High performance optocoupler circuits

Abstract

EXPERIMENTAL AND ANALYTICAL results for a highly linear, temperature-insensitive optocoupler circuit will be presented in this report. In this feedback arrangement one GaP LED shines almost equally on two photodetecting silicon diodes. The photon current generated from one detector is used for feedback control of the nonlinear and temperature sensitive LED light generator, while the light-generated current in the other diode equals that of the first diode. Significantly, this arrangement lends itself to mass fabrication since no preselection or prematching of the single LED nor the two photodiodes is required. At audio frequencies the measured gain stability versus temperature is typically about 1% from -50" C to +looo C. For close to full modulation of the dynamic range the harmonic distortion measures around -70 dB.as a function of the GaP LED bias current. The high temperature sensitivity of the LED is also indicated. The photongenerated diode current is independent of the carrier sweepout voltage and it is about five orders of magnitude lager than the thermally generated I, of the diodes. Significantly, the current difference between DA and D, amounts to a constant gain factor difference due to the finite differences in the amount of light received. Figure 2 shows the simple dual diode detector singte LED DIP structure.Figure 3. The measured transfer linearity is better than resolvable with a fivc-place digital voltmeter. Figure 4 indicates a typical measured static transfer function; it will be noted that the feedback adjusts the LED bias current, and thus the transfer function of the interface becomes linear. Figure 1 also shows a horizontal trace, the measured gain tracking of about 1% from -50° C to +looo C. This finite tracking is independent of the LED current level and is possibly due to mechanical instabilities of the optocouplers.coupler driven by a simple operational amplifier. The GaP LED has a flat response for light generation u p to about 100 kHz.That frequency response in conjunction with the 20-dB/decade rolloff of the op amp determines the maximum feedback possible. The feedback gain is increased by reducing the resistor in series with the LED, as illustrated by the transmission peak emerging for R = 20 a at 100 kHz. It follows indirectly that a reasonable loop gain of 40 dB is possible a t audio frequencies. Analysis, by stipulating that the sum of the currents to the input node of the operational amplifiers equals zero in the circuit, indicated in Figure 1 shows representative silicon diode detector currents This optocoupler is operated in the feedback circuit shown in Figure 5 shows the measured frequency response of the opto-For typical parameters, Arranging the photo current buffer as shown in Figure 3 results, by a similar analysis, in an output signalFor a random insertion of nonselected optocouplers into a fixed circuit the absolute gain has a spread of less than 15%. One resistance adjustment during the final assembly and test sets the absolute circuit gain with precision.cuits as sh...

A monolithic isolation amplifier in silicon-on-insulator CMOS: Testing and applications

“…power supply feedback signals, command and control signals, etc.) two principal methods are used: optocouplers and magnetic coupling using pulse transformers [1][2][3]. Both of these options, however, have important drawbacks that limit their effectiveness in certain applications.…”