MOS-Switched Resistor Divider for Precise Low-Noise Trimming
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Solution Overview
Problem
Conventional variable resistor circuits face challenges in achieving high accuracy and reducing thermal noise while maintaining a low layout area, particularly when applied to differential amplifiers, due to the need for low resistance values and high common mode rejection ratio (CMRR) requirements.
Innovation Solution
A variable resistor circuit design featuring resistor unit circuits with a first resistor connected in parallel to a second resistor in series with a MOS switch, and a third resistor in series with the unit circuits, allowing for resistance value changes through the on/off states of MOS switches, enabling precise trimming and reducing the number of resistors needed, thus minimizing layout area and thermal noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional variable resistor circuits are used to achieve high accuracy and low thermal noise, then resistance value precision is improved, but layout area increases
Solution Approach 1:
The variable resistor circuit is segmented into multiple resistor unit circuits (first, second, third, and fourth resistor unit circuits) with different resistance change ratios. Each unit circuit contains resistors and switching elements that can be independently controlled, allowing the total resistance to be adjusted in precise increments while maintaining a compact layout by distributing the resistance adjustment across multiple smaller units rather than requiring one large resistor.
2Area of stationary object
If the number of resistors is reduced to minimize layout area, then layout area is improved, but resistance value change precision deteriorates
Solution Approach 1:
The circuit employs switching elements (such as transistors) that can dynamically change the resistance values of the resistor units by switching between different connection states. This dynamic switching capability allows a small number of physical resistors to achieve multiple resistance values, providing fine adjustment precision without requiring a large number of resistors that would increase layout area.
Solution Approach 2:
The circuit achieves multiple resistance values by changing the connection parameters and switching states of the switching elements rather than using multiple fixed resistors. By varying which switching elements are on or off, the effective resistance of each resistor unit circuit changes, enabling precise resistance adjustment with fewer physical components.
3Object-generated harmful factors
If low resistance values are used to reduce thermal noise, then thermal noise is reduced, but common mode rejection ratio requirements become more difficult to meet
Solution Approach 1:
Different resistor unit circuits are designed with different resistance change ratios tailored to specific positions in the differential amplifier circuit. The first and second resistor unit circuits (with larger resistance change ratios) are configured for coarse adjustment where larger resistance changes are acceptable, while the third and fourth resistor unit circuits (with smaller resistance change ratios) are configured for fine adjustment where precision is critical. This local optimization allows low resistance values to be used throughout, reducing thermal noise, while still meeting CMRR requirements through position-specific adjustment capabilities.
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 design achieves a high degree of resistance value change precision with a smaller number of resistors, improving accuracy and reducing thermal noise, while maintaining a compact layout, effectively enhancing the CMRR and accuracy in differential amplifiers.
Implementation Method 1
a first resistor connected in parallel to a second resistor in series with a MOS switch
Data Source
AI summary
A variable resistor circuit is configured to change its resistance value based on the resistance value change ratio given by (Δ/RA)×100% in each resistor unit circuit, where A represents the resistance value change in each resistor unit circuit between a first combined resistance value of that resistor unit circuit including at least the resistance value of the resistor unit circuit with a MOS switch off and a second combined resistance value of the resistor unit circuit including at least the resistance value of the resistor unit circuit with the MOS switch on, and RA represents the sum of the total combined resistance values of the plurality of resistor unit circuits with the MOS switches in all the resistor unit circuits off and the resistance value of the third resistor.