Out in Front: June 20, 1996

Fiber-optic receivers for synchronous-optical-network (SONET) and synchronous-digital-hierarchy (SDH) applications need transimpedance amplifiers to amplify the weak and widely varying photodiode current and convert it into a voltage. For the 155-Mbps OC-3 rate, as well as 266-Mbps fiber-channel applications, the 165-MHz bandwidth SA5223 from Philips Semiconductors includes an AGC amplifier with an internal loop-hold capacitor; the AGC minimizes pulse-width distortion at high signal levels. The amplifier handles signals from 0.5 µA to 2 mA with bit error rates of less than 1×10^-10; recovers data without overshoot errors, despite as much as 10 dB of gain change; and provides error-free output, even when received signals have asymmetrical duty cycles. Spectral noise is 1.17 pA/([square root]Hz] for this eight-pin SOIC, which costs $6.30 (10,000).

For 622-Mbps applications, the MAX3664 from Maxim Integrated Products is a 590-MHz amplifier that requires only 25 mA from a 3.3V supply. It includes a dc-cancellation circuit that reduces pulse-width distortion to 200 psec (maximum) by providing a true differential output for input currents ranging as high as 300 µA. Transimpedance gain is 6 k[ohm], and the differential output, which back-terminates each output line with 60[ohm], can drive a 100[ohm] load. Typical sensitivity is -33.2 dBm, due to the low input-noise figure of 55 nA (rms) with 1300-nm, 622-Mbps receivers. The MAX3664 is available in eight-lead SOIC and µMAX packages and costs $15 (1000).

—by Bill Schweber

Philips Semiconductors, Sunnyvale, CA. (800) 447-1500, http://www.semiconductors.philips.com/ps/.

Maxim Integrated Products, Sunnyvale, CA. (408) 737-7600, ext 6087, http://www.maxim-ic.com.