A 1.6Gb/s, 3mW Integrating CMOS Optical Receiver with Hybrid GaAs Photo-Detectors

Bonding arrays of hundreds of photo-detectors and VCSELs to silicon substrates has been demonstrated [1-2] and scaling to thousands of devices is possible. To support these large numbers of optical IO, very small and low-power IO interface electronics is required. To remove both the speed and power trade-offs intrinsic in receiver that uses a transimpedance amplifier to convert the photo-current to a voltage, our receiver integrates the photo current directly on to the photodetector parasitic capacitance, and uses double sampling to create an integrating input receiver. Figure 1 illustrates the block diagram of the designed receiver. Since the capacitor integrates the photo current, the value of each bit is then determined by comparing the voltage on that capacitor after the bit arrived to the value before the bit. These two voltages are provided by the two samplers connected to photo-detector. The use of two samplers and two StrongArm latch/sense amps, allows the clock to run at the bit rate divided by 2, resulting in receiver timing that is very similar to a standard high-speed IO. After the top sampler is updated the first sense amp is triggered, and then a half cycle later, this sense amp is reset, the other sampler is updated, and the second sense amp is triggered. Directly integrating the current on the photo capacitor means there is no amplification of the signal path before it is sampled, so this circuit's DC current is roughly the average optical current received. The resulting circuit is small and low power, and the bit rate is limited by the sample bandwidth and input sensitivity of the sense amp, and not the gain-band-width of an amplifier. Higher data rates can be achieved using more samplers, clocked sense amps, and multi-phase clocks. The key issue in the design of this receiver is dealing with small input signals needed for low optical power detection, thus the clocks and the sense amps need to be designed to minimize the noise and offset of the receiver. The circuit also needs to create a reference current to convert the single-ended, positive photo current to a bipolar current.