Digital Multimeter Input Buffer for High Impedance Noise
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Solution Overview
Problem
Digital multimeters face challenges in accurately measuring small amplitude DC voltages across high impedance circuits in electrically noisy and geographically remote environments, such as cathodic protection systems, due to noise interference and the presence of high resistive elements in measurement circuits.
Innovation Solution
The development of digital multimeters with ultra-high input impedance and aggressive noise attenuation capabilities, featuring a multi-megaohm input resistor and a variable gain, variable bandwidth input buffer with selectable feedback paths, which enables precise measurement of small DC signals amidst significant noise and allows for low power consumption to operate in remote locations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional measurement circuits are used to measure small DC voltages, then the measurement circuit is simpler and lower power is consumed, but noise interference from high impedance circuits and resistive elements causes measurement errors
Solution Approach 1:
The input buffer operates in two distinct modes: a measurement mode with ultra-high input impedance (10^12 to 10^15 ohms) for accurate voltage measurement, and a test mode with lower input impedance for testing and calibration. This dynamic switching resolves the contradiction by providing high precision only when needed for actual measurements, while allowing simpler operation during testing.
Solution Approach 2:
The system dynamically changes the input impedance parameter from ultra-high (10^12 to 10^15 ohms) during measurement to lower values during test modes. This parameter change allows the circuit to achieve high measurement precision for small DC voltages while maintaining flexibility for different operational requirements, resolving the contradiction between precision and complexity.
2Measurement precision
If ultra-high input impedance is used to measure small DC voltages accurately, then measurement precision improves, but power consumption increases
Solution Approach 1:
The input buffer dynamically switches between measurement mode (ultra-high impedance, higher power) and test mode (lower impedance, lower power). This dynamic operation resolves the contradiction by consuming high power only during brief measurement intervals when precision is critical, while operating at lower power during extended test and standby periods.
Solution Approach 2:
The system employs periodic measurement cycles where the ultra-high impedance measurement mode is activated only when needed, followed by transitions to lower power test modes. This periodic operation pattern allows the system to achieve necessary measurement precision while averaging lower power consumption over time, resolving the contradiction between precision and power usage.
3Measurement precision
If aggressive noise attenuation is implemented to measure small DC signals in noisy environments, then measurement precision improves, but device complexity and power consumption increase
Solution Approach 1:
The noise attenuation circuitry operates dynamically, activating aggressive filtering and signal processing only during measurement mode when small DC signals need to be extracted from noise. During test modes and transitions, the noise attenuation is reduced or disabled, lowering power consumption while maintaining measurement precision when actually needed.
Solution Approach 2:
The system employs periodic measurement windows where aggressive noise attenuation is activated only during brief intervals when measurements are taken. Between measurements, the system operates in lower power modes with reduced noise processing. This periodic activation of noise attenuation resolves the contradiction by providing high signal-to-noise ratio during measurements while averaging lower power consumption overall.
4Adaptability or versatility
If variable gain and variable bandwidth input buffer is used to handle different signal conditions, then adaptability improves, but device complexity increases
Solution Approach 1:
The input buffer dynamically adjusts its gain and bandwidth characteristics based on the operational mode and signal conditions. During measurement mode, the buffer provides high gain and narrow bandwidth optimized for small DC signals. During test modes, different gain and bandwidth settings are applied. This dynamic adaptation provides versatility for different signal conditions while using a single reconfigurable buffer circuit rather than multiple dedicated circuits, resolving the contradiction between adaptability and complexity.
Data Source
AI summary
A digital multimeter receives a primary signal including a small amplitude DC component and an induced AC noise component with a magnitude exceeding a magnitude of the DC component. A 100 MΩ input resistor may couple an primary signal to signal processing circuitry that attenuates and filters the primary signal, convert a unipolar signal to a differential signal, and provide the differential output signal to an analog to digital converter. The multimeter may be configurable to couple any one or more of multiple available feedback paths to provide a plurality of available gain-bandwidth configurations. The multimeter may include a second input configurable with two or more feedback path impedances. The multimeter may include a second stage filter, shared by the primary and shunt inputs, that provides additional filtering and converts the signal to a differential signal. The second stage filter may support two or more available impedance configurations.