Rainbow Electronics MAX1273 - Manual (Page 17)

MAX1272/MAX1273
Fault-Protected, 12-Bit ADCs
with Software-Selectable Input Range
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Layout, Grounding, and Bypassing
For best performance, use printed circuit (PC) boards.
Wire-wrap configurations are not recommended since
the layout should ensure proper separation of analog
and digital traces. Do not run analog and digital lines
parallel to each other, and do not lay out digital signal
paths underneath the ADC package. Use separate
analog and digital PC board ground sections with only
one star point (Figure 14), connecting the two ground
systems (analog and digital). For lowest-noise opera-
tion, ensure that the ground return to the star ground’s
power supply is low impedance and as short as possi-
ble. Route digital signals far away from sensitive analog
and reference inputs.
High-frequency noise in the power supply (V
DD ) can
degrade the performance of the ADC’s fast compara-
tor. Bypass V
DD to the star ground with a 0.1µF capaci-
tor located as close as possible to the MAX1272/
MAX1273’s power-supply input. Minimize capacitor lead
length for best supply-noise rejection. Add an attenua-
tion resistor (5
Ω) to extremely noisy power supplies.
Definitions
Integral Nonlinearity
Integral nonlinearity (INL) is the deviation of the values
on an actual transfer function from a straight line. This
straight line can be either a best-straight-line fit or a line
drawn between the endpoints of the transfer function,
once offset and gain errors have been nullified. The
static linearity parameters for the MAX1272/MAX1273
are measured using the endpoint method.
Differential Nonlinearity
Differential nonlinearity (DNL) is the difference between
an actual step-width and the ideal value of 1 LSB. A
DNL error specification of 1 LSB guarantees no missing
codes and a monotonic transfer function.
Aperture Definitions
Aperture jitter (t AJ ) is the sample-to-sample variation in
the time between samples. Aperture delay (t AD ) is the
time between the falling edge of the sampling clock
and the instant when the actual sample is taken.
Signal-to-Noise Ratio
For a waveform perfectly reconstructed from digital
samples, signal-to-noise ratio (SNR) is the ratio of the
full-scale analog input (RMS value) to the RMS quanti-
zation error (residual error).
The ideal, theoretical minimum analog-to-digital noise is
caused by quantization noise error only and results
directly from the ADC’s resolution (N-bits):
SNR = (6.02 ✕ N + 1.76) dB
In reality, there are other noise sources besides quanti-
zation noise: thermal noise, reference noise, clock jitter,
etc. SNR is computed by taking the ratio of the RMS
signal to the RMS noise, which includes all spectral
components minus the fundamental, the first five har-
monics, and the DC offset.
Signal-to-Noise Plus Distortion
Signal-to-noise plus distortion (SINAD) is the ratio of the
fundamental input frequency’s RMS amplitude to the
RMS equivalent of all the other ADC output signals:
SINAD (dB) = 20 ✕ log [Signal RMS / (Noise +
Distortion) RMS ]
Effective Number of Bits
Effective number of bits (ENOB) indicates the global
accuracy of an ADC at a specific input frequency and
sampling rate. An ideal ADC’s error consists of quanti-
zation noise only. With an input range equal to the full-
scale range of the ADC, calculate the effective number
of bits as follows:
ENOB = (SINAD - 1.76) / 6.02
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the RMS
sum of the first five harmonics of the input signal to the
fundamental itself. This is expressed as:
where V 1 is the fundamental amplitude and V 2 through
V 5 are the 2nd- through 5th-order harmonics.
Spurious-Free Dynamic Range
Spurious-free dynamic range (SFDR) is the ratio of the
RMS amplitude of the fundamental (maximum signal
component) to the RMS value of the next largest fre-
quency component, excluding DC offset.
THD
V V V V
V
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+ + +
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