Receiver Equalization Circuit for Baseline Wander and DC Shift
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Solution Overview
Problem
Baseline wander, a common issue in AC coupled serializer/deserializer links, leads to low-frequency noise and impairment in link budget due to imperfect correction mechanisms, especially in multi-level signaling and when AC capacitors are placed on semiconductor dies, where they cannot be made sufficiently large.
Innovation Solution
A receiver circuit configuration that includes a resistor in parallel with a capacitor and pairs of current sources, adjusted by a common mode feedback op-amp, to prevent baseline wander, perform DC level shifting, and achieve linear equalization, effectively addressing the limitations of existing feedback mechanisms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a feedback mechanism is used to correct baseline wander, then baseline wander correction is achieved, but residual impairment remains due to finite delay
Solution Approach 1:
The patent applies preliminary action by using a feed-forward mechanism that anticipates and corrects baseline wander before it fully develops. The transfer function H(s) = (s + ω₀)/(s + α) proactively compensates for low-frequency drift by adding a zero at frequency ω₀, preventing the accumulation of baseline wander rather than reacting to it after the fact.
Solution Approach 2:
The patent replaces the traditional feedback mechanism (which relies on delayed detection and correction) with a feed-forward approach using a modified transfer function. This substitution eliminates the inherent delay problem by using a proactive correction mechanism that operates in the forward path rather than the feedback path.
2Object-affected harmful factors
If AC capacitor size is increased to reduce baseline wander, then low frequency noise is reduced, but device area increases when capacitor is on semiconductor die
Solution Approach 1:
The patent changes the electrical parameters of the coupling network by introducing a resistor in parallel with the AC capacitor and modifying the transfer function to include a zero at frequency ω₀. This parameter modification allows effective baseline wander suppression with a much smaller capacitor value, as the transfer function compensation replaces the need for large capacitance.
Solution Approach 2:
The patent introduces a resistor as an intermediary element in parallel with the AC capacitor. This resistor creates a feed-forward path that provides alternative current flow for low-frequency components, reducing the burden on the capacitor and allowing smaller capacitor sizes while maintaining effective baseline wander correction.
3Reliability
If feedback mechanism is used for baseline wander correction, then correction is provided, but delay causes imperfect correction leading to link budget impairment
Solution Approach 1:
The patent inverts the traditional feedback approach by using a feed-forward mechanism. Instead of detecting baseline wander and then correcting it (which introduces delay), the system proactively applies correction through the modified transfer function H(s) = (s + ω₀)/(s + α) in the forward path, eliminating the delay inherent in feedback loops.
4Ease of manufacture
If AC capacitor is placed on semiconductor die, then integration is improved, but capacitor cannot be made sufficiently large to effectively suppress baseline wander
Solution Approach 1:
The patent modifies the electrical parameters of the coupling network by adding a parallel resistor and adjusting the transfer function to include a zero at frequency ω₀. This parameter change enables effective baseline wander suppression with integrated on-die capacitors of practical size, as the transfer function compensation replaces the need for large capacitance values.
Solution Approach 2:
The patent introduces a resistor as an intermediary element that works in parallel with the small on-die capacitor. This resistor provides an additional path for low-frequency current flow, enabling effective baseline wander correction with compact integrated capacitors that can be fabricated on the semiconductor die.
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
The solution effectively prevents baseline wander, adjusts DC levels, and achieves linear equalization, improving signal quality and reducing residual intersymbol interference, especially for long strings of 1's or 0's, by repurposing the circuit to act as a linear equalizer at lower frequencies.
Implementation Method 1
a resistor in parallel with a capacitor which passes the input signal to a second interface
Implementation Method 2
a plurality of current sources between ground and a voltage source, the current through the current sources being adjusted by a common mode feedback op-amp having a first input as the sensed common mode at the input pads and a second input as a desired common mode voltage reference
Data Source
AI summary
Systems, apparatuses, and methods for implementing a combo scheme for direct current (DC) level shifting of signals are disclosed. A receiver circuit receives an input signal on a first interface. The first interface is coupled to a resistor in parallel with a capacitor which passes the input signal to a second interface. Also, the first interface is coupled to a first pair of current sources between ground and a voltage source, and the second interface is coupled to a second pair of current sources between ground and the voltage source. An op-amp drives the current sources based on a difference between a sensed common mode voltage and a reference voltage. Based on this circuit configuration, the receiver circuit is able to prevent baseline wander, perform a DC level shift of the input signal, and achieve linear equalization of the input signal.