When evaluating noise performance, the noise value can differ when components are analyzed independently versus when they are included as part of a complete signal chain. While datasheet noise values describe individual components in isolation, system‑level noise is influenced by gain distribution, bandwidth, filtering, and interactions between stages. Signal Chain Designer (SCD), a design tool for simulating circuit performance, helps bridge this gap by allowing noise to be evaluated at both the single‑stage and full signal‑chain levels, helping explain why the numbers don’t always match.
Stage Noise and Signal Chain Noise Plots in Signal Chain Designer
Why Stage and Signal Chain Noise Can Differ
Here are a few key factors that influence how noise can differ depending on how and where it is viewed.
- Reference Point Matters - Noise can be referred to either the input or the output of a stage. As gain and attenuation are applied across the chain, the same noise source can appear very different depending on the reference point. Signal Chain Designer lets you choose whether total noise is input‑referred or output‑referred, making it easier to interpret results consistently across stages.
Viewing Input-referred and Output-referred Noise in Signal Chain Designer
- Filtering Changes Noise Bandwidth - Adding filters in your signal chain can help limit noise bandwidth, which reduces integrated RMS noise. In a full signal chain view, noise from earlier stages is shaped by the succeeding filter, an effect that isn’t visible when a stage is evaluated alone. Viewing noise at the signal‑chain level in SCD clearly shows how filter placement impacts overall noise performance.
Viewing the Signal Chain Noise and How Adding a Filter Affected it in Signal Chain Designer
- Loading of the Next Stage - At the stage level, the noise performance of a block, such as an amplifier, can be influenced by the input impedance of the next stage. Signal Chain Designer allows you to either include or ignore this loading effect, helping you understand how real system interactions affect noise.
Viewing the Amplifier Stage Noise and How Input Impedance of the Next Stage Affects it in Signal Chain Designer
- Aliasing Effects in Sampled Systems - In ADC‑based signal chains, out‑of‑band noise can fold back into the signal band due to aliasing. The amount of aliased noise depends on sampling rate and filter placement, which can cause noise values to differ between stage‑level and signal‑chain views.
Example Scenario where Stage and Signal Chain Noise Differ (Brought by Aliasing)
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- Signal Chain Designer includes an option to enable or disable aliasing in the noise spectrum, making this effect easy to visualize and understand.
Viewing the Effect of Aliasing in Noise Spectrum Plots in Signal Chain Designer
Signal Chain Designer is intended for system‑level exploration, not just individual block evaluation. Viewing noise at the signal‑chain level reveals how architecture choices, gain distribution, filter placement, and ADC configuration interact in ways that are not obvious from single‑stage analysis.
If stage‑level noise numbers don’t match what you see in the full signal chain, check the following before assuming an error:
- Check the noise reference point (if input or output-referred).
- Consider the effects of filtering and ADC sampling.
- Use Signal Chain Designer to validate system‑level performance, then export to LTspice for detailed verification as needed.
To learn more about this topic and see a walkthrough on how SCD can help you in checking both stage and signal chain noise and how they can differ, check out this video.