ADC Input Stage With Differential Feedback for Channel Gain Matching
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Solution Overview
Problem
Existing A/D converters face challenges in achieving high linearity and low noise while minimizing power consumption, particularly in sensor applications where gain matching between channels is essential to prevent angular errors.
Innovation Solution
The input stage design for A/D converters includes transconductance elements receiving analog input and feedback signals of the same polarity, reducing signal swing and eliminating the need for additional resistors, thereby reducing noise and power consumption, and incorporating a chopper for mode switching between differential and common-mode operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If degeneration of the input transistor pair is used to increase linearity, then linearity is improved, but noise increases
Solution Approach 1:
Instead of applying degeneration (adding resistance) to improve linearity, the patent inverts the approach by using a fully differential architecture where the transconductance elements operate without degeneration. The linearity is achieved through the differential signaling and matching of the fully differential path, eliminating the need for noisy degeneration resistors while maintaining high linearity.
2Measurement precision
If feedback to input of the amplifier is used to improve linearity, then linearity is improved, but power consumption increases
Solution Approach 1:
The patent implements feedback through the fully differential architecture where the output signals are fed back to the input stage. The differential feedback paths enable the amplifier to self-correct non-linearities, achieving high linearity without requiring additional power-consuming correction circuits or heavy degeneration.
3Measurement precision
If feedback to input of the amplifier is used to improve linearity, then linearity is improved, but input impedance becomes finite
Solution Approach 1:
Instead of using a single-ended amplifier with feedback that inherently creates finite input impedance, the patent inverts to a fully differential architecture. The differential input stage presents high impedance to both input signals simultaneously, maintaining high input impedance while achieving linearity through the differential operation rather than feedback alone.
4Measurement precision
If highly linear transconductance elements are used to reduce signal swing, then linearity is improved, but manufacturing complexity increases
Solution Approach 1:
The patent changes the operating parameters of the transconductance elements by operating them in a fully differential mode with balanced signal swings. This parameter change allows the use of standard, easily manufacturable transconductance elements without requiring highly linear specialized devices, as the differential operation inherently cancels non-linearities.
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 high linearity and low noise with reduced power consumption, enabling effective gain matching between channels and suitable for applications requiring high input impedance, such as sensors, without the need for highly linear transconductance elements.
Implementation Method 1
The input stage comprises a first transconductance element adapted to receive, at a first input of the transconductance element, a first analog input signal of an analog input signal differential pair that is to be converted to a digital signal by an A/D converter; a second transconductance element adapted to receive, at a first input of the second transconductance element, a second analog input signal of the analog input signal differential pair
Implementation Method 2
an integrator for integrating a sum of an output current of the first transconductance element and an output current of the second transconductance element, and generating an integrator output signal representative of the integrated sum of output currents
Implementation Method 3
incorporating a chopper for mode switching between differential and common-mode operations
Data Source
Figure 1~2
Figure 3
Figure 4
AI summary
There is described an input stage for an A/D converter, comprising (a) a transconductance element (306, 406) adapted to receive, at a first input (302, 402) of the transconductance element, an analog input signal that is to be converted to a digital signal by the A/D converter, (b) a feedback path for providing an analog feedback signal to a second input (304, 404) of the transconductance element, the analog feedback signal being based on a digital output signal of the A/D converter, and (c) an integrator (308, 408) for integrating an output current of the transconductance element, wherein the integrating element is adapted to generate an integrator output signal representative of the integrated output current. There is also described an A/D converter comprising such an input stage and a system comprising a plurality of such A/D converters.