Link: reviewed by Matt Bonaccio on *SoundStage! Hi-Fi* on January 15, 2023

**General Information**

All measurements taken using an Audio Precision APx555 B Series analyzer.

The RME ADI-2 DAC FS was conditioned for 30 minutes at 0dBFS (3.6Vrms out) into 200k ohms before any measurements were taken.

The ADI-2 DAC FS offers three digital inputs: one coaxial S/PDIF (RCA), one optical SPDIF (TosLink), and one USB. There are two sets of line-level outputs (balanced XLR and unbalanced RCA) and two headphone outputs (1/4″ TRS labelled “PHONES” and 1/8″ TRS labelled “IEM”). There is a digital volume control for the headphone and line-level outputs. Comparisons were made between unbalanced and balanced line-level outputs, and aside from the 6dB extra voltage over balanced, no differences were seen in terms of THD and noise.

The ADI-2 DAC FS offers a dizzying array of features and settings. The following are the default settings used for the coaxial input, balanced line-level outputs, PHONES and IEM headphone outputs, using a 0dBFS input, unless otherwise specified:

Line-level output:

- Ref level: +7dBu (3.6Vrms out over XLR)
- Auto ref level: off
- Mono: off
- Width: 1.0 (full stereo)
- M/S proc: off
- Polarity: off
- Crossfeed: off
- DA filter: SD Sharp
- De-emphasis: Auto
- Dual EQ: off
- Volume: 0dB
- Lock volume: off
- Balance: center
- Loopback to USB: off

PHONES output:

- Hi-power: on
- Auto ref level: off
- Mono: off
- Width: 1.0 (full stereo)
- M/S proc: off
- Polarity: off
- Crossfeed: off
- DA filter: SD Sharp
- De-emphasis: Auto
- Dual EQ: off
- Volume: set to output 2Vrms
- Lock volume: off
- Balance: center
- Loopback to USB: off

IEM output:

- Hi-power: off
- Auto ref level: off
- Mono: off
- Width: 1.0 (full stereo)
- M/S proc: off
- Polarity: off
- Crossfeed: off
- DA filter: SD Sharp
- De-emphasis: Auto
- Dual EQ: off
- Volume: set to output 0.5Vrms
- Lock volume: off
- Balance: center
- Loopback to USB: off

There are six digital filter settings, labelled: Short Delay (SD) Sharp, SD Slow, Sharp, Slow, Non-oversampling (NOS), and Brickwall. Here are RME’s descriptions for each:

- SD Sharp: the most linear frequency response and lowest latency
- SD Slow: causes a small drop in the higher frequency range
- Sharp and Slow: same as SD Sharp and SD Slow respectively but with higher latency but linear phase over the entire audioband
- NOS: smallest steepness and therefore affects treble more than the other filters, but offers the best impulse response
- Brickwall: sharp filtering and phase-linear

The ADI-2 DAC FS volume control ranges from -93.8dB to +6dB, in steps of 6dB to 0.5dB (most of the range is 0.5dB steps). Channel-to-channel deviation proved exceptional, at around 0.01dB throughout the range.

**Volume-control accuracy (measured at line-level outputs): left-right channel tracking**

Volume position | Channel deviation |

-93.8dB | 0.01dB |

-60dB | 0.012dB |

-30dB | 0.012dB |

-20dB | 0.013dB |

-10dB | 0.013dB |

0dB | 0.014dB |

+6dB | 0.013dB |

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by RME for the ADI-2 DAC FS compared directly against our own. The published specifications are sourced from RME’s website, either directly or from the supplied manual, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), assume, unless otherwise stated, the coaxial digital input (24/96 1kHz sine wave at 0dBFS), the balanced line-level or unbalanced headphone outputs into 200k ohms (line-level) and 300 ohms (headphone) using a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

Parameter | Manufacturer | SoundStage! Lab |

XLR line-out | ||

SNR (A-weighted, +7dBu, 24/96) | 123dB | 124dB |

Frequency response (16/44.1, SD Sharp filter) | 0-20.7kHz (-0.1dB) | 0-20.2kHz (-0.1dB) |

Frequency response (24/96, SD Sharp filter) | 0-46.1kHz (-0.5dB) | 0-45.5kHz (-0.5dB) |

Frequency response (24/192, SD Sharp filter) | 0 - 91.5kHz (-1dB) | 0 - 88.9kHz (-1dB) |

THD (0dBFS, 24/96) | <0.0001% | <0.00006% |

THD+N (0dBFS, 24/96, A-weighted) | <0.00016% | <0.0001% |

Channel separation (10kHz, 24/96) | >120 dB | 129.2dB |

Output impedance | 200 ohms | 213 ohms |

RCA Line-out | ||

SNR (A-weighted, +7dBu, 24/96) | 122dB | 122dB |

Output impedance | 100 ohms | 108 ohms |

PHONES output | ||

Output impedance | 0.1 ohm | 0.86 ohm |

Output level (0dBFS, Hi-Power, 300 ohm) | 10Vrms | 10.1Vrms |

Output level (0dBFS, Low-Power, 32 ohm) | 1.73Vrms | 1.8Vrms |

SNR (A-weighted, +22dBu, 24/96) | 123dB | 124dB |

THD (+18dBu, 32-ohm load) | <0.0001% | <0.00017% |

THD+N (+18dBu, 32-ohm load, A-weighted) | <0.0002% | <0.00022% |

IEM output | ||

Output level (0dBFS) | 0.55Vrms | 0.57Vrms |

SNR (A-weighted, -3dBu, 24/96) | 121dB | 116dB* |

*measured value is likely under-reporting performance of device under test because analyzer noise and device noise are of similar magnitude

Our primary measurements revealed the following using the coaxial input and the balanced line-level outputs (unless specified, assume a 1kHz sine wave at 0dBFS, 200k ohms loading, 10Hz to 90kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz, 16/44.1) | -120.4dB | -119.9dB |

Crosstalk, one channel driven (10kHz, 24/96) | -129.2dB | -150.5dB |

DC offset | <-0.15mV | <-0.32mV |

Dynamic range (A-weighted, 16/44.1) | 96dB | 96dB |

Dynamic range (unweighted, 16/44.1) | 93.5dB | 93.5dB |

Dynamic range (A-weighted, 24/96) | 124.4dB | 124.5dB |

Dynamic range (unweighted, 24/96) | 115.6dB | 115.7dB |

IMD ratio (CCIF, 18kHz and 19kHz stimulus tones, 1:1, 16/44.1) | <-106dB | <-106dB |

IMD ratio (CCIF, 18kHz and 19kHz stimulus tones, 1:1, 24/96) | <-118dB | <-118dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1, 16/44.1) | <-92dB | <-92dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1, 24/96) | <-107dB | <-110dB |

Maximum output voltage (+13dBu setting, 1% THD) | 9.9Vrms | 9.9Vrms |

Output impedance (XLR) | 213 ohms | 213 ohms |

Output impedance (RCA) | 108 ohms | 108 ohms |

Noise level (A-weighted, 16/44.1) | <56uVrms | <56uVrms |

Noise level (unweighted, 16/44.1) | <75uVrms | <75uVrms |

Noise level (A-weighted, 24/96) | <3.3uVrms | <3.2uVrms |

Noise level (unweighted, 24/96) | <10uVrms | <9.2uVrms |

THD ratio (unweighted, 16/44.1) | <0.00035% | <0.00035% |

THD+N ratio (A-weighted, 16/44.1) | <0.0016% | <0.0016% |

THD+N ratio (unweighted, 16/44.1) | <0.0021% | <0.0021% |

THD ratio (unweighted, 24/96) | <0.00006% | <0.00005% |

THD+N ratio (A-weighted, 24/96) | <0.0001% | <0.0001% |

THD+N ratio (unweighted, 24/96) | <0.00029% | <0.00026% |

Our primary measurements revealed the following using the coaxial input and the headphone output (unless specified, assume a 1kHz sine wave at 0dBFS, 300-ohm loading, 10Hz to 90kHz bandwidth, and 2Vrms output for the PHONES output, and 0.5Vrms for the IEM output):

PHONES output / Hi-Power setting | ||

Parameter | Left channel | Right channel |

Maximum Vrms/0dBFS | 11.4Vrms | 11.4Vrms |

Maximum output power into 600 ohms (1% THD+N, unweighted) | 211mW | 211mW |

Maximum output power into 300 ohms (1% THD+N, unweighted) | 389mW | 389mW |

Maximum output power into 32 ohms (1% THD+N, unweighted) | 1.48W | 1.48W |

Output impedance | 0.66 ohm | 0.86 ohm |

Noise level (A-weighted, 16/44.1) | <33uVrms | <32uVrms |

Noise level (unweighted, 16/44.1) | <48uVrms | <47uVrms |

Noise level (A-weighted, 24/96) | <10uVrms | <7uVrms |

Noise level (unweighted, 24/96) | <22uVrms | <19uVrms |

Dynamic range (A-weighted, 16/44.1, max output) | 96.0dB | 95.9dB |

Dynamic range (A-weighted, 24/96, max output) | 125.1dB | 125.3dB |

THD ratio (unweighted, 16/44.1) | <0.0004% | <0.0004% |

THD+N ratio (A-weighted, 16/44.1) | <0.0017% | <0.0017% |

THD+N ratio (unweighted, 16/44.1) | <0.0024% | <0.0024% |

THD ratio (unweighted, 24/96) | <0.00025% | <0.00019% |

THD+N ratio (A-weighted, 24/96) | <0.00054% | <0.00038% |

THD+N ratio (unweighted, 24/96) | <0.0011% | <0.001% |

IEM output | ||

Parameter | Left channel | Right channel |

Maximum Vrms/0dBFS | 788mVrms | 788mVrms |

Maximum output power into 600 ohms (1% THD+N, unweighted) | 0.99mW | 0.99mW |

Maximum output power into 300 ohms (1% THD+N, unweighted) | 1.97mW | 1.97mW |

Maximum output power into 32 ohms (1% THD+N, unweighted) | 17.9mW | 17.9mW |

Output impedance | 0.66 ohm | 0.86 ohm |

Noise level (A-weighted, 16/44.1) | <8.4uVrms | <8.4uVrms |

Noise level (unweighted, 16/44.1) | <12uVrms | <12uVrms |

Noise level (A-weighted, 24/96) | <0.9uVrms* | <0.9uVrms* |

Noise level (unweighted, 24/96) | <3.2uVrms* | <3.2uVrms* |

Dynamic range (A-weighted, 16/44.1, max output) | 96.0dB | 95.8dB |

Dynamic range (A-weighted, 24/96, max output) | 119.6dB* | 119.7dB* |

THD ratio (unweighted, 16/44.1) | <0.0004% | <0.0004% |

THD+N ratio (A-weighted, 16/44.1) | <0.0016% | <0.0016% |

THD+N ratio (unweighted, 16/44.1) | <0.0022% | <0.0022% |

THD ratio (unweighted, 24/96) | <0.00016% | <0.00013% |

THD+N ratio (A-weighted, 24/96) | <0.00023% | <0.00023% |

THD+N ratio (unweighted, 24/96) | <0.00064% | <0.00064% |

*measured value is likely under-reporting performance of device under test because analyzer noise and device noise are of similar magnitude

**Frequency response (16/44.1, 24/96, 24/192 with SD Sharp filter)**

The plot above shows the ADI-2 DAC FS’s frequency response as a function of sample rate. The blue/red traces are for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple/green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and, finally, orange/pink represents 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same for the all input resolutions—perfectly flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is as expected, offering sharp filtering around 22k, 48k, and 96kHz (half the respective sample rate), although the 24/192 data shows softer attenuation around the corner frequency. The -3dB point for each sample rate is roughly 21.2, 46.6, and 92.6kHz, respectively. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue, purple or orange trace) is performing identically to the right channel (red, green or pink trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

**Frequency response (bass and treble)**

Above are frequency-response plots measured at the balanced outputs with the bass and treble controls set to maximum (blue/red plots) and minimum (purple/green plots). We see that roughly +/- 6dB of gain/cut is available for each.

**Frequency response (16/44.1 with SD Sharp, SD Slow, and Sharp fiters)**

The plots above show frequency responses for a 0dBFS input signal sampled at 44.1kHz for the SD Sharp (blue), SD Slow filter (red), and Sharp (green) filters into a 200k ohm load for the left channel only. The graph is zoomed in from 1kHz to 22kHz to highlight the various responses of the three filters. We can see that the SD Sharp and Sharp filters offer essentially the same frequency response, with a -3dB point at 21.2 kHz. The SD Slow filter offers a much shallower attenuation, with a -3dB point at 17.4kHz.

**Frequency response (16/44.1 with Slow, NOS, and Brickwall fiters)**

The plots above show frequency responses for a 0dBFS input signal sampled at 44.1kHz for the Slow (blue), NOS (red), and Brickwall (green) filters into a 200k ohm load for the left channel only. The graph is zoomed in from 1kHz to 22kHz to highlight the various responses of the three filters. We can see that the Slow filter is similar to the SD Slow filter above, but with a -3dB point at 20kHz. The NOS filter, predictably, exhibits significant high-frequency roll-off with a -0.8dB response at 10kHz, and -3.4dB at 20kHz. The Brickwall filter exhibits the most, *well*, brickwall-type behavior, although with a lower -3dB point (19.8kHz) than the filters shown above.

**Phase response vs. sample rate (16/44.1, 24/96, 24/192 with SD Sharp filter)**

Above are the phase-response plots from 20Hz to 20kHz for the coaxial input, measured at the balanced output, using the SD Sharp filter setting. The blue/red traces are for a dithered 16/44.1 input at 0dBFS, the purple/green for 24/96, and the orange/pink for 24/192. We can see that the ADI-2 DAC FS does not invert polarity, with a worst-case phase shift of 92 degrees at 13kHz for the 16/44.1, 60 degrees at 20kHz for the 24/96 input data, and just 20 degrees of phase shift at 20kHz for the 24/192 input data.

**Phase response vs. filter type (16/44.1 with SD Sharp, SD Slow, and Sharp filters)**

Above are the phase-response plots from 20Hz to 20kHz for a 0dBFS input signal sampled at 44.1kHz for the SD Sharp (blue), SD Slow (red), and Sharp (green) filters into a 200k ohm load for the left channel only. The SD Slow and Sharp filters yielded significantly less phase shift than the default SD Sharp filter, with -10 and +40 degrees respectively of phase shift at 20kHz.

**Phase response vs. filter type (16/44.1 with Slow, NOS, and Brickwall filters)**

Above are the phase-response plots from 20Hz to 20kHz for a 0dBFS input signal sampled at 44.1kHz for the Slow (blue), NOS (red), and Brickwall (green) filters into a 200k ohm load for the left channel only. Predictably, the Brickwall filter yields the highest phase shift at over +180 degrees at 20kHz, the Slow filter is at +120 degrees at 20kHz, while the NOS filter is +80 degrees at 20kHz.

**Digital linearity (16/44.1 and 24/96 data)**

The graph above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the balanced line-level output of the ADI-2 DAC FS. The digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555. The ideal response is a straight flat line at 0dB (*i.e.*, the amplitude of the digital input perfectly matches the amplitude of the measured analog output). The 24/96 input data is essentially pefect down to -120dBFS, while the 16/44.1 input data performed well, only over-responding by 2.5dB at -120dBFS. The 24/96 data yielded such superb results that we extended the sweep down to . . .

**Digital linearity (16/44.1 and 24/96 data)**

. . . -140dBFS. Above we see that even at -140dBFS, the ADI-2 DAC FS is only undershooting by -1 to -2 dB. This is an exemplary linearity test result.

**Impulse response (SD Sharp, SD Slow, Sharp filters)**

The graph above shows the impulse responses for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period, then back to digital silence, fed to the coaxial digital input, measured at the balanced outputs, for the SD sharp (blue), SD Slow (red), and Sharp (green) filters into a 200k ohm load for the left channel only. We can see that the SD Sharp filter has no pre-ringing, but significant post-ringing, the SD Slow filter has only very minor post-ringing, and the Sharp filter exhibits symmetrical pre- and post-ringing, as seen in a typical sinc function.

**Impulse response (Slow, NOS, Brickwall filters)**

The graph above shows the impulse responses for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period, then back to digital silence, fed to the coaxial digital input, measured at the balanced outputs, for the Slow (blue), NOS (red), and Brickwall (green) filters into a 200k ohm load for the left channel only. We can see that the Slow filter has very minor pre- and post-ringing, the NOS filter is as advertised and shows a single pulse with essentially no pre- or post-ringing, and the Brickwall filter exhibits symmetrical pre/post ringing, as seen in a typical sinc function.

**Impulse response (NOS filter)**

We decided to investigate the impulse response of the NOS filter in more detail. The graph above shows the impulse responses for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s), for one sample period then back to digital silence, fed to the coaxial digital input, measured at the balanced outputs, for the NOS (red) filter only into a 200k ohm load for the left channel only. The graph is zoomed in to show that the NOS filter is as advertised, and yields a single pulse with essentially no pre/post ringing.

**J-Test (coaxial input)**

The plot above shows the results of the J-Test test for the coaxial digital input measured at the balanced line-level output of the ADI-2 DAC FS. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (*i.e.*, 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sine wave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least-significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (*e.g.*, 250Hz, 750Hz, 1250Hz, etc). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA (fundamental at 250Hz) down to -170dBrA for the odd harmonics. The test file can also be used in conjunction with artificially injected sine-wave jitter by the Audio Precision, to also show how well the DAC rejects jitter.

The coaxial input shows an extremely clean J-Test result, with only minor peaks at the -150dBrA level. This is an indication that the ADI-2 DAC FS should not be sensitive to jitter through this input.

**J-Test (optical input)**

The plot above shows the results of the J-Test test for the coaxial digital input measured at the balanced line-level output of the ADI-2 DAC FS. The optical input shows an extremely clean J-Test result, with only minor peaks at the -150dBrA level. This is an indication that the ADI-2 DAC FS should not be sensitive to jitter through this input.

**J-Test (coaxial input, 2kHz sine-wave jitter at 500ns)**

The plot above shows the results of the J-Test test for the coaxial digital input (the optical input behaved similarly), measured at the balanced line-level output, with an additional 500ns of 2kHz sine-wave jitter injected by the APx555. The result remains clean with no visible sidebands at 10kHz and 14kHz (12kHz main signal +/-2kHz jitter signal). This is further evidence of the ADI-2 DAC FS’s superb jitter immunity.

**J-Test (coaxial input, 2kHz sine-wave jitter at 900ns)**

The plot above shows the results of the J-Test test for the coaxial digital input (the optical input behaved similarly), measured at the balanced line-level output, with an additional 900ns of 2kHz sine-wave jitter injected by the APx555. Here sidebands are visible at 10kHz and 14kHz (12kHz main signal +/- 2kHz jitter signal), but remain relatively low at -110dBrA. With jitter above this level, the ADI-2 DAC FS lost sync with the signal.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (SD Sharp filter)**

The plot above shows a fast Fourier transform (FFT) of the ADI-2 DAC FS’s balanced line-level output with white noise at -4dBFS (blue/red) and a 19.1kHz sine wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using the SD Sharp filter setting. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of the brick-wall-type reconstruction filter. There are absolutely no imaged aliasing artifacts in the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is at -105dBrA.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (SD Slow filter)**

The plot above shows a fast Fourier transform (FFT) of the ADI-2 DAC FS’s balanced line-level output with white noise at -4dBFS (blue/red) and a 19.1kHz sine wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using the SD Slow filter setting. The slow roll-off above 20kHz in the white-noise spectrum shows the implementation of a reconstruction filter with a slow roll-off. Despite this, there are absolutely no imaged aliasing artifacts in the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is near -10dBrA.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (Sharp filter)**

The plot above shows a fast Fourier transform (FFT) of the ADI-2 DAC FS’s balanced line-level output with white noise at -4dBFS (blue/red) and a 19.1kHz sine wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using the Sharp filter setting. The sharp roll-off above 20kHz in the white noise spectrum shows the implementation of the brick-wall-type reconstruction filter. There are absolutely no imaged aliasing artifacts in the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is at -105dBrA.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (Slow filter)**

The plot above shows a fast Fourier transform (FFT) of the ADI-2 DAC FS’s balanced line-level output with white noise at -4dBFS (blue/red) and a 19.1kHz sine wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using the Slow filter setting. The slow roll-off above 20kHz in the white-noise spectrum shows the implementation of a reconstruction filter with slow roll-off. Despite this, there are absolutely no imaged aliasing artifacts in the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is near -10dBrA.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (NOS filter)**

The plot above shows a fast Fourier transform (FFT) of the ADI-2 DAC FS’s balanced line-level output with white noise at -4dBFS (blue/red) and a 19.1kHz sine wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using the NOS filter setting. Due to the lack of a filter, the noise spectrum is mostly constant (*i.e.*, un-attenuated), except at multiples of the 44.1kHz sample rate. Despite this, the imaged aliasing artifacts in the audioband at 13.2kHz is only at -120dBrA. The primary aliasing signal at 25kHz is near -5dBrA.

**Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (Brickwall filter)**

The plot above shows a fast Fourier transform (FFT) of the ADI-2 DAC FS’s balanced line-level output with white noise at -4dBFS (blue/red) and a 19.1 kHz sine wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green), using the Brickwall filter setting. The very sharp roll-off above 20kHz in the white noise spectrum shows the implementation of the brick-wall-type reconstruction filter. There are absolutely no imaged aliasing artifacts in the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is at roughly -100dBrA.

**THD ratio (unweighted) vs. frequency vs. load (24/96)**

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for a 24/96 dithered 1kHz signal at the coaxial input. The 200k- and 600-ohm data are nearly identical throughout the audioband, which is in indication that ADI-2 DAC FS’s outputs are robust and can handle loads below 1k ohms with no difficultly. The right channel does outperform the left by about 5dB from 20Hz to 1kHz; however, at these THD levels (0.00005% to 0.0001%), the differences are of absolutely no consequence. It should also be noted that these THD ratios are pushing up against the limits of the AP analyzer, which exhibits just under 0.00002% THD at 3.6Vrms in loopback mode. Above 3kHz, there is a rise in THD, up to 0.0004% at 20kHz.

**THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1 and 24/96)**

The chart above shows THD ratios at the balanced line-level output into 200k ohms as a function of frequency for a 16/44.1 (blue/red) and a 24/96 (purple/green) dithered 1kHz signal at the coaxial input. The 24/96 data consistently outperformed the 16/44.1 data by about 10dB from 20Hz to about 2kHz due to the inherently higher noise floor at 16 bits (*i.e.*, the analyzer cannot assign a THD value below the noise floor). THD ratios with 16/44.1 data range from 0.0002% at 20Hz, down to 0.0001% at 5kHz, back up to 0.0004% at 20kHz. THD ratios with 24/96 data range from 0.00005% at 20Hz, up to 0.0004% at 20kHz.

**THD ratio (unweighted) vs. output (16/44.1 and 24/96)**

The chart above shows THD ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green). Once again, the 24/96 outperformed the 16/44.1 data, with a THD range from 0.2% to 0.00005%, while the 16/44.1 ranged from 5% down to nearly 0.0002%.

**THD+N ratio (unweighted) vs. output (16/44.1 and 24/96)**

The chart above shows THD+N ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green). The 24/96 outperformed the 16/44.1 data, with a THD+N range from 3% down to 0.0003%, while the 16/44.1 ranged from 40% down to 0.002% at the maximum output voltage of 3.6Vrms.

**THD ratio (unweighted) vs. output (24/96) at maximum gain**

The chart above shows THD ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS at 24/96 (blue/red), this time with the ADI-2 DAC FS gain set to maximum (*i.e.*, Ref Level set +19dBu, volume set to +6dB). The THD ratios ranged from 0.1% at 0.5mVrms, down to 0.00005% at the “knee” at 7Vrms, with the 1% THD mark hit at roughly 10Vrms at the output.

**THD+N ratio (unweighted) vs. output (24/96) at maximum gain**

Similar to the chart above, this chart shows THD+N ratios (the addition of noise to THD) measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS at 24/96 (blue/red), with the ADI-2 DAC FS gain set to maximum (*i.e.*, Ref Level set +19dBu, volume set to +6dB). The THD+N ratios ranged from 2% at 0.5mVrms, down to 0.0003% at the “knee” at 7Vrms, with the 1% THD+N mark hit at roughly 10Vrms at the output.

**FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1. We see no sign of signal harmonics above the -135dBrA noise floor. There are also no power-supply noise peaks to speak of to the left of the main signal peak.

**FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96. Due to the increased bit-depth, the noise floor is much lower compared to the 16/44.1 FFT, at a very low -160dBrA. We see very low signal harmonics ranging from -130dBrA, or 0.00003%, at 7kHz, down to below -150dBrA, or 0.000003%. Here also, there are no power-supply noise peaks to speak of to the left of the main signal peak.

**FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1 at -90dBFS. We see the signal peak at the correct amplitude, and no signal harmonics above the noise floor within the audioband.

**FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)**

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/961 at -90dBFS. We see the signal peak at the correct amplitude, and no signal harmonics above the noise floor within the audioband.

**Intermodulation distortion vs generator level (SMPTE, 60Hz:4kHz, 4:1, 16/44.1, 24/96)**

The chart above shows intermodulation distortion (IMD) ratios measured at balanced output for 16/44.1 (blue/red) and 24/96 input data (purple/green), from -60dBFS to 0dBFS. Here, the SMPTE IMD method is used, where the primary frequency (F1 = 60Hz), and the secondary frequency (F2 = 7kHz), are mixed at a ratio of 4:1. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 16/44.1 data yields IMD ratios from 2% down to 0.002% at 0dBFS. The 24/96 data yields IMD ratios from 0.1% down to 0.0003% at 0dBFS.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 16/44.1)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the balanced output into 200k ohms for the coaxial input at 16/44.1. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 3.6Vrms (0dBrA) at the output. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz), as well as the third-order modulation products, at 17kHz and 20kHz, are not visible above the -135dBrA noise floor.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 24/96)**

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the balanced output into 200k ohms for the coaxial input at 24/96. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 3.6Vrms (0dBrA) at the output. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at -135dBrA, or 0.00002%, for the left channel (right channel peak cannot be seen above -150dBrA noise floor), while the third-order modulation products, at 17kHz and 20kHz, are slightly higher, at around -120dBrA to -130dBrA, or 0.0001% to 0.00003%. This is an exceptionally clean IMD result.

*Diego Estan*

Electronics Measurement Specialist