Differential Capacitive Sensor Common-Mode Control Circuit

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Solution Overview

Problem

Fully differential detection circuits for differential capacitive sensors face challenges in reducing common-mode noise and current consumption, particularly in low supply voltage applications, where the high feedback capacitance required for noise rejection can increase output noise and current consumption, making them unsuitable for portable devices.

Innovation Solution

A passive input-common-mode control circuit is introduced, which generates a balanced common-mode charge to counteract the common-mode signal, reducing noise and current consumption, and an optional active feedback loop can be used to compensate for capacitance mismatches, allowing for smaller feedback capacitance and further reducing noise and power consumption.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If high feedback capacitance is used in fully differential detection circuits, then common-mode noise rejection is improved, but output noise and current consumption increase

Engineering Contradiction:

Improvecommon-mode noise rejectionVSAvoidcurrent consumption

Core Design Contradiction:

Object-affected harmful factorsVSUse of energy by moving object

Solution Approach 1:

The feedback capacitance is segmented into two separate capacitances (first feedback capacitance and second feedback capacitance) connected to different inputs of the differential sensor. This segmentation allows the circuit to achieve common-mode noise rejection through the differential configuration while using smaller individual capacitance values, thereby reducing the total charge transfer and current consumption compared to using a single large feedback capacitance.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent applies different capacitance values locally to different inputs of the differential sensor based on the specific common-mode noise conditions. By optimizing the first and second feedback capacitances independently for each input, the circuit achieves effective common-mode rejection while minimizing the overall capacitance required, thus reducing power consumption in low supply voltage applications.

Inventive Principle:

Principle #3Local quality

2Object-affected harmful factors

If high feedback capacitance is used in fully differential detection circuits, then common-mode noise rejection is improved, but output noise increases

Engineering Contradiction:

Improvecommon-mode noise rejectionVSAvoidoutput noise

Core Design Contradiction:

Object-affected harmful factorsVSManufacturing precision

Solution Approach 1:

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent employs feedback mechanisms where the output signal is fed back through the first and second feedback capacitances to the respective inputs. This feedback action helps to stabilize the output and reduce noise by continuously adjusting the input signals to counteract noise variations, thereby improving output noise performance while maintaining common-mode rejection.

Inventive Principle:

Principle #23Feedback

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

The passive input-common-mode control circuit effectively reduces noise and current consumption, maintaining performance while enabling the use of differential capacitive sensors in low power portable devices, and the optional active feedback loop ensures stability across technological variations.

Implementation Method 1

the interface circuit comprises a first feedback capacitance having a first capacitance value, and a second feedback capacitance having a second capacitance value

Methodology Applied

Scientific EffectCapacitance: Capacitance

Data Source

PatentUS9201090B2Detection circuit using a differential capacitive sensor with input-common-mode control in a sense interface

Publication Date: 2015.12.01 STMICROELECTRONICS SRL

AI summary

A detection circuit is provided with a differential capacitive sensor and with an interface circuit having a first sense input and a second sense input, electrically connected to the differential capacitive sensor. Provided in the interface circuit are: a sense amplifier connected at input to the first sense input and to the second sense input and supplying an output signal related to a capacitive unbalancing of the differential capacitive sensor; and a common-mode control circuit, connected to the first sense input and to the second sense input and configured to control a common-mode electrical quantity present on the first sense input and on the second sense input. The common-mode control circuit is of a totally passive type and is provided with a capacitive circuit, which is substantially identical to an equivalent electrical circuit of the differential capacitive sensor and is driven with a driving signal in phase opposition with respect to a read signal supplied to the differential capacitive sensor.