Differential Receiver Input Impedance Matching for High PSRR
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Solution Overview
Problem
Differential pairs in analog circuits, particularly those using PMOS transistors in deep N-well processes, suffer from reduced immunity to supply disturbances due to inability to short source and bulk connections, leading to inferior power supply rejection ratio (PSRR) similar to NMOS counterparts.
Innovation Solution
The implementation of an impedance matching network at the inputs of the differential pair to force equal impedances, utilizing a combination of resistors and capacitors to transform supply disturbance currents into common-mode disturbances that can be rejected by the differential pair, thereby enhancing PSRR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If deep N-well isolation is used to isolate sensitive analog blocks from noisy substrates, then substrate noise immunity is improved, but the ability to short source and bulk of PMOS transistors is lost, worsening supply disturbance rejection
Solution Approach 1:
The patent introduces an intermediary circuit (bulk connection circuit) that mediates between the bulk terminals and the source terminal. This circuit includes transistors and resistors configured to dynamically connect the bulk to the source when supply disturbances are detected, while maintaining the deep N-well isolation structure. The intermediary circuit enables the system to have both substrate noise immunity (through maintained isolation) and supply disturbance rejection (through dynamic bulk-source connection).
Solution Approach 2:
The patent implements dynamic control of the bulk connection state. The bulk connection circuit is controlled by detection circuits that monitor supply disturbances and adjust the bulk-source connection accordingly. When disturbances are detected, the circuit dynamically switches to connect bulk to source; when not disturbed, it maintains isolation. This dynamic behavior resolves the contradiction by adapting the system state to operating conditions.
2Reliability
If source and bulk of PMOS transistors are shorted to eliminate supply disturbances coupling, then supply disturbance rejection is improved, but deep N-well isolation is compromised, worsening substrate noise immunity
Solution Approach 1:
Instead of a static short connection, the patent uses dynamic switching controlled by disturbance detection circuits. The bulk connection circuit remains disconnected during normal operation to maintain deep N-well isolation, and only connects when supply disturbances are detected. This temporal separation resolves the contradiction by having both isolation and connection states at different times.
Solution Approach 2:
The patent segments the bulk connection function into separate controllable stages: the deep N-well isolation structure provides substrate noise immunity, while the added bulk connection circuit provides supply disturbance rejection when activated. This segmentation allows each function to operate independently, resolving the contradiction by enabling both functions through separate circuit elements working in coordination.
3Reliability
If impedance matching elements are added to improve PSRR, then supply disturbance rejection is improved, but circuit complexity increases
Solution Approach 1:
The impedance matching elements (resistors and capacitors) are integrated into existing circuit nodes and serve multiple functions: they provide impedance matching for AC signals, establish DC bias points, and contribute to the bulk connection circuit operation. This multi-functionality reduces the net complexity increase by making existing elements serve additional purposes rather than adding purely dedicated components.
Solution Approach 2:
The patent merges the impedance matching function with the bulk connection circuitry. The same resistors and capacitors that provide impedance matching for signal integrity also serve as part of the bulk connection network that rejects supply disturbances. This merging of functions achieves PSRR improvement without proportionally increasing circuit complexity, as one set of components performs multiple roles.
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 achieves high immunity to supply and substrate disturbances, improving PSRR by greater than 60 dB, maintaining deep N-well isolation while preventing adverse effects on loop dynamics.
Implementation Method 1
a first impedance matching element coupled to the differential pair, wherein the first impedance matching element provides DC impedance matching from the inverting input and non-inverting input of the differential pair
Implementation Method 2
a second impedance matching element coupled to the differential pair, wherein the second impedance matching element provides AC impedance matching from the inverting input and non-inverting input of the differential pair
Implementation Method 3
a differential amplifier having an inverting input and a non-inverting input
Data Source
AI summary
A circuit for receiving an input signal is described. The receiver comprises a first receiver input configured to receive a first input of a differential input signal; a second receiver input configured to receive a second input of a differential input signal; a differential pair having an inverting input and a non-inverting input; a first impedance matching element coupled to the differential pair, wherein the first impedance matching element provides DC impedance matching from the inverting input and non-inverting input of the differential pair; and a second impedance matching element coupled to the differential pair, wherein the second impedance matching element provides AC impedance matching from the inverting input and non-inverting input of the differential pair.