Signal Chain Analysis Focus on Audio Distortion in Conversion Processes

Signal Chain Analysis Focus on Audio Distortion in Conversion Processes

Reducing Noise in High-Speed Analog-to-Digital Converter Signal Chains

In the realm of high-speed, high-performance analog-to-digital converter (ADC) systems, managing noise is crucial for preserving signal integrity and maintaining converter performance. Here's a breakdown of the key principles for minimizing noise in ADC signal chains.

1. Noise Bandwidth Definition and Control

Noise bandwidth, different from the traditional –3 dB bandwidth, is a rectangular bandwidth used to integrate noise power. For instance, a 150 MHz bandwidth defined by a filter actually corresponds to a noise bandwidth of about 183 MHz (1.22 times the 150 MHz), increasing the integrated noise. Designers should use the noise bandwidth rather than –3 dB bandwidth when calculating expected noise levels and selecting filtering stages to reduce noise entering the ADC.

2. Resistor Noise

Thermal (Johnson) noise from resistors is proportional to their resistance and noise bandwidth. To minimize resistor noise at ADC inputs, keep resistor values low where possible, and use precision, low-noise resistors. Using lower resistor values at input nodes reduces thermal noise power, directly lowering noise contributions to the ADC input.

3. Amplifier Noise Contributions

Amplifiers preceding ADCs add voltage and current noise. To minimize amplifier noise at the ADC input, select low-noise amplifiers optimized for the required bandwidth and impedance. The amplifier bandwidth should be limited to the needed signal bandwidth using filters to reduce wideband noise. Proper matching of amplifier input impedance and careful layout reduce noise coupling.

4. Clocking Noise

ADC clock signals can inject jitter and switching noise, degrading ADC noise performance. Noise minimization includes careful clock design, shielding, and separation of clock traces from sensitive analog inputs. Using low-jitter clock sources and differential clocking helps reduce noise pickup and distortion at ADC inputs.

5. Power-Supply Noise

Power supplies can introduce ripple and switching noise coupling into ADC analog inputs. Use local decoupling capacitors placed very close to the ADC power pins to filter noise. Employ low-noise voltage regulators or dedicated analog power domains for the ADC. Physically separate analog and digital grounds, and use star grounding or split planes with single-point connections to avoid ground loops.

Practical Applications at Each ADC Input Node

• Define the noise bandwidth carefully, using appropriate anti-aliasing filters to limit the noise frequency range reaching the ADC.
• Use low-value, low-noise resistors in input networks to minimize resistor noise.
• Select and configure low-noise amplifiers tailored to the expected signal bandwidth and impedance.
• Implement meticulous clock signal routing and filtering to minimize jitter and switching noise at the ADC.
• Ensure robust power supply filtering very close to the ADC pins, separating analog and digital ground and power domains to reduce noise coupling.

By collectively reducing noise convolving onto the ADC output spectrum, these principles preserve dynamic range and fidelity in high-speed, high-performance ADC systems. For example, for a 1 kΩ resistor, resistor noise is equal to 4 nV/√(Hz) for 1-Hz bandwidth. To quantify the regulator's noise and its effect on the ADC, the designer can use the regulator's measured noise band, which is usually found in the regulator's datasheet.

Understanding signal-chain noise tradeoffs and root causes of noise can lead to easier design tradeoffs upfront when designing high-speed signal chains. In this example, a two-pole inductor-capacitor filter (second-order system) for the AAF implementation was used, yielding a noise bandwidth of 128.1 MHz. Regulator noise can couple through the power-supply pins of the ADC or any active device that requires a power-supply connection. The noise spectral density of 150 dB/Hz is challenging for many noise sources in high-speed radio-frequency (RF) analog signal chains.

References

[1] Analog Devices, Inc. (2021). Minimizing Noise in Analog-to-Digital Converter (ADC) Signal Chains. Retrieved from https://www.analog.com/en/articles/minimizing-noise-in-analog-to-digital-converter-adc-signal-chains.html

[2] Texas Instruments (2021). Analog-to-Digital Converter (ADC) Noise. Retrieved from https://www.ti.com/analog/products/adc/noise.html

Technology plays a significant role in minimizing noise in high-speed analog-to-digital converter (ADC) signal chains. For instance, designers can use low-noise amplifiers, precision resistors, and low-jitter clock sources, all of which are advancements offered by modern technology.

Moreover, understanding the principles of noise bandwidth control, reducing resistor and amplifier noise, dealing with clocking and power-supply noise are crucial steps in utilizing technology effectively to improve the performance of high-speed ADC systems.

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