MAX152
Conversion Rate
The maximum sampling rate (f
max
) for the MAX152 is
achieved in the WR-RD mode (t
RD
< t
INTL
) and is cal-
culated as follows:
Signal-to-Noise Ratio and Effective
Number of Bits
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.
The theoretical minimum A/D noise is caused by quan-
tization error, and results directly from the ADC's reso-
lution: SNR = (6.02N + 1.76)dB, where N is the number
of bits of resolution. Therefore, a perfect 8-bit ADC can
do no better than 50dB.
The FFT plot (
Typical Operation Characteristics
) shows
the result of sampling a pure 30.27kHz sinusoid at a
400kHz rate. This FFT plot of the output shows the out-
put level in various spectral bands.
The effective resolution, or "effective number of bits,"
the ADC provides can be measured by transposing the
equation that converts resolution to SNR: N = (SINAD -
1.76)/6.02 (see
Typical Operating Characteristics
).
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the RMS
sum of all harmonics of the input signal (in the frequen-
cy band above DC and below one-half the sample rate)
to the fundamental itself. This is expressed as:
where V
1
is the fundamental RMS amplitude, and V
2
to
V
N
are the amplitudes of the 2nd through Nth harmonics.
Spurious-Free Dynamic Range
Spurious-free dynamic range is the ratio of the funda-
mental RMS amplitude to the amplitude of the next
largest spectral component (in the frequency band
above DC and below one-half the sample rate).
Usually the next largest spectral component occurs at
some harmonic of the input frequency. However, if the
ADC is exceptionally linear, it may occur only at a ran-
dom peak in the ADC's noise floor. See "Signal to Noise
Ratio" plot in
Typical Operating Characteristics
.
THD 20 log
(V
2
2
V
3
2
V
4
2
V
N
2
)
V
1
=
++++
L
f
1
tttt
e.g. at T 25 C, V 3.0V:
f
max
1
600ns 800ns 300ns 450ns
f 465kHz
where t Write pulse width
t Delay between WR and RD pulses
t = RD to INT delay
t = Delay time between conversons.
max
WR RD RI P
ADD
max
WR
RD
RI
P
=
+++
=+ ° =+
=
+++
=
=
=
+3V, 8-Bit ADC with 1µA Power-Down
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