User Manual

Applications Information (Continued)
7.0 DYNAMIC PERFORMANCE
Many applications require the A/D converter to digitize AC
signals, but conventional DC integral and differential nonlin-
earity specifications don’t accurately predict the A/D con-
verter’s performance with AC input signals. The important
specifications for AC applications reflect the converter’s
ability to digitize AC signals without significant spectral er-
rors and without adding noise to the digitized signal. Dynam-
ic characteristics such as signal-to-noise ratio (SNR) and
total harmonic distortion (THD), are quantitative measures
of this capability.
An A/D converter’s AC performance can be measured us-
ing Fast Fourier Transform (FFT) methods. A sinusoidal
waveform is applied to the A/D converter’s input, and the
transform is then performed on the digitized waveform. The
resulting spectral plot might look like the ones shown in the
typical performance curves. The large peak is the funda-
mental frequency, and the noise and distortion components
(if any are present) are visible above and below the funda-
mental 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. Typical values for THD are given in the
table of Electrical Characteristics.
Signal-to-noise ratio is the ratio of the amplitude at the fun-
damental frequency to the rms value at all other frequen-
cies, excluding any harmonic distortion components. Typical
values are given in the Electrical Characteristics table. An
alternative definition of signal-to-noise ratio includes the dis-
tortion components along with the random noise to yield a
signal-to-noise-plus-distortion ration, or S/(N
a
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’’ ver-
sus frequency. An ideal A/D converter with no linearity er-
rors or self-generated noise will have a signal-to-noise ratio
equal to (6.02n
a
1.8) 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:
n (effective)
e
S/(N
a
D) (dB)
b
1.8
6.02
where S/(N
a
D) is the ratio of signal to noise and distor-
tion, which can vary with frequency.
As an example, an ADC10061 witha5V
P-P
, 100 kHz sine
wave input signal will typically have a signal-to-noise-plus-
distortion ratio of 59.2 dB, which is equivalent to 9.53 effec-
tive bits. As the input frequency increases, noise and distor-
tion gradually increase, yielding a plot of effective bits or
S/(N
a
D) as shown in the typical performance curves.
8.0 SPEED ADJUST
In applications that require faster conversion times, the
Speed Adjust pin (pin 14 on the ADC10062, pin 17 on the
ADC10064) can significantly reduce the conversion time.
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 cur-
rent is increased, which reduces the conversion time. As an
example, an 18k resistor reduces the conversion time of a
typical part from 600 ns to 350 ns with no significant effect
on linearity. Using smaller resistors to further decrease the
conversion time is possible as well, although the linearity
will begin to degrade somewhat (see curves). Note that the
resistor value needed to obtain a given conversion time will
vary from part to part, so this technique will generally require
some ‘‘tweaking’’ to obtain satisfactory results.
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