power supply and photocoupler required in the conventional insulated RS 485 and other products are no longer required. As a result, this product reduces costs and is ideal for interfacing with industrial measurement equipment.

This article introduces the following features of LTC6820.

1.    Simple Isolated Differential Conversion of SPI Interface without Software Changes
2.    General purpose transformer and general purpose twisted pair cable enable 100m communication even in high noise environment
3.    Insulation power supply, photocoupler, and no format conversion are required to reduce cost.

Simple Isolated Differential Conversion of SPI Interface without Software Changes

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

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

Block Diagram

General purpose transformer and general purpose twisted pair cable enable 100m communication even in high noise environment

The isolation barrier can be bridged with a simple pulse transformer to achieve 100 of volts of isolation.

The LTC6820 uses matched sink and source currents to drive the differential signal, eliminating the need for transformer intermediate taps and reducing EMI.

Standard application

Insulation power supply, photocoupler, and no format conversion are required to reduce cost

Compared with the insulated RS 485, the number of cables, insulated power supply and photocoupler are reduced, simplifying the system configuration and reducing costs.

Comparison with RS-485
 

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

However, when the sensor and the control board are sufficiently far apart (1m or more), it is a popular method to use an interface such as UART / RS485 to transmit the sensor board data to the MCU of the control board. To do this, the MCU must convert the sensor interface (SPI / I2C) into UART / RS485.

LT6820 allows you to pass SPI data around without having to format it into another interface.

"DC1941D" Evaluation Kit for quick evaluation

You can quickly evaluate LTC6820 by using the DC1941D evaluation kit.

The DC1941D can be set to either master or slave by jumper. 
SPI communication can be enabled by connecting 2 DC1941D with an Ethernet cable.

"DC1941D" Evaluation Kit

The evaluation board has a 14 pin ribbon cable connector that can be connected to another evaluation board for evaluation.

Run isolated LTC 2983 using 2 DC1941D
 

It is also possible to monitor master-slave SPI communication using the DC1907A Evaluation Kit (isoSPI2 - Wire Serial Analyzer).

By connecting the SPI output of the DC1941D to the DC1907A and the DC1907A to the PC via USB, the SPI communication can be analyzed using the application for the DC1907A.

Connect DC1907A and monitor SPI communication between Master and Slave

Target applications

• Sensor based system with SPI interface 
• Industrial network communications
• Battery monitor system
• Remote sensor

Target applications

Features of LTC6820

• 1 Mbps isolated SPI data communication
• Simple electrical isolation using a standard transformer
• 1 twisted pair bi-directional interface
• Supports cable lengths up to 100 meters
• Very resistant to EMI and emits very little EMI
• High noise exclusion and low power consumption configurations are possible.
• Designed for ISO26262 compliant systems
• No software changes required for most SPI systems
• Ultra-low idle current：2μA
• Auto start detection
• Operating temperature range：-40℃～125℃
• Power supply voltage：2.7V～5.5V
• Interface to all logic from 1.7V～5.5V
• Available in 16pin package and MSOP package