Comparator Diode Precharge for Data-Dependent Jitter Control
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Solution Overview
Problem
Conventional high-speed comparators experience data dependent jitter due to varying voltage swings of diodes associated with different data patterns in differential input signals, leading to duty cycle distortion in single-ended output signals.
Innovation Solution
A comparator circuit with a differential stage, diodes, and a current steering circuit that selectively provides a reference current to diodes in the off state to reduce voltage swing, thereby minimizing jitter, and an output stage that converts the differential signal to a single-ended output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the differential input signal does not rapidly change, then the diode voltage decays to a very low voltage level, but this causes large voltage swings and data dependent jitter in the output signal
Solution Approach 1:
The patent applies preliminary action by pre-charging the diode to a predetermined voltage level (e.g., half of the supply voltage) before the comparison operation. This is achieved through a precharge circuit that activates before the differential input signal is applied, ensuring the diode starts from a known voltage state rather than allowing it to decay to a very low level. This preliminary voltage establishment eliminates the large voltage swings that cause data dependent jitter, while the overall comparator structure remains relatively simple.
2Reliability
If a reference current is provided to the diode in the off state to reduce voltage swing, then jitter is reduced, but the device complexity increases
Solution Approach 1:
The patent implements self-service by using the existing tail current from the differential pair to automatically maintain the diode voltage above the threshold voltage. The current steering circuit is configured to redirect the tail current to the off-state diode when needed, without requiring an external reference current source. This allows the comparator to self-regulate the diode voltage based on its own operating conditions, reducing jitter while minimizing additional circuit complexity.
3Productivity
If the diode voltage swings from a very low voltage to threshold voltage, then the comparator can switch the diode on, but this causes varying data propagation delays appearing as jitter
Solution Approach 1:
The patent applies parameter changes by modifying the initial voltage parameter of the diode before switching operation. Instead of allowing the diode voltage to decay to a very low level, the precharge circuit establishes it at a predetermined higher voltage level (e.g., half of the supply voltage). This parameter change ensures that when the differential input signal causes the diode to switch, the voltage swing is reduced and the propagation delay becomes consistent across different data patterns, eliminating jitter while maintaining fast switching speed.
Data Source
AI summary
In one example, a comparator circuit includes a differential stage adapted to receive a differential input signal. The comparator circuit includes first and second diodes coupled to the differential stage. The first and second diodes are adapted to selectively switch on and off to provide a differential output signal at first and second differential output nodes in response to the differential input signal. The comparator circuit includes a current steering circuit adapted to selectively provide a reference current from a current source to the first or second diode in an off state to reduce a voltage swing of the first or second diode between the off state and an on state. The comparator circuit includes an output stage coupled to the first and second diodes at the first and second differential output nodes. The output stage is adapted to convert the differential output signal to a single ended output signal.