ADC Reference Circuit Using Precharged Capacitor for Fast Settling
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Solution Overview
Problem
Analog to digital converters face challenges in providing a stable and accurate voltage reference, especially at high sampling rates, due to the need for fast settling of the reference signal, which often requires high-power circuitry that consumes excessive chip space and power.
Innovation Solution
The use of a low-bandwidth amplifier in combination with a large capacitor or a comparator and switch circuit to maintain the reference voltage, allowing for quick settling with minimal power consumption by reducing the need for charge from an external reference during high-speed conversions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If a fast, wide bandwidth on-chip amplifier is used to drive the reference input to the ADC, then the reference signal settles quickly to the required accuracy, but the power consumption increases significantly
Solution Approach 1:
The patent applies preliminary action by pre-charging a capacitor to the reference voltage level before the ADC conversion phase. This capacitor is charged during a calibration phase when the reference signal is stable, storing energy that can be quickly transferred to the ADC reference input during conversion. This eliminates the need for a high-power fast amplifier during the critical conversion phase, as the pre-charged capacitor provides the reference voltage instantaneously.
Solution Approach 2:
The patent introduces a capacitor as an intermediary energy storage element between the reference voltage source and the ADC reference input. This capacitor acts as a buffer that can be charged slowly from a low-power source and then discharge quickly to provide the reference voltage during ADC conversion. The capacitor mediates between the low-power reference source and the high-speed ADC requirements, eliminating the need for a high-power amplifier.
2Use of energy by moving object
If a low-bandwidth amplifier is used instead of a fast amplifier, then the power consumption is reduced, but the reference signal cannot settle quickly enough for high-speed ADC conversions
Solution Approach 1:
The patent uses preliminary action by performing the reference voltage preparation in advance during a calibration phase. A low-bandwidth amplifier charges a capacitor to the required reference voltage level when time is not critical. During the subsequent ADC conversion phase, the pre-charged capacitor provides the reference voltage instantaneously, achieving fast settling without requiring a high-bandwidth amplifier.
Solution Approach 2:
The patent implements periodic action by alternating between a calibration phase (where the reference capacitor is charged) and a conversion phase (where the capacitor provides the reference). This periodic operation allows the system to use a low-bandwidth amplifier for charging during calibration, then switch to capacitor discharge mode during conversion, achieving both low power consumption and fast reference settling.
3Measurement precision
If off-chip voltage reference sources are used to provide temperature compensated references, then the reference accuracy is improved, but additional circuitry is required on-chip to isolate the reference from dynamic loading and ensure sufficient settling
Solution Approach 1:
The patent extracts the energy storage function from the complex isolation circuitry and places it in a dedicated capacitor. By using a capacitor to store the reference voltage energy, the system simplifies the isolation requirements. The capacitor can be switched between the reference source and the ADC, providing both isolation during charging and fast reference provision during conversion, thereby reducing the complexity of continuous isolation circuitry.
Solution Approach 2:
The patent uses periodic switching between calibration and conversion phases to simplify isolation circuitry. During calibration, the capacitor is connected to the off-chip reference source for charging. During conversion, the capacitor is switched to provide the reference to the ADC, isolating the off-chip reference from dynamic loading. This periodic switching eliminates the need for complex continuous isolation circuitry.
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
This approach reduces power consumption and simplifies the design by allowing the reference signal to settle quickly with minimal charge drawn from the external reference, improving the stability and accuracy of ADC conversions while reducing chip space requirements.
Implementation Method 1
a relatively large capacitor connected to the same node as the amplifier output and connectable to the ADC reference input. The capacitor is charged substantially to the external reference voltage
Data Source
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AI summary
The present disclosure provides alternative solutions to the problem of providing a stable voltage reference to high speed ADCs that possess high sampling rates. In one example the high speed amplifier is replaced by a smaller, slower, lower power amplifier in combination with a relatively large capacitor connected to the same node as the amplifier output and the ADC reference input. The capacitor is charged substantially to the external reference voltage and hence keeps the reference input of the ADC almost at the external reference voltage between conversions, such that when conversion is about to occur and the external reference is switched in then very little charge is required from the external reference, and hence the reference signal quickly settles. An alternative arrangement is to replace the amplifier with a comparator that takes as one of its inputs the external reference signal, and as the other of its inputs the internal reference to the ADC, and makes use of a control circuit that adjusts the threshold of the comparator from bit-trial to bit-trial until the internal reference is brought up to substantially the same signal level as the external reference. When the external reference is then switched in to supply the ADC circuit it settles very quickly and draws very little power therefrom.