Fluorescence Detector Gain Gating to Prevent Amplifier Saturation
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Solution Overview
Problem
Current fluorescence detection systems face challenges with amplifier saturation and non-linearity due to high spectral power density of excitation sources, leading to degraded bandwidth and linearity, especially when detecting materials with short fluorescence lifetimes.
Innovation Solution
A pulsed light source is gated ON and OFF with a control signal, coupled with a two-stage amplifier system where the first stage is a transimpedance amplifier with reduced gain during excitation and balanced source impedances to minimize charge injection effects, and the second stage has programmable gain and offset for digital control, preventing saturation and non-linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the gain of the amplifier is increased to detect weak fluorescence signals, then the sensitivity is improved, but the amplifier saturates due to the high spectral power density of the excitation source
Solution Approach 1:
The patent applies periodic action by using a pulsed excitation source that is gated ON and OFF, and synchronously switching the amplifier gain between low and high states. During the excitation pulse (ON state), the gain is reduced to prevent saturation from the high spectral power density. During the OFF state, the gain is increased to detect the weak fluorescence emission signals. This periodic switching resolves the contradiction between detection sensitivity and amplifier linearity.
2Speed
If the bandwidth of the detection system is increased to capture short lifetime fluorescence signals, then the detection speed is improved, but the noise and crosstalk increase
Solution Approach 1:
The system uses periodic gating synchronized to the excitation pulse timing. The detector operates at high bandwidth during the excitation pulse to capture fast decay signals, but the gain is reduced during this period to minimize noise and crosstalk from the excitation source. During the OFF period, the gain is increased to detect weak signals. This synchronized periodic operation resolves the contradiction between detection speed and noise/crosstalk levels.
3Device complexity
If a single light source is used to simplify the system, then the device complexity is reduced, but the power and pulse width control precision become challenging
Solution Approach 1:
The patent applies dynamics by implementing an active control system that dynamically adjusts the drive current to the single light source based on feedback from a reference detector. The controller monitors the actual pulse width and power output, and adjusts the drive signal to maintain precise control. This dynamic control mechanism enables precise pulse width and power control with a single light source, resolving the contradiction between device simplicity and control precision.
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 minimizes distortion and crosstalk, achieving a decaying exponential voltage output with improved bandwidth, linearity, and dynamic range, enabling accurate detection of resonant fluorescence signals with minimal instrumental artifacts.
Implementation Method 1
a single light emitting diode (LED) or multiple LEDs with sufficient power and a sufficiently short duration or pulse width
Implementation Method 2
Resonant fluorescence emissions from the test material are coupled into a photodiode that converts the radiation to an electrical current
Implementation Method 3
The current is amplified in an amplifier system, which may comprise first and second stage amplifiers. The first stage amplifier may be a current-to-voltage converter
Data Source
AI summary
A light source is gated ON and OFF in response to a pulsed signal. Photo emissions from the light source are coupled to a material under test. Resonant fluorescent emissions from the material are coupled to a photodiode. Current from the photodiode is coupled into an amplifier system comprising a first and second amplifier stages. The first amplifier stage is gated to a low gain when the light source is turned ON and the gain is increased when the light source goes from ON to OFF. The second amplifier stage has digitally programmable offset and gain settings in response to control signals. The output of the second amplifier stage is digitized by an analog to digital converter. A controller generates the pulse control signal and the control signals.