The electrically isolated micropower 12-bit data-acquisition system in Fig 1 is ideal for digitizing the output of a transducer that operates in a hostile environment. The 1:1 transformer and three digital isolators provide electrical isolation of 500V rms between the host system and the data-acquisition circuitry and sensor. The circuit operates on a single 5V supply and communicates to a host system by using just two serial lines.
IC1's micropower switcher, operating in flyback mode, drives the primary of a 1:1 transformer that transfers power to the isolated circuitry. The circuit rectifies and filters the voltage on the transformer secondary and applies the result to IC2, a fixed 5V micropower linear regulator. Q1 generates the feedback signal for IC1. Isolator IC3, drawing a maximum of 700 µA, transmits this feedback across the isolation barrier. Q1 turns on when the transformer's secondary voltage rises to approximately 600 mV above the regulated 5V output voltage, which is just above IC2's dropout voltage. This feature keeps IC2 operating at its peak efficiency. D2 reduces Q1's output voltage to 5V before it reaches the input pin 1 of IC3. IC3's output, pin 11, feeds back to IC1, closing the feedback loop. In this application, IC1 operates over an input range of 3 to 8V.
The micropower 12-bit serial ADC, IC4, requires signal connections for CS, serial clock, and output data. Isolators IC5 and IC7 transfer the serial clock and data, respectively, across the isolation barrier. IC6, D1, R1, and C1 generate a local CS whenever the serial clock is active. The CS signal becomes active when the host's serial clock signal makes its first low-to-high transition. CS remains low as long as the clock signal is active and the time between low-to-high transitions is less than 2250, 225, or 22.5 µsec for C1 equal to 1, 0.1, or 0.01 µF, respectively.
At a 10-sample/sec conversion rate, IC4 and IC6 together typically draw 20 µA. At 14k samples/sec, they draw 1 mA. For serial clock frequencies below 20 kHz, a lower current version of IC3, IC5, and IC7 is available. (DI#1676)