Texas Instruments ADS9120, User Manual, Page: 19 / 38

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ADS9120EVM-PDK Operation
19 SBAU262 – August 2016
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ADS9120EVM-PDK
6.4 Spectral Analysis Tool
The spectral analysis tool is intended to evaluate the dynamic performance (SNR, THD, SFDR, SINAD,
and ENOB) of the ADS9120 SAR ADC through single-tone sinusoidal signal FFT analysis using the 7-
term Blackman-Harris window setting. Also, the window setting of
None can be used to search for noise
spurs over frequency in dc inputs.
For dynamic performance evaluation, the external differential source must have better specifications than
the ADC itself to ensure that the measured system performance is not limited by the performance of the
signal source. Therefore, the external reference source must meet the source requirements mentioned in
Table 4 .
Table 4. External Source Requirements for Evaluation of the ADS9120
Specification Description Specification Value
Signal frequency 2 kHz
External source type Balanced differential
External source common-mode
0 V or floating
(see Section 2.2.2 for jumper settings)
External source impedance (R
S
) 10
Ω
–30
Ω
External source differential impedance
(R
S_DIFF
= 2 × R
S
)
20 Ω –60 Ω
Source differential signal
(V
PP
Amplitude for –0.1 dBFS)
(2 × R
S
× 4.45 × 10
–3
) + 8.9 V
or
(R
S_DIFF
× 4.45 × 10
–3
) + 8.9 V
Maximum noise 10 µV
RMS
Maximum SNR 110 dB
Maximum THD –130 dB
For 2-kHz SNR and ENOB evaluation at a maximum throughput of 2.5 MSPS, the number of samples
must be 32768 or 65536. More samples brings the noise floor so low that the external source phase noise
can dominate the SNR and ENOB calculations. On the contrary, for THD and SFDR evaluation, a much
large number of samples must be used to reduce the noise floor below –140 dBc to analyze noise-free
harmonics and spurs in the order of –120 dBc. Such analysis requires at least 262144 samples.