Optical Receiver Cascode Front End for Noise Immunity
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Solution Overview
Problem
Existing optical receivers face challenges in generating a fully differential signal quickly and accurately, especially at high speeds, which is crucial for DC applications due to issues with pulse width distortion and electromagnetic interference immunity.
Innovation Solution
The optical receiver design incorporates a differential TIA linked with a photodiode, utilizing cascode circuits and a peak detector to provide a proportion of the maximum sense signal, along with a replica circuit and transconductance block, to generate a fully differential output voltage with minimal delay, and an automatic gain control loop for noise immunity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a fully differential TIA is used to improve noise and EMI immunity, then noise immunity is improved, but creating an accurate comparator reference point for pseudo differential signal at high speed becomes difficult
Solution Approach 1:
The patent applies preliminary action by generating the fully differential signal through cascode circuits and peak detector before the signal reaches the comparator. The cascode circuits convert the single-ended photodiode current into differential currents (I_sig_tia and I_dark_tia) in advance, and the peak detector pre-establishes the reference voltage level. This preliminary conversion eliminates the need for complex real-time reference point generation at the comparator, resolving the contradiction between noise immunity and device complexity.
2Measurement precision
If feedback via error amplifier is used to generate differential signal, then DC rejection is improved, but significant lag is introduced before differential signal is correctly generated
Solution Approach 1:
The patent uses preliminary action by implementing cascode circuits that directly convert the photodiode current into differential signals without waiting for feedback loops. The cascode circuits immediately generate I_sig_tia and I_dark_tia from the photodiode current, and the peak detector pre-establishes reference levels. This eliminates the significant lag inherent in feedback-based error amplifier approaches, while still achieving DC rejection through the differential architecture.
Solution Approach 2:
The patent introduces intermediary elements (cascode circuits and peak detector) between the photodiode and the output stage. These intermediaries perform the differential signal conversion and reference establishment in advance, acting as mediators that eliminate the need for slow feedback loops. The cascode circuits serve as intermediary converters that transform single-ended signals into differential signals without requiring error amplifier feedback, thus reducing delay while maintaining DC rejection capability.
3Reliability
If peak detector responds to output voltage to create fully differential signal, then differential signal is generated, but significant delay occurs before fully differential signal is created
Solution Approach 1:
The patent applies preliminary action by having the peak detector respond to the photodiode current directly through the cascode circuits, rather than waiting for the TIA output voltage. The cascode circuits immediately convert the photodiode current into differential currents, and the peak detector simultaneously establishes the reference level. This preliminary parallel operation eliminates the sequential delay where the peak detector would otherwise wait for TIA output, significantly reducing the time to create the fully differential signal while maintaining reliability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables the generation of a fully differential output signal with reduced delay, suitable for DC to multiple MHz applications, improving noise immunity and reducing pulse width distortion, making it suitable for high-speed data processing.
Implementation Method 1
The conversion of optical light into a voltage is usually implemented using a transimpedance amplifier (TIA)
Data Source
Figure 1~2
Figure 3~4
Figure 5~6
AI summary
An optical receiver (1) comprises a differential TIA (4) linked with a photodiode (2, 3) providing a current sense signal (Isig_tia). The receiver is configured to provide to the TIA a sense signal as a sense TIA input (Isig_tia) and a second input (Idark_tia) which is a proportion of the maximum sense signal. The proportion input is half of said maximum sense signal. The inputs to the TIA are via cascode circuits (5, 6), thereby providing the advantages of a low input impedance for large area photodiodes at the TIA input, while creating a fully differential signal at the output, and the reduction of TIA bandwidth in burst mode applications, which filters out high frequency noise.