MAX1438 Datasheet, PDF (19/22 Pages) Maxim Integrated Products – Octal, 12-Bit, 65Msps, 1.8V ADC with Serial LVDS Outputs Serial LVDS Outputs

Octal, 12-Bit, 65Msps, 1.8V ADC
with Serial LVDS Outputs
Offset Error
Offset error is a figure of merit that indicates how well
the actual transfer function matches the ideal transfer
function at a single point. For the MAX1438, the ideal
midscale digital output transition occurs when there is
-1/2 LSBs across the analog inputs (Figures 6 and 7).
Bipolar offset error is the amount of deviation between
the measured midscale transition point and the ideal
midscale transition point.
Gain Error
Gain error is a figure of merit that indicates how well the
slope of the actual transfer function matches the slope
of the ideal transfer function. For the MAX1438 the gain
error is the difference of the measured full-scale and
zero-scale transition points minus the difference of the
ideal full-scale and zero-scale transition points.
For the bipolar devices (MAX1438), the full-scale transi-
tion point is from 0x7FE to 0x7FF for two’s-complement
output format (0xFFE to 0xFFF for offset binary) and the
zero-scale transition point is from 0x800 to 0x801 for
two’s complement (0x000 to 0x001 for offset binary).
Crosstalk
Crosstalk indicates how well each analog input is isolated
from the others. For the MAX1438, a 5.3MHz, -0.5dBFS
analog signal is applied to one channel while a 19.3MHz,
-0.5dBFS analog signal is applied to another channel. An
FFT is taken on the channel with the 5.3MHz analog sig-
nal. From this FFT, the crosstalk is measured as the dif-
ference in the 5.3MHz and 19.3MHz amplitudes.
Aperture Delay
Aperture delay (t_AD) is the time defined between the
rising edge of the sampling clock and the instant when
an actual sample is taken. See Figure 11.
Aperture Jitter
Aperture jitter (t_AJ) is the sample-to-sample variation in
the aperture delay. See Figure 11.
Signal-to-Noise Ratio (SNR)
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 quantiza-
tion error only and results directly from the ADC’s reso-
lution (N bits):
SNR_dB[max] = 6.02_dB x N x 1.76_dB
In reality, there are other noise sources besides quantiza-
tion noise: thermal noise, reference noise, clock jitter, etc.
CLK
ANALOG
INPUT
SAMPLED
DATA
T/H
HOLD
t_AD
t_AJ
TRACK
HOLD
Figure 11. Aperture Jitter/Delay Specifications
For the MAX1438, SNR is computed by taking the ratio
of the RMS signal to the RMS noise. RMS noise
includes all spectral components to the Nyquist fre-
quency excluding the fundamental, the first six harmon-
ics (HD2–HD7), and the DC offset.
Signal-to-Noise Plus Distortion (SINAD)
SINAD is computed by taking the ratio of the RMS signal
to the RMS noise plus distortion. RMS noise plus distor-
tion includes all spectral components to the Nyquist fre-
quency, excluding the fundamental and the DC offset.
Effective Number of Bits (ENOB)
ENOB specifies the dynamic performance of an ADC at
a specific input frequency and sampling rate. An ideal
ADC’s error consists of quantization noise only. ENOB for
a full-scale sinusoidal input waveform is computed from:
ENOB
=
⎛
⎝⎜
SINAD −1.76
6.02
⎞
⎠⎟
Total Harmonic Distortion (THD)
THD is the ratio of the RMS sum of the first six harmon-
ics of the input signal to the fundamental itself. This is
expressed as:
⎛
THD
=
20
×
log
⎜
⎜
⎝
V₂₂
+
V₃₂
+
V₄₂
+
V₅₂
+
V₆₂
+
V₇₂
⎞
⎟
V₁
⎟
⎠
Spurious-Free Dynamic Range (SFDR)
SFDR is the ratio expressed in decibels of the RMS
amplitude of the fundamental (maximum signal compo-
nent) to the RMS value of the next-largest spurious
component, excluding DC offset. SFDR is specified in
decibels relative to the carrier (dBc).
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