Switched Capacitor Current Detection Circuit
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Solution Overview
Problem
Conventional current detection circuits face challenges in accurately detecting small currents due to low voltage drops across current detection resistors, which can be exacerbated by thermal noise and offset voltages from operational amplifiers, particularly in portable devices where smaller resistors are preferred to maximize battery capacity.
Innovation Solution
A switched capacitor circuit that inputs the voltage drop across the current detection resistor to an operational amplifier, canceling offset voltages and reducing noise by integrating the voltage drop, thereby improving the signal-to-noise ratio through repeated switching states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a smaller current detection resistor is used to minimize voltage drop and maximize battery capacity, then energy loss is reduced, but measurement precision deteriorates due to smaller voltage drop
Solution Approach 1:
The patent replaces the conventional operational amplifier-based voltage amplification system with a switched capacitor system. The switched capacitor circuit converts the small voltage drop across the current detection resistor into a measurable voltage signal through capacitive switching and integration, eliminating the need for high-gain amplification that would introduce noise. This substitution enables accurate current measurement even with very small voltage drops, thus allowing the use of smaller current detection resistors to minimize energy loss.
2Measurement precision
If an operational amplifier is used to amplify small voltage difference, then measurement precision improves, but reliability deteriorates due to thermal noise and offset voltage
Solution Approach 1:
The patent substitutes the operational amplifier with a switched capacitor circuit that performs signal conditioning through capacitive switching rather than resistive amplification. The switched capacitor integrates the voltage drop signal over time and produces an amplified output without introducing the thermal noise and offset voltage characteristics of operational amplifiers. This is achieved by using capacitors to store and transfer charge proportional to the input voltage, avoiding the noisy resistive elements inherent in op-amp circuits.
Solution Approach 2:
The switched capacitor circuit operates by periodically switching the capacitors between different states (charging, discharging, integrating) at a specific switching frequency. This periodic action allows the circuit to accumulate the voltage drop signal over multiple switching cycles, effectively amplifying the small signal while averaging out noise components. The regular switching pattern enables precise control of the signal processing without the instability introduced by operational amplifiers.
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 circuit effectively detects small voltage differences with enhanced accuracy and reduced noise, allowing for precise current detection while minimizing voltage drop and thermal noise effects, suitable for portable devices.
Implementation Method 1
an operational amplifier can amplify a voltage difference between two ends of a current detection resistor
Implementation Method 2
In general, the resistors generate thermal noise. The amount of generated thermal noise is equal to Vn2=4·k·T·B·R
Data Source
AI summary
An upper end voltage and a lower end voltage of a current detection resistor Rs are supplied, via first and second switches S1 and S2, to one end of a main capacitor Ci. A reference voltage VREF is supplied, via a third switch S3, to the other end of the main capacitor Ci. The operational amplifier OP has a negative input terminal to which a voltage of the other end of the main capacitor Ci is supplied and a positive input terminal to which the reference voltage VREF is supplied. The circuit performs an operation for charging the main capacitor Ci with a voltage corresponding to a difference between the lower end voltage and the reference voltage in a state where the first and third switches S1 and S3 are turned on and the second switch S2 is turned off. The circuit obtains a voltage applied to the other end of the main capacitor Ci which is equivalent to a sum of the reference voltage VREF and a difference between the upper end voltage and the lower end voltage in a state where the first and third switches S1 and S3 are turned off and the second switch S2 is turned on. The circuit detects a voltage difference between two ends of the current detection resistor Rs based on a value obtained by the operational amplifier OP that subtracts the reference voltage VREF from the obtained voltage.