Photodiode Current Sample-and-Hold Circuit for Background Offset Range
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Solution Overview
Problem
Conventional current sample-and-hold circuits are limited in offsetting a wide range of background photocurrents due to the risk of capacitor voltage becoming too large, which reduces the efficiency of photodiodes used in heart rate detection circuits, especially when light intensity or photodiode area changes.
Innovation Solution
A current sample-and-hold circuit with a first transconductance amplifier having adjustable transconductance is used, connected between the capacitor and the photodiode, allowing the transconductance to be adjusted to maintain a stable capacitor voltage while offsetting larger background photocurrents, incorporating a bias circuit and electronic switches for control and feedback.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a differential amplifier is used to amplify the differential signal from the bridge circuit, then the output signal level is improved, but offset voltage and temperature dependence are introduced
Solution Approach 1:
A virtual ground node is introduced as an intermediary between the bridge circuit and the differential amplifier. This virtual ground, maintained at a stable reference potential through operational amplifier feedback, mediates the signal transmission while blocking offset voltage and temperature drift from propagating to the output. The virtual ground serves as a stable reference point that eliminates the harmful effects of amplifier offset and thermal drift.
2Measurement precision
If the sampling resistor value is increased to improve signal level, then the output signal is improved, but power consumption and circuit time constant increase
Solution Approach 1:
The circuit dynamically changes the effective sampling resistance through capacitive coupling and switching mechanisms. During the sampling phase, a low resistance path is provided to quickly charge the holding capacitor with minimal power consumption. During the holding phase, the capacitor maintains the voltage without requiring continuous current flow. This parameter switching allows achieving adequate signal level without the continuous high power consumption that would result from using a permanently high-value sampling resistor.
3Device complexity
If a single operational amplifier is used for both signal amplification and offset compensation, then device complexity is reduced, but signal integrity is compromised
Solution Approach 1:
The signal processing function is segmented into distinct stages: the first operational amplifier handles offset compensation and virtual ground maintenance, while the second operational amplifier handles signal amplification. This segmentation allows each amplifier to be optimized for its specific function, preventing offset and drift from being amplified along with the signal, thereby maintaining signal integrity while managing device complexity through functional separation.
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 solution enables the current sample-and-hold circuit to effectively offset a wider range of background photocurrents without significant changes in capacitor voltage, maintaining photodiode efficiency across varying light intensities and photodiode areas.
Implementation Method 1
a photodiode is generally used for receiving reflected light from human body, and then an integrator is utilized to convert an induced current of the photodiode into a voltage signal
Implementation Method 2
a current output circuit in the current sample-and-hold circuit is formed by a P-channel Metal Oxide Semiconductor (PMOS) transistor M of a fixed size. The PMOS transistor M, in a sampling stage, is used for sampling the background photocurrent of the photodiode PD and converting the sampled current into a voltage of a capacitor CSH
Data Source
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AI summary
Some embodiments of this disclosure provide a current sample-and-hold circuit and a sensor. The current sample-and-hold circuit is used for offsetting a background photocurrent of a photodiode, and includes a capacitor and a first transconductance amplifier which has adjustable transconductance and is used for outputting a sampled current to the photodiode to offset the background photocurrent of the photodiode; one end of the capacitor is connected with a power supply VDD, the other end of the capacitor is connected with one end of the first transconductance amplifier; and the other end of the first transconductance amplifier is connected with the photodiode to output the sampled current to the photodiode to offset the background photocurrent of the photodiode. According to the embodiments of this disclosure, compared to existing technologies, the first transconductance amplifier with the adjustable transconductance is arranged in the current sample-and-hold circuit, and when the background photocurrent of the photodiode is increased, a change of a voltage of the capacitor within a large range can be avoided by increasing the transconductance of the first transconductance amplifier, so that the current sample-and-hold circuit can offset a larger background photocurrent.