Low-Noise Current Source Using Switched Capacitor Offset Cancellation
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Solution Overview
Problem
Current sources in switched capacitor circuits face challenges in minimizing noise and offset variations due to internal and external factors, making it difficult to provide a constant, noiseless direct current.
Innovation Solution
The proposed solution involves a current source configuration using transconductors with a cascode structure and capacitors to reduce noise and offset, where switches operate in clamp and gain modes to refresh and filter noise, and a resistor is used to increase output impedance, minimizing noise and offset contributions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional current source configurations are used in switched capacitor circuits, then the circuit structure is simple, but noise and offset variations increase
Solution Approach 1:
The current source is divided into two distinct transconductors (first and second transconductors) with separate functions. The first transconductor handles the primary current conversion while the second transconductor specifically addresses noise and offset issues. This segmentation allows each component to be optimized for its specific function, reducing overall noise and offset variations without requiring complete redesign of the entire circuit.
Solution Approach 2:
The first transconductor acts as an intermediary element between the input voltage and the output current. By introducing this intermediate conversion stage, the circuit can better control and filter noise and offset variations before they reach the final output. The first transconductor converts input voltage to an intermediate current that is then processed by the second transconductor, effectively isolating and reducing harmful noise and offset effects.
2Object-affected harmful factors
If switches operate continuously to refresh noise, then noise reduction is improved, but energy consumption increases
Solution Approach 1:
The switches in the circuit operate periodically rather than continuously, refreshing the noise filtering function at specific intervals. The first and second transconductors are activated in alternating phases, with switches periodically switching between clamp mode and gain mode. This periodic operation maintains effective noise reduction while significantly reducing energy consumption compared to continuous operation, as the switches remain inactive during non-refresh periods.
Data Source
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Figure 4~6
AI summary
In one example embodiment, a current source is provided to limit noise and offset. In one embodiment, a source transistor is provided, with current sourced at the drain. A feedback network runs from the source node to the gate. The feedback network produces voltage gain by a transconductance, such as a transistor. Appropriate capacitors are also provided, and two pairs of switches are disposed to provide offset cancelation by toggling between gain and clamp modes in the switched capacitor architecture.