Common-Mode Control in Sample-and-Hold Circuits With AC-Coupled Inputs
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Sample-and-hold circuits in analog-to-digital converters (ADCs) face issues with common mode voltage drift due to leakage currents, which can cause the common mode voltage to drop towards 0 volts, disrupting the operation of the circuit.
Innovation Solution
A common mode controller is introduced to monitor and maintain the common mode voltage within a desired range by providing a current to counteract leakage currents, using a differential amplifier and resistors to establish and sustain the correct voltage level, ensuring the circuit operates within a predefined common mode voltage range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If AC coupling is used to provide differential input signals to the sample-and-hold circuit, then the circuit can operate with differential signals, but the common mode voltage drifts towards 0 volt due to leakage currents
Solution Approach 1:
The patent implements a common mode feedback mechanism where the common mode voltage is continuously monitored and compared against a reference voltage. The error signal generated by this comparison controls a feedback amplifier that adjusts the common mode voltage level, counteracting the drift caused by leakage currents and maintaining stable operation
Solution Approach 2:
The patent introduces a common mode feedback amplifier as an intermediary component between the input stage and the rest of the circuit. This amplifier acts as a mediator that detects common mode voltage deviations and generates corrective signals to maintain the proper common mode level, isolating the effects of leakage currents from the signal path
2Device complexity
If the common mode voltage is allowed to drop towards 0 volt due to leakage currents, then the circuit structure remains simple, but the sample-and-hold circuit cannot operate correctly
Solution Approach 1:
The common mode feedback system operates autonomously to maintain proper common mode voltage levels. The feedback amplifier automatically detects voltage drops caused by leakage currents and generates corrective signals without external intervention, allowing the circuit to self-correct and maintain reliable operation
Solution Approach 2:
The patent dynamically adjusts the common mode voltage parameter through feedback control. By continuously monitoring the common mode voltage and adjusting it via the feedback amplifier, the system maintains the voltage within the required range despite variations in leakage currents, ensuring reliable operation
Data Source
AI summary
A common mode controller circuit (60) for maintaining a common mode voltage (Vcm) at a first node (52) and a second node (54) in a sample-and-hold circuit receiving a pair of AC coupled differential input signals (Vinp, Vinn) includes first and second resistors (R1/R2) and third and fourth resistors (R3/R4), each set of resistors connected in series between the first and second nodes, and a differential amplifier (A1) having an inverting input terminal coupled to a third node (62) between the first and second resistors, a non-inverting input terminal coupled to a reference voltage (Vref) and an output terminal coupled to a fourth node (64) between the third and fourth resistors. The common mode voltage is sampled at the third node and the differential amplifier provides a sourcing output current indicative of the difference between the sampled common mode voltage and the reference voltage to drive the fourth node.