Feedback Integrator Circuit for Stable IR Sensor AFE Settling
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Solution Overview
Problem
Current analog front-end (AFE) configurations for multichannel infrared (IR) sensor systems face challenges in stabilizing loop bandwidth and settling speed due to variations in photocurrent and environmental conditions, leading to amplifier saturation and instability.
Innovation Solution
The proposed AFE incorporates a precharge mode using switches to isolate the amplifier during transient periods, and a feedback integrator with an NMOS transistor and capacitors to maintain unity gain feedback, shifting the dominant pole to improve stability and reduce noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the loop bandwidth is increased to obtain rapid settling, then the settling speed is improved, but the gain bandwidth of the amplifier is severely limited by the resistor ratio
Solution Approach 1:
The patent divides the I2V converter into two separate integrators: a first integrator for signal integration and a second integrator for feedback control. This segmentation allows independent optimization of each integrator's bandwidth and settling characteristics, resolving the contradiction between rapid settling and amplifier bandwidth limitations.
Solution Approach 2:
The patent introduces a feedback transconductor as an intermediary element that mediates between the photocurrent input and the integrator feedback. This transconductor converts voltage feedback signals into current, enabling precise control of the feedback loop without directly loading the integrator capacitor, thus maintaining fast settling while avoiding bandwidth limitations.
2Measurement precision
If the photocurrent is several orders of magnitude larger than the received pulse, then the receiver transresistance is improved, but the settling of the photocurrent is significantly reduced due to saturation of the amplifier
Solution Approach 1:
The patent applies a precharge mode that activates before the actual measurement phase. During precharge, the integrator capacitor is precharged to the expected DC level of the photocurrent, and the feedback transconductor is enabled to establish the feedback loop in advance. This preliminary action prevents amplifier saturation when the large photocurrent is applied, allowing the system to maintain high transresistance without stability issues.
Solution Approach 2:
The patent implements a feedback mechanism where the output of the first integrator is fed back through a feedback transconductor to the input of the first integrator. This feedback loop automatically adjusts to maintain the integrator output at a virtual ground, preventing saturation even when handling large photocurrents several orders of magnitude larger than the received pulse signal.
3Adaptability or versatility
If the front end loop bandwidth varies depending on the photocurrent, then the adaptability to different current levels is improved, but the stability of the system is reduced
Solution Approach 1:
The patent employs dynamic control of the feedback transconductor gain based on the detected photocurrent level. The system automatically adjusts the transconductor's transfer function to maintain optimal loop bandwidth across varying photocurrent conditions. This dynamic adaptation ensures stability while allowing the system to handle different current levels effectively.
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 configuration enhances the stability and settling speed of the AFE, reducing amplifier saturation and maintaining accurate touch detection in IR sensor systems by precharging capacitors and using feedback to control the current source, independent of photocurrent bias.
Implementation Method 1
a capacitor coupled between the inverting input of the amplifier and the second resistor
Implementation Method 2
a transistor coupled between the current source and the second resistor
Implementation Method 3
an input terminal configured to receive an input signal from a optical receiver
Data Source
AI summary
An apparatus is provided. There is an input terminal that is configured to receive an input signal from a optical receiver and an output terminal. First and second integrators are coupled between the input and output terminals. In the second integrator, there is a current source that is coupled to the input terminal, a first resistor that is coupled to the output terminal, and a second resistor. Also, there is an amplifier having a first input, a second input, and an output, where the first resistor is coupled to the first input of the amplifier and where the second input of the amplifier is configured to receive a reference voltage. A transistor is coupled between the current source the second resistor and is coupled to the output of the amplifier at its gate. A capacitor is also coupled between the first input of the amplifier and the second resistor.