Noise spectral density (NSD) expresses an ADC’s input-referred noise power per hertz of bandwidth. It helps compare converters at different sample rates and estimate noise in a specific signal band. It is a useful complement to SNR and ENOB, not a complete measure of converter quality.
What ADC noise spectral density measures
NSD describes noise power normalized to bandwidth. For a Nyquist-rate ADC, the noise is distributed across the Nyquist bandwidth, from DC to half the sampling rate, or fs/2. Rather than quoting noise only as a total across that bandwidth, NSD states its level per 1 Hz.
Analog Devices author Ian Beavers defines it this way: “NSD defines the entire noise power, per unit of bandwidth, sampled at an ADC input.” The 1 Hz reference is a normalization convention, not a claim that the ADC is measured using a literal 1 Hz FFT bin.
What the units mean
- dBFS/Hz expresses noise power density relative to the ADC’s full-scale power, normalized to 1 Hz. It is relative to the converter’s full-scale reference, not an absolute input power.
- dBm/Hz expresses absolute input-referred noise power density. Converting a full-scale-relative figure to dBm/Hz requires the ADC’s full-scale input power and the applicable impedance or a direct measurement; the unit labels alone are not interchangeable.
Keep the reference and conditions with any NSD figure. Two values in dBFS/Hz may use different full-scale references or test conditions, while dBm/Hz depends on the stated input-power basis.
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How to estimate noise in an application bandwidth
For approximately flat, white noise, integrating a power density over bandwidth adds the bandwidth’s logarithmic contribution. If an ADC has a flat NSD of N dBFS/Hz across a bandwidth of B Hz, the integrated noise power is approximately N + 10 log10(B) dBFS. For example, integrating over 1 MHz adds 60 dB to the per-hertz power-density value.
This estimate assumes the noise is approximately flat across the band and that the bandwidth being integrated is the one relevant to the application. If the spectrum is shaped, or digital filters alter the passband, integrate the actual noise spectrum through the applicable bandwidth or filter response instead. Be consistent about whether a figure represents power or amplitude; the conversion is not the same.
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Why NSD complements SNR and ENOB
SNR and ENOB summarize converter performance under a particular signal, sample rate and measurement bandwidth. They remain useful, but a single SNR value can be difficult to compare across different sampling conditions because total noise is measured over different bandwidths.
NSD normalizes noise by bandwidth, making it more useful when the system has a defined signal band or when comparing converters that run at different sample rates. Texas Instruments’ SBAA625A brief presents NSD as a better metric for modern data converters when the system concern is noise within a defined bandwidth. “Better” here is application-dependent: SNR and ENOB still answer other useful questions, and NSD does not include every performance limit.
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| Metric | What it summarizes | Useful when |
|---|---|---|
| NSD, in dBFS/Hz or dBm/Hz | Noise power per unit bandwidth, with the reference indicated by the unit | Estimating noise in a specified band or comparing bandwidth-normalized noise across sampling conditions |
| SNR | Signal power relative to noise power under stated test conditions | Assessing signal-to-noise performance for a particular signal and measurement setup |
| ENOB | Effective resolution inferred from converter performance, commonly tied to a stated measurement | Summarizing effective converter resolution under the specified test conditions |
For Nyquist-rate ADCs under comparable conditions, doubling the sample rate doubles the Nyquist bandwidth. If approximately the same total noise is spread across that wider band, the noise density falls by about 3 dB. This is a bandwidth-normalization effect, not evidence by itself that the converter’s in-band noise has improved.
Does FFT length change the ADC noise floor?
Changing FFT record length changes bin width and how noise appears across displayed bins; it does not change the ADC’s underlying spectral noise density. Analog Devices notes that changing FFT sampling depth does not alter an ADC’s NSD. A narrower bin can display less integrated noise per bin even though the converter itself has not become quieter.
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FFT plots can also reflect windowing, averaging and processing choices. Processing gain and filtering can lower the measured in-band floor when the system has excess bandwidth and rejects out-of-band noise. That displayed, processed floor should not be confused with the converter’s intrinsic density or compared without matching measurement conditions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What else to check when comparing ADCs
NSD is not a standalone quality score. A low noise density may not help if distortion, spurs, clock jitter, input bandwidth, power constraints or application filtering set the system limit. Compare converters using the conditions that matter to the design:
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- All-Digital IF Technology
- Frequency Range from 9 kHz up to 2.1 GHz
- -161 dBm/Hz Displayed Average Noise Level (Typ.)
- -98 dBc/Hz @10 kHz Offset Phase Noise (1 GHz, Typ.)
- 1 Hz Minimum Resolution Bandwidth (RBW)
- Noise in the actual band: Check the NSD across the signal band, not just a headline number that may represent a different range.
- Sample rate and usable analog bandwidth: Confirm the intended input frequency is within the converter’s supported analog bandwidth and that the stated sampling conditions match the application.
- Noise shape and filtering: Determine whether noise is flat, shaped or affected by digital filtering; a single density figure may not predict integrated noise through a non-flat response.
- Clock-jitter sensitivity: Jitter becomes more restrictive as input frequency rises. In an Analog Devices example from 2017, 200 fs rms clock jitter limits SNR to about 70 dB at a 250 MHz input, while a 1 GHz input requires 50 fs rms or less for the same 70 dB SNR.
- Reference and test conditions: Distinguish dBFS/Hz from dBm/Hz and retain the full-scale reference, measurement bandwidth and other stated test conditions with each number.
Analog Devices gave a typical ADC NSD range of −140 to −165 dBFS/Hz in 2017. Treat that as a dated, typical range rather than a specification for every current ADC; the useful comparison is the figure for the actual converter, band and conditions under consideration. AMD’s RF Data Converter documentation also lists NSD in dBFS/Hz and defines SNR separately using RMS signal and noise quantities, underscoring that the metrics are related but not interchangeable.
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