Logarithmic ADC Circuit for Wide-Range APD Current Monitoring
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
The dynamic range of avalanche photodiode (APD) current poses challenges in designing circuits for real-time monitoring, as it requires a wide range of current measurement, which is difficult to achieve with existing technologies.
Innovation Solution
A logarithmic current-to-voltage conversion using an NPN-type bipolar transistor as both an amplifier and comparator, integrated with an ADC circuit, allowing for compact and accurate monitoring within a small module space.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a linear ADC circuit is used to monitor APD current, then the circuit can measure current accurately, but it cannot handle the wide dynamic range of 10,000:1
Solution Approach 1:
The patent transforms the APD current measurement from a linear scale to a logarithmic scale by using a bipolar transistor in its exponential region. The transistor converts the wide dynamic range current (10,000:1) into a compressed voltage range that can be accurately measured by a standard 8-bit ADC, effectively changing the measurement parameter from linear current to logarithmic voltage.
Solution Approach 2:
The bipolar transistor serves as an intermediary device between the APD current source and the ADC converter. It acts as a log converter that mediates the transformation of the wide dynamic range current signal into a compressed voltage signal suitable for digital conversion, bridging the gap between the current's dynamic range and the ADC's measurement capability.
2Measurement precision
If a high-resolution ADC is used to cover the full dynamic range, then measurement accuracy is improved, but circuit complexity and size increase
Solution Approach 1:
By changing the measurement parameter from linear current to logarithmic voltage through the bipolar transistor, the patent enables the use of a simple 8-bit ADC instead of a complex high-resolution ADC. This parameter transformation allows standard low-resolution converters to achieve effective high precision across the wide dynamic range.
Solution Approach 2:
The patent replaces the need for complex high-resolution ADC hardware with a simpler system consisting of a bipolar transistor log converter followed by a standard 8-bit ADC. This substitution achieves the same measurement capability with reduced circuit complexity and smaller size.
3Area of stationary object
If the module space is reduced for integration, then ease of integration is improved, but measurement accuracy may deteriorate
Solution Approach 1:
The patent merges the log conversion function and ADC conversion function into a single integrated circuit block. The bipolar transistor log converter and the 8-bit ADC are combined in one module, reducing the overall module space while maintaining measurement accuracy through the logarithmic compression approach.
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 compresses the dynamic range of APD current, achieving 3.663% relative accuracy with an 8-bit quantizer across four decades, reducing circuit complexity and size while maintaining accuracy, suitable for integration with monolithic power switchers in optical communications.
Implementation Method 1
The pre-amplifying transistor is configured to receive an input voltage at the base that varies logarithmically; and produce an output voltage at the collector according to a comparison of a reference voltage and a difference between the input voltage and a voltage at the emitter
Data Source
AI summary
An electronic circuit comprises an analog-to-digital converter (ADC) circuit. The ADC circuit includes a pre-amplifying transistor and a quantizer circuit. The pre-amplifying transistor includes a base, an emitter and a collector. The pre-amplifying transistor is configured to receive an input voltage at the base that varies logarithmically; and produce an output voltage at the collector according to a comparison of a reference voltage and a difference between the input voltage and a voltage at the emitter. The quantizer circuit is operatively coupled to the pre-amplifying transistor and is configured to generate a digital value for the input voltage using the output voltage produced by the pre-amplifying transistor.