Sub-sampling Weakly-Driven Nodes With Source-Follower Buffer
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Solution Overview
Problem
Traditional on-chip sampling circuits are not suitable for sampling weakly-driven or floating nodes in semiconductor devices, as they often result in significant switching effects, high leakage currents, and clock feed-through, making it difficult to accurately monitor voltages at these nodes.
Innovation Solution
A method and apparatus for voltage sub-sampling of weakly-driven nodes, which includes isolating the signal using a source-follower amplifier to reduce leakage, employing a sample and hold circuit with master and slave pass gates, and using a biasing circuit for calibration, allowing for accurate sampling of high-frequency signals without significant distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If traditional sampler designs are used for weakly-driven nodes, then the sampling circuit can be simple, but switching effects become large compared to signal amplitude
Solution Approach 1:
A source-follower amplifier is introduced as an intermediary buffer between the weakly-driven node and the sampling circuit. This buffer isolates the node from the sampling circuit's switching effects and leakage currents, allowing accurate sampling without directly coupling the sampler to the weak driver.
Solution Approach 2:
The sampling circuit is divided into multiple stages: a source-follower amplifier stage for buffering, followed by a sampling stage with master and slave pass gates. This segmentation allows each stage to be optimized independently - the buffer handles the weak signal while the sampler handles the sampling function.
2Device complexity
If traditional sampler designs are used for weakly-driven nodes, then the sampler structure can be simple, but leakage currents become unacceptably high
Solution Approach 1:
The source-follower amplifier acts as an intermediary that prevents direct leakage paths from the sampling circuit to the weakly-driven node. The high input impedance of the source-follower minimizes loading effects and leakage current draw from the node being sampled.
3Device complexity
If traditional sampler designs are used for weakly-driven nodes, then the circuit can be simple, but clock feed-through effects occur
Solution Approach 1:
The source-follower amplifier serves as a buffer that isolates the clock signals from the weakly-driven node. The sampling operation occurs at the amplifier's output rather than directly at the node, preventing clock feed-through from contaminating the original signal source.
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
The solution effectively reduces switching effects and leakage currents, enabling accurate sub-sampling of weakly-driven nodes, including those involved in proximity-based communication, without the need for complex calibration circuits, and allows for the reconstruction of original waveforms at lower frequencies for testing purposes.
Implementation Method 1
Sampling a weakly-driven node may in some embodiments include isolating the signal to be sampled from diffusion regions in order to reduce degradation of the signal due to leakage. In some embodiments, this isolation may be implemented by passing the signal though a source-follower amplifier before presenting it to the input of the sample and hold circuit of the sampler.
Implementation Method 2
The sample and hold circuit of the sampler may in some embodiments include a master pass gate and a slave pass gate, clocked on opposite phases of a sampling clock.
Implementation Method 3
An amplifier may convert the measured voltage to a current, which may in some embodiments be multiplied using a current mirror. The output of the sampler may be a current communicated off-chip, such as for testing purposes.
Implementation Method 4
The biasing circuit used may depend on the desired bias voltage. For example, a PMOS transistor may be used to bias a capacitively-driven node to a high voltage, while an NMOS transistor may be used to bias a capacitively-driven node to ground.
Data Source
AI summary
A method and apparatus for performing on-chip voltage sampling of a weakly-driven node of a semiconductor device are disclosed. In some embodiments, the node is a floating node or is capacitively-driven. In some embodiments, it is involved in proximity-based communication. Sampling the node may include isolating the signal to be sampled using a source-follower amplifier before passing it to the sampling circuit. Sampling the node may include biasing the node to a desired voltage using a leaky transistor or other biasing circuit. In some embodiments, the biasing circuit may also be used to calibrate the sampler by coupling one or more calibration voltages to the node in place of a biasing voltage and measuring the sampler output. The sampler may be suitable for sub-sampling high frequency signals to produce a time-expanded, lower frequency version of the signals. The output of the sampler may be a current communicated off-chip for testing.