Analog Isolation in the 21st Century

Arlie Stonestreet II, Chief Design Engineer at Ultra Electronics - ICE Corporation -September 04, 2014

Micropower Digital Isolators

 

The NVE IL500, IL700 series isolator and the equivalent Avago HCPL-9000/-0900 series, use giant magnetoresistive (GMR) technology for high-speed digital isolation (Figure 2). The advantage these devices have over other isolators is that for the transmitting channels the quiescent power is less than 30uA per channel and the additional dynamic power consumption is around 14uA/100kbps per channel. If the data rate is equivalent to 100kbps the total isolator supply current is only 44uA, which can be easily supplied by the 8A100 Dionics-USA high-current PVI mW power source while still leaving plenty of current in reserve for powering the associated data converter.

 

 

Figure 2: IL711 Two-Channel Digital Isolator - Input channels are micropower

 

For applications requiring bi-directional digital isolation such as SPI interfaces, Analog Devices recently released the micropower ADuM144x series quad-channel digital isolators based on their iCoupler® technology (Figure 3). For these devices, the quiescent supply current is under 10uA/channel and the dynamic current is under 10uA/100kbps and again are easily powered by the 8A100 devices.

       

 

Figure 3: ADuM144x Micropower Digital Isolator

         
 

Isolated Analog Voltage-to-PWM

 

The Linear Technology LTC6992 micropower PWM TimerBlox® accepts a 0 to 1V analog input and produces an output duty cycle proportional to the input voltage. The PWM output is isolated via the IL510 and demodulated with a simple RC filter (Figure 4). Alternatively, the PWM output may be read directly in digital form by the input capture port of a microcontroller to determine duty cycle and hence voltage.


Even when configured for a PWM frequency of 100kHz the device current for the TimerBlox was around 112uA and the device current for the IL510 isolator was 64uA for a total supply current of 176uA. This supply is easily furnished by the Dionics 8A100 high-current PVI and proper voltage regulation on the isolated side is set using the 3.0V shunt reference. A TimerBloxDesigner Excel Spreadsheet is available from the Linear Technology website to ease component selection for frequency, duty cycle, and control voltage range.


      

Figure 4: Isolated PWM

 

 

Isolated Analog-to-Digital Converters

 

There are a number of micropower data converters available in both delta-sigma (Table 1) and SAR flavors (Table 2) that can be used to create a custom isolated ADC signal path typical of Figure 5. In this example the micropower ADuM1441 provides isolation of the SPI interface between the ADC and the microcontroller and the Dionics 8A100 high current PVI again provides a clean source of isolated power to the ADC and the digital isolator. For most ADC’s the highest power draw is often consumed only during conversion mode, or when operating at the highest sampling rates.


Even though many ADC’s are not advertised explicitly as micropower, often times they can achieve micropower operation by scaling back the conversion rate duty cycle or operating at lower sample rates. Alternatively, the PVI power source can be set to run at a relatively lean bias current during converter sleep mode and then during periods of high conversion current the PVI can be biased at considerably higher currents on a duty-cycle limited basis. For example, the 8A100-2 PVI could deliver in excess of a full 1mA (i.e. 1000uA) supply current at 3.3V under a 75mA LED input bias having a 25% duty cycle.

   

Figure 5: Isolated ADC

     

Table 1: Micropower Delta-Sigma ADCs

 

 

 

Table 2: Micropower SAR ADCs

 

   

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