Isolation Makes Sense

This post sponsored by Texas Instruments.

Industrial machines use a lot of heavy inductive loads. From solenoids and actuators to motors and blowers, it is not surprising that these materials can introduce noise, spike, glitches, and inductive reactions that can threaten more sensitive microcontrollers and logic. 

Circuit protection techniques can go a long way to assuring reliable operations even under varying loads and startup conditions. Transorbs, suppressors, MOVs, gas discharge tubes, and even neon bulbs can be used to absorb a lot of potentially damaging spikes. 

One proven protection technique is isolation. It is true that any integrated isolation solution always has the potential to arc across if voltages are too high. But, galvanic isolation is very desirable for driving signals into high-noise environments. Photovoltaic isolation (also called opto-isolators), provide logic-level drive control across power barriers to serve just this purpose. 

While useful for driving, optical isolation techniques are not as effective at conveying analog information back to the protected micro. Some linear opto-isolators do exist but do not provide much resolution. On top of that, biasing is difficult, especially with drift. 

Pulse-width modulation (PWM) techniques are effective using standard opto-isolators. Analog signals can control a VCO, which drives a single-bit digital line to the host. The host decodes the duty cycle to recreate the value. But, when signals vary rapidly, the resolution is lost at some point because the VCO just cannot keep up. 

An interesting technique from Texas Instruments™ called the capacitive double isolation barrier is showing how reinforced semiconductor isolation barriers can be set up that deliver the 7,000 V peak levels needed to comply with CSA, UL, and VDE standards. Based on a Delta-Sigma Modulator, the AMC1305 family of shunt resistor-based sensing circuits (Fig. 1) is ideally suited to target industrial motor drive, uninterruptible power supplies, and reactive controllers that depend on real-time sensors to optimize performance. 

Fig. 1: The isolated barrier protects the micro while providing high-level common-mode noise immunity. (Source: Texas Instruments)

By using a digital filter like the one integrated onto the TMS320F2837 family of DSP processors, an algorithm can decimate the bitstream to achieve a 16-bit resolution at 78 K samples/second. Note, an analog high-side voltage of 5 Volts is used, and on the isolated low side, a 3.3 or 5 Volt supply can bias the isolated modulator.

To help demonstrate this, TI has an Isolated Current Shunt and Voltage Measurement Kit that combines the AMC130x isolated modulator with the dual-core 32-bit 200 MHz Floating Point TMS320F28377 Delfino™ Processor. The eval board handles an isolated power supply onboard to support real-time current monitoring (and voltage) of an external 3-phase motor. A run-time GUI helps configure, monitor, and test the SINC filters, PLL frequency, and data values (Fig. 2). It also provides the voltage and current waveforms while maintaining a calibrated accuracy of +- 0.2% with response times of Fig. 2: A graphical GUI allows parameter settings as well as waveform display. (Source: Texas Instruments)

Note, circuit protection is not the only benefit of real-time isolated-motor sensing. Because of energy concerns, the demand for higher-efficiency motors means that real-time monitoring and control can better optimize torque, rotor speed, and reduce current consumption, especially when not fully loaded. 

A reference design and complete Schematic, Bill of Materials, and Design Files are also available online. 

TMS320F28377D
For more information about this product, click here.
To request a sample, click here. 
To buy now at Mouser, click here. 

AMC1305M25
For more information about this product, click here.
To request a sample, click here.
To buy now at Mouser, click here. 

CDCE906
For more information about this product, click here.
To request a sample, click here. 
To buy now at Mouser, click here. 

DAC8564
For more information about this product, click here.
To request a sample, click here. 
To buy now at Mouser, click here. 

INA826
For more information about this product, click here.
To request a sample, click here. 
To buy now at Mouser, click here. 

LP38798
For more information about this product, click here.
To request a sample, click here. 
To buy now at Mouser, click here. 

OPA211
For more information about this product, click here.
To request a sample, click here. 
To buy now at Mouser, click here. 

By Jon Gabay, Hearst Business Media