ADC10662CIWMX/NOPB National Semiconductor, ADC10662CIWMX/NOPB Datasheet - Page 15

• Manufacturer Part Number: ADC10662CIWMX/NOPB
• Manufacturer: National Semiconductor

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Applications Information
An A/D converter’s AC performance can be measured using
Fast Fourier Transform (FFT) methods. A sinusoidal wave-
form is applied to the A/D converter’s input, and the trans-
form is then performed on the digitized waveform. The re-
sulting spectral plot might look like the ones shown in the
typical performance curves. The large peak is the fundamen-
tal frequency, and the noise and distortion components (if
any are present) are visible above and below the fundamen-
tal frequency. Harmonic distortion components appear at
whole multiples of the input frequency. Their amplitudes are
combined as the square root of the sum of the squares and
compared to the fundamental amplitude to yield the THD
specification. Guaranteed limits for THD are given in the
table of Electrical Characteristics.
Signal-to-noise ratio is the ratio of the amplitude at the
fundamental frequency to the rms value at all other frequen-
cies, excluding any harmonic distortion components. Guar-
anteed limits are given in the Electrical Characteristics table.
An alternative definition of signal-to-noise ratio includes the
distortion components along with the random noise to yield a
signal-to-noise-plus-distortion ration, or S/(N + D).
The THD and noise performance of the A/D converter will
change with the frequency of the input signal, with more
distortion and noise occurring at higher signal frequencies.
One way of describing the A/D’s performance as a function
of signal frequency is to make a plot of “effective bits” versus
frequency. An ideal A/D converter with no linearity errors or
(Continued)
14
self-generated noise will have a signal-to-noise ratio equal to
(6.02n + 1.76) dB, where n is the resolution in bits of the A/D
converter. A real A/D converter will have some amount of
noise and distortion, and the effective bits can be found by:
where S/(N + D) is the ratio of signal to noise and distortion,
which can vary with frequency.
As an example, an ADC10662 with a 4.85 V
sine wave input signal will typically have a signal-to-noise-
plus-distortion ratio of 59.2 dB, which is equivalent to 9.54
effective bits. As the input frequency increases, noise and
distortion gradually increase, yielding a plot of effective bits
or S/(N + D) as shown in the typical performance curves.
8.0 SPEED ADJUST
The speed adjust pin is connected to an on-chip current
source that determines the converter’s internal timing. By
connecting a resistor between the speed adjust pin and
ground as shown in Figure 4, the internal programming
current is increased, which reduces the conversion time. The
ADC10662 and ADC10664 are specified and guaranteed for
operation with R
2). Smaller resistors will result in faster conversion times, but
linearity will begin to degrade as R
curves).
SA
= 14.0 kΩ (Mode 1) or R
SA
becomes smaller (see
SA
= 8.26k (Mode
P-P
, 100 kHz