Capacitive Voltage Divider Circuit for Quiescent-Free Measurement
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Solution Overview
Problem
Conventional methods for measuring battery or super-capacitor voltage in low power devices result in quiescent current discharge, which is inefficient and inaccurate due to parasitic influences and process variations.
Innovation Solution
A voltage measurement circuit utilizing a capacitive voltage divider (CVD) and analog-to-digital converter (ADC) with a control circuit to measure bandgap/reference voltage, charge capacitors to known voltages, and apply code values to calculate the actual voltage, eliminating quiescent current and accounting for parasitic capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional voltage measurement methods are used, then voltage can be measured, but quiescent current is generated that discharges the battery
Solution Approach 1:
The measurement circuit operates periodically by charging the capacitor to the battery voltage only when measurement is needed, then disconnecting it. This periodic charging action eliminates continuous quiescent current while still providing voltage measurement capability when required.
Solution Approach 2:
A capacitor is introduced as an intermediary energy storage element between the battery and the measurement circuit. The capacitor temporarily stores the battery voltage during measurement and isolates the battery from continuous current draw, acting as a buffer that enables measurement without direct continuous connection.
2Measurement precision
If conventional measurement circuits are used, then voltage measurement is achieved, but parasitic influences and process variations reduce accuracy
Solution Approach 1:
The circuit measures both the battery voltage (Vbat) and the reference voltage (Vref) using the same ADC and capacitor, then calculates the actual battery voltage by compensating for parasitic effects through the ratio of measured values. This feedback-based calculation approach eliminates the need for precise knowledge of parasitic capacitances and resistance values.
Solution Approach 2:
The invention changes the measurement parameter from directly measuring battery voltage through a potentially biased circuit to measuring the ratio of battery voltage to reference voltage. By using the reference voltage as a comparison baseline, the system compensates for process variations and parasitic influences that would otherwise affect absolute voltage measurements.
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
Accurately measures battery or super-capacitor voltage without quiescent current, providing precise readings by compensating for parasitic influences and process variations, suitable for low power devices with tight power requirements.
Implementation Method 1
a first capacitor (C1) to a first voltage (V1) and a second capacitor (C2) to a second voltage (V2), wherein C1 and C2 are flying capacitors of the ADC
Data Source
Figure 1~2
Figure 3
AI summary
A method for measuring a voltage using a capacitive voltage divider (CVD) and an analog-to-digital converter includes the steps of measuring a bandgap or reference voltage and determining a first code value of the bandgap or reference voltage, charging a first capacitor to a voltage to be measured and determining a second code value of voltage of the first capacitor, charging a second capacitor to a second known voltage and determining a third code value of voltage of the second capacitor, and determining the voltage to be measured by applying the first, second, and third code values.