Programmable Capacitor Array Switching for Low-Distortion Linearity
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Solution Overview
Problem
Programmable capacitor arrays experience linearity degradation due to the non-linearity of switching devices, particularly when MOSFET transistors are in the off state, leading to distortion of input signals.
Innovation Solution
A low distortion programmable capacitor array is implemented using control circuitry to manage the MOSFET switch, ensuring it remains on continuously when loading capacitance and briefly turns on when unloading, while periodically shorting the drain voltage to the source voltage to minimize linearity degradation, thereby preventing the turn-on of p-well or p-substrate diodes at high frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If MOSFET switches are used in the programmable capacitor array, then the capacitance can be programmably connected or disconnected from the input, but the non-linearity of the MOSFET in the off state causes distortion and degradation of the input signal
Solution Approach 1:
A diode-connected MOSFET is introduced as an intermediary component between the input signal and the programmable capacitor array. This diode-connected device acts as a buffer that linearizes the input signal by providing a consistent impedance interface, preventing the non-linear effects of the switching MOSFETs from directly affecting the signal. The diode connection ensures that the input signal sees a predictable impedance regardless of the switching state of other MOSFETs in the array.
Solution Approach 2:
The invention changes the operating parameters of the MOSFET switches by controlling their gate voltages to specific levels. When a MOSFET is turned off, its gate voltage is maintained at a level that ensures the device remains in a non-conducting state with minimal leakage and non-linear effects. This parameter control allows the MOSFET to function as an ideal switch while minimizing signal distortion during the off state.
2Adaptability or versatility
If the MOSFET is turned off to remove capacitance from the input, then the programmable capacitance function is achieved, but linearity degradation occurs on the input signal
Solution Approach 1:
The diode-connected MOSFET maintains an equipotential condition at the input node by providing a consistent voltage reference. When other MOSFETs in the array are turned off to remove capacitance, the diode-connected device ensures that the input node remains at a stable potential, preventing voltage fluctuations and maintaining signal linearity. This equipotential buffering effect decouples the input signal from the switching actions of other array elements.
Solution Approach 2:
The diode-connected MOSFET provides beforehand cushioning by establishing a protective impedance buffer before the signal encounters the potentially non-linear switching MOSFETs. This pre-established buffer cushions the input signal against the non-linear effects that would otherwise occur when MOSFETs are turned off, ensuring linearity is maintained even as capacitance is removed from the array.
3Manufacturing precision
If the MOSFET is periodically turned on to prevent linearity degradation, then signal linearity is improved, but additional switching operations are required
Solution Approach 1:
The diode-connected MOSFET serves multiple functions simultaneously: it provides input impedance buffering, linearizes the signal path, and stabilizes the input node voltage. This multi-functional component eliminates the need for separate linearization circuits or complex switching control mechanisms, as the diode connection inherently provides all necessary functions through its passive electrical characteristics.
Data Source
Figure 1A~1B
Figure 2A~2B
Figure 3~4
AI summary
In one example embodiment, a programmable capacitor array (300) is provided for low distortion and minimizing linearity degradation of an input (Vin) by utilizing control circuitry (110) to switch on and off an array of MOSFET switches. The control circuitry (110) turns on a MOSFET (104) to load a capacitance (C1) on Vin and turns off the MOSFET (104) to remove the capacitance (C1) from Vin in response to a Din control signal. When the intention is to load the capacitance (C1) with Vin, the MOSFET is left on continuously. When the intention is to remove or unload the capacitance (C1) from Vin, the MOSFET (104) is primarily turned off, however, the MOSFET (104) is still periodically turned on with appropriate voltage levels in response to a clock signal (CLK) for periods of time when the loading of the capacitance (C1) on Vin is tolerable to the system, thereby ensuring minimal linearity degradation of Vin due to the programmable capacitor array system.