What is SPI isolation, SPI Isolator IC

What is SPI

SPI is (Serial Peripheral Interface) and synchronous serial communication. SPI communication is used for short distance communication between Master (microcontroller/FPGA) and Slave (SPI devices). Large range IC’s , ADC, DAC , memory IC’s, Sensor, sensor controllers, power controllers, motion and position controller modules and IC’s etc use SPI protocol for control and communication. Many of these devices communicate at very high speed, upto 100MHz clock. However this is not SPI protocol limit. SPI bus consist on following signal lines

  • SCLK/SCK : Generated by Master and applies on all SPI devices or Single SPI device at SCLK/SCL input
  • MOSI: Master output and slave input, data flow from master to slave by this line. Some slave devices uses SDI, DI, DIN, SI for this line and some master uses SIMO also.
  • MISO: Master input and slave output, date flow from slave to master by this line. Some slave devices uses SDO, DO, DOUT, SO for this line and some master uses SOMI also.
  • CS/SS: Chip select or slave select. Master uses to select slave for communication or control. some slave devices uses this line for some other purpose parallel like some ADC start sampling at CS pin assertion.

Why and when we need isolation for SPI communication

  • When one of master or slave circuit uses high voltage that may vulnerable to another circuit. Then we use SPI isolation IC. SPI Isolation IC have galvanic/optical isolation barrier between master and slave devices. This isolation barrier can bear 1KV-5KV without leakage. And protect such high voltage to transfer to another side. For example a SPI thermometer sensor ic attached to human body is monitor by and equipment which operates on 220VAC. so any utility line leakage can be lethal for patient. In such equipment SPI isolation is necessary to create high voltage barrier b/w patient and equipment.
  • When two circuits or device which uses SPI communication. But both uses different power sources. And we are not desirous to connect them in any situation. so SPI isolator use two isolated supplies and does not mix them. For example and external battery voltage charger / monitor ADC SPI link should be isolated from micro controller.

What is integrated digital isolation technology

For digital isolation opto-coupler was popular method. But there are many design constraints specially where we need isolation for high speed signal. Opto-coupler’s different channels offers different CTR (current transfer ratio). Which makes signals lags and leads eventually resultant data may be corrupted. opto-coupler transient parameter always vary for component to component. some time exact delay is required.

Now many semiconductor manufacturers offer integrated digital isolation IC’s in small packages. these IC’s use galvanic isolation barrier between two circuits. This barrier is consist on on-chip transformer, which primary and secondary coils are separated by a poly-amide thin layer. Poly-amide is outstanding electrical insulator and very stable at high temperature. this few micron thick poly-amid layer can bear up to 5KV without breakdown. Poly-amid can also bear soldering temperature unlike other polymers.

Above mentioned on-chip transformer have little inductance but favorable mutual inductance due to small distance between primary and secondary coils. these coils are spiral-like single trace. Input digital signal is modulated with high carrier frequency and put into Primary coil. At secondary coil this signal demodulated, and converted to logic level for output by output circuitry. Multiple and bidirectional digital isolators are available.

iCoupler Technology for Digital isolation by Analog Devices inc
Texas Instruments ISOW7840 Series Digital isoloators

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