Capacitive AC Amplifier Front End for Flat DMM Frequency Response
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Solution Overview
Problem
Current digital multimeter (DMM) designs face challenges in achieving a flat frequency response and fast settling time due to parasitic capacitances and resistances, leading to increased noise and complexity, especially when handling high-impedance signals.
Innovation Solution
A front-end circuit for DMMs replaces resistive attenuators with strictly capacitive attenuators, utilizing an inverting programmable-gain capacitive amplifier built around an operational amplifier with a single resistor for DC feedback, providing a broad bandwidth and minimizing parasitic effects, and optionally using RC networks or additional op-amps for improved response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If resistive attenuators are used in the AC signal path, then signal attenuation is achieved, but parasitic capacitances and resistances cause non-flat frequency response and increased noise
Solution Approach 1:
The patent removes the resistive attenuator from the AC signal path entirely, extracting the harmful resistive element that causes parasitic effects. The attenuation function is replaced by a capacitive attenuator that does not introduce the same parasitic resistance-capacitance interactions, thereby eliminating the need for complex compensation circuits while maintaining frequency response flatness
Solution Approach 2:
The patent changes the fundamental parameter of the attenuator from resistive to capacitive. By using a capacitive attenuator instead of a resistive one, the patent alters the electrical characteristics of the signal path to eliminate parasitic RC time constants that cause frequency response deviations, achieving flat response without additional compensation components
2Reliability
If high-impedance (1 MΩ or higher) environment is used, then signal source loading is reduced, but maintaining desired frequency response becomes challenging
Solution Approach 1:
The patent introduces a capacitive attenuator as an intermediary element between the high-impedance signal source and the measurement circuitry. This capacitive attenuator serves as a mediator that maintains the high-impedance characteristic (reducing signal source loading) while avoiding the parasitic RC effects that would otherwise degrade frequency response in high-impedance environments
3Measurement precision
If measured signal is small relative to ADC dynamic range, then measurement sensitivity is improved, but signal must be amplified which increases noise
Solution Approach 1:
The patent performs preliminary signal conditioning by using the capacitive attenuator to establish proper signal levels before amplification. By pre-adjusting the signal attenuation in a noise-free capacitive stage, the subsequent amplifier operates at optimal signal levels, minimizing the need for high gain amplification and thereby reducing amplifier noise contribution
4Adaptability or versatility
If signal attenuation is performed before buffering, then dynamic range matching is achieved, but heavy load is presented to signal source
Solution Approach 1:
The patent applies local quality by making the attenuator capacitive rather than resistive. This localized change in the attenuator's electrical characteristic allows the circuit to present a high-impedance (low-loading) interface to the signal source while still achieving the necessary signal attenuation for dynamic range matching with the ADC
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 achieves a flat frequency response without adjustments, reduces noise, and simplifies design while maintaining high bandwidth, eliminating the need for complex compensation techniques and minimizing DC load on input signals.
Implementation Method 1
an inverting programmable-gain capacitive amplifier built around an operational amplifier
Implementation Method 2
featuring a single resistor for DC feedback providing a 1st-order response for AC coupling
Data Source
AI summary
A front-end circuit suitable for digital multimeters and measurement devices may be configured with strictly capacitive attenuators replacing the resistive attenuators in the AC signal path. An inverting programmable-gain amplifier (PGA) may be built around an operational amplifier (op-amp), with a single resistor for DC feedback providing a 1st-order response for AC coupling. The single resistor may be replaced by a more complex RC network to provide a 2nd-order response. The feedback circuit may also include active components such as additional op-amps, which may assist in shaping the overall response and/or reducing the DC offset at the output of the circuit. The capacitive load presented to the input by the front-end circuit may be minimized to compare to capacitances present in resistive implementations. The circuit would not present a DC load to the input since DC loads are typically the result of a resistive attenuator in the DC signal path.