Sigma-Delta ADC Bias Compensation for Low-THD+N Microphone Input
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Solution Overview
Problem
Sigma-delta analog-to-digital converters (ADCs) with single-ended microphones suffer from suboptimal common mode rejection (CMR) and high Total Harmonic Distortion Plus Noise (THD+N), particularly in applications with longer wires or adaptive active noise canceling, which increases power consumption.
Innovation Solution
Incorporating a pair of operational transconductance amplifiers (OTAs) with a differential digital-to-analog converter (DAC) and a bias compensation circuit that measures and applies biasing conditions between OTAs to reduce THD+N, allowing for better CMR performance by converting single-ended microphone signals into differential inputs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If a single-ended input is used with ground as differential input in sigma-delta ADC, then the circuit provides high integration and low power consumption, but the common mode rejection (CMR) is generally not optimal
Solution Approach 1:
The single-ended input signal is segmented into two differential paths by using a pair of OTAs. The first OTA processes the microphone signal directly, while the second OTA processes the same signal through an inverted path. This segmentation allows the system to maintain differential operation for better CMR while keeping the input single-ended for low power consumption.
Solution Approach 2:
The bias compensation circuit introduces asymmetric control to the differential pair of OTAs. By measuring the biasing condition of one OTA and applying it to the other OTA with appropriate inversion, the system creates symmetric differential operation from an asymmetric single-ended input, thereby improving common mode rejection.
2Device complexity
If biasing conditions of OTAs are not compensated in sigma-delta ADC, then the circuit operation is simple, but Total Harmonic Distortion Plus Noise (THD+N) is high
Solution Approach 1:
The bias compensation circuit implements feedback by measuring the biasing condition (such as current or voltage) of one OTA and using this information to adjust the biasing of the other OTA. This feedback mechanism ensures that both OTAs operate at optimal bias points, reducing THD+N while adding minimal complexity to the overall circuit operation.
Solution Approach 2:
The bias compensation circuit acts as an intermediary between the two OTAs. It measures the biasing condition of one OTA and applies the compensated biasing to the other OTA, mediating the interaction between the differential pair to ensure balanced operation and minimize distortion.
3Adaptability or versatility
If longer wires or cables are used with microphone and inputs including adaptive active noise canceling, then the application functionality is enhanced, but common mode noise increases power consumption
Solution Approach 1:
The invention inverts the traditional approach by using a single-ended input with ground reference instead of a differential input. By processing the single-ended signal through a differential OTA pair with bias compensation, the system achieves better noise rejection for longer cables while maintaining low power consumption, effectively doing things 'the other way round' compared to conventional differential input designs.
Data Source
AI summary
Embodiments of sigma-delta analog-to-digital converter (ADC) circuits and a microphone circuit are disclosed. In an embodiment, a sigma-delta ADC circuit includes a pair of operational transconductance amplifiers (OTAs), a filter connected to the pair of OTAs, a quantizer connected to the filter, a differential digital-to-analog converter (DAC) connected to the quantizer, and a bias compensation circuit configured to measure a biasing condition of a first OTA of the pair of OTAs and to apply the biasing condition of the first OTA to a second OTA of the pair of OTAs to reduce Total Harmonic Distortion Plus Noise (THD+N) in the sigma-delta ADC circuit. An output of a microphone and a differential output of the differential DAC are inputted into input terminals of the pair of OTAs.