MAX120EAG Ver la hoja de datos (PDF) - Maxim Integrated
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MAX120EAG Datasheet PDF : 15 Pages
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MAX120/MAX122
500ksps, 12-Bit ADCs with Track/Hold
and Reference
Figure 17. Bipolar Transfer Function
Figure 19. Offset and Gain Adjustment (Noninverting)
Figure 18. Trim Circuit for Gain Only
ADCs have traditionally been evaluated by specifications
such as zero and full-scale error, integral nonlinearity
(INL), and differential nonlinearity (DNL). Such parame
ters are widely accepted for specifying performance with
DC and slowly varying signals, but are less useful in
signal processing applications where the ADC’s impact
on the system transfer function is the main concern. The
significance of various DC errors does not translate well
to the dynamic case, so different tests are required.
Signal-to-Noise Ratio and
Effective Number of Bits
The signal-to-noise plus distortion ratio (SINAD) is the
ratio of the fundamental input frequency’s RMS amplitude
to the RMS amplitude of all other ADC output signals. The
output band is limited to frequencies above DC and below
one-half the ADC sample rate.
Figure 19. Offset and Gain Adjustment (Noninverting)
The theoretical minimum ADC noise is caused by quanti-
zation error and is a direct result of the ADC’s resolution:
SNR = (6.02N + 1.76)dB, where N is the number of bits
of resolution. A perfect 12-bit ADC can, therefore, do no
better than 74dB. An FFT plot shows the output level in
various spectral bands. Figure 22 shows the result of
sampling a pure 100kHz sinusoid at a 500ksps rate with
the MAX120.
By transposing the equation that converts resolution to
SNR, we can, from the measured SINAD, determine the
effective resolution (or effective number of bits) the ADC
provides: N = (SINAD - 1.76)/6.02. Figure 22 shows the
effective number of bits as a function of the input fre-
quency for the MAX120. The MAX122 performs similarly.
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