Signal-to-Noise Ratio (SNR)
SNR is a dynamic figure of merit that indicates the con-
verter’s noise performance. For a waveform perfectly
reconstructed from digital samples, the theoretical
maximum SNR is the ratio of the full-scale analog input
(RMS value) to the RMS quantization error (residual
error). The ideal, theoretical minimum analog-to-digital
noise is caused by quantization error only and results
directly from the ADC’s resolution (N bits):
SNR
dB[max]
= 6.02
dB
x N + 1.76
dB
In reality, there are other noise sources such as thermal
noise, reference noise, and clock jitter that also
degrade SNR. SNR is computed by taking the ratio of
the RMS signal to the RMS noise. RMS noise includes
all spectral components to the Nyquist frequency
excluding the fundamental, the first five harmonics, and
the DC offset.
Total Harmonic Distortion (THD)
THD is a dynamic figure of merit that indicates how much
harmonic distortion the converter adds to the signal.
THD is the ratio of the RMS sum of the first five harmon-
ics of the fundamental signal to the fundamental itself.
This is expressed as:
where V
1
is the fundamental amplitude, and V
2
through
V
6
are the amplitudes of the 2nd- through 6th-order
harmonics.
Spurious-Free Dynamic Range (SFDR)
SFDR is a dynamic figure of merit that indicates the
lowest usable input signal amplitude. SFDR is the ratio
of the RMS amplitude of the fundamental (maximum
signal component) to the RMS value of the next-largest
spurious component, excluding DC offset. SFDR is
specified in decibels relative to the carrier (dBc).
Intermodulation Distortion (IMD)
IMD is the ratio of the RMS sum of the intermodulation
products to the RMS sum of the two fundamental input
tones. This is expressed as:
The fundamental input tone amplitudes (V
1
and V
2
) are
at -6.5dBFS. 14 intermodulation products (V
IM
_) are
used in the MAX1391/MAX1394 IMD calculation. The
intermodulation products are the amplitudes of the out-
put spectrum at the following frequencies, where f
IN1
and f
IN2
are the fundamental input tone frequencies:
• 2nd-order intermodulation products:
f
IN1
+ f
IN2
, f
IN2
- f
IN1
• 3rd-order intermodulation products:
2 x f
IN1
- f
IN2
, 2 x f
IN2
- f
IN1
, 2 x f
IN1
+ f
IN2
, 2 x f
IN2
+ f
IN1
• 4th-order intermodulation products:
3 x f
IN1
- f
IN2
, 3 x f
IN2
- f
IN1
, 3 x f
IN1
+ f
IN2
, 3 x f
IN2
+ f
IN1
• 5th-order intermodulation products:
3 x f
IN1
- 2 x f
IN2
, 3 x f
IN2
- 2 x f
IN1
, 3 x f
IN1
+ 2 x f
IN2
,
3 x f
IN2
+ 2 x f
IN1
Channel-to-Channel Crosstalk
Channel-to-channel crosstalk indicates how well each
analog input is isolated from the others. The channel-to-
channel crosstalk for the MAX1394 is measured by
applying DC to channel 2 while a sine wave is applied
to channel 1. An FFT is taken for channels 1 and 2, and
the difference (in dB) is reported as the channel-to-
channel crosstalk.
Aperture Delay
The MAX1391/MAX1394 sample data on the falling
edge of its third SCLK cycle (Figure 14). In actuality,
there is a small delay between the falling edge of the
sampling clock and the actual sampling instant.
Aperture delay (t
AD
) is the time defined between the
