Design Ideas: December 21, 1995

Isolation amplifiers provide insulation for low-frequency analog signals at a high cost. Optocouplers cost less and are widely used with digital signals. However, you have to factor in limitations from distortion and bandwidth restrictions when using optocouplers. The bandwidth limits result from a declining current-transfer ratio as a function of frequency. You can use feedback techniques to improve the linearity, but the circuits are usually complex. The circuit in Figure 1, which applies current drive to the optocoupler diode, improves bandwidth and allows easy frequency-response equalization.

A low-cost optocoupler, such as the 4N25, has a cutoff frequency of about 80 kHz (Figure 2, curve a). The transconductance amplifier in Figure 1 drives the LED in the 4N25 optocoupler. The differential-current output of the MAX435 amplifier connects directly to the LED. This current is given by the formula:

This structure allows an easy insertion of a zero in the frequency response, which Z_T=R₃C₂ determines. The values shown place a zero at 80 kHz. With the highpass filter in Section 1 of Figure 1 absent, the cutoff frequency of the amplifier and the optocoupler is 250 kHz. The differential input of the MAX435 provides an easy way to set the dc bias current for the optocoupler. In the circuit in Figure 1, adjusting R₄ sets the bias current at 3 mA.

To improve the frequency response, you can add the highpass filter shown in Section 1 of Figure 1. This filter provides a second zero determined by R₁ and C₁. If you place this zero at 120 kHz, the cutoff frequency is 1 MHz (Figure 2, curve b). This frequency is a fourfold improvement over the single-zero response. The measured second-harmonic distortion of the circuit is –50 dB at 100 kHz and –40 dB at 500 kHz, for a 50-mV p-p sinusoidal signal. Note that the 4N25 can exhibit wide device-to-device variations in frequency response, so you might want to either select optocouplers or tailor the component values in the circuit to obtain the desired response. (DI #1798)