Datasheet

ADC10061, ADC10062, ADC10064
SNAS069E –JUNE 1999–REVISED MARCH 2013 www.ti.com
Because they incorporate a direct sample/hold control input, the ADC10061, ADC10062, and ADC10064 are
suitable for use in DSP-based systems. The S/H input allows synchronization of the A/D converter to the DSP
system's sampling rate and to other ADC10061s, ADC10062s, and ADC10064s.
POWER SUPPLY CONSIDERATIONS
The ADC10061, ADC10062, and ADC10064 are designed to operate from a +5V (nominal) power supply. There
are two supply pins, AV
CC
and DV
CC
. These pins allow separate external bypass capacitors for the analog and
digital portions of the circuit. To ensure accurate conversions, the two supply pins should be connected to the
same voltage source, and each should be bypassed with a 0.1 µF ceramic capacitor in parallel with a 10 µF
tantalum capacitor. Depending upon the circuit board layout and other system considerations, more bypassing
may be necessary.
The ADC10061 has a single ground pin, and the ADC10062 and ADC10064 each have separate analog and
digital ground pins for separate bypassing of the analog and digital supplies. The devices with separate analog
and digital ground pins should have their ground pins connected to the same potential, and all grounds should be
“clean” and free of noise.
In systems with multiple power supplies, careful attention to power supply sequencing may be necessary to avoid
over-driving inputs. The A/D converter's power supply pins should be at the proper voltage before digital or
analog signals are applied to any of the other pins.
LAYOUT AND GROUNDING
In order to ensure fast, accurate conversions from the ADC10061, ADC10062, and ADC10064, it is necessary to
use appropriate circuit board layout techniques. The analog ground return path should be low-impedance and
free of noise from other parts of the system. Noise from digital circuitry can be especially troublesome.
All bypass capacitors should be located as close to the converter as possible and should connect to the
converter and to ground with short traces. The analog input should be isolated from noisy signal traces to avoid
having spurious signals couple to the input. Any external component (e.g., a filter capacitor) connected across
the converter's input should be connected to a very clean ground return point. Grounding the component at the
wrong point will result in reduced conversion accuracy.
DYNAMIC PERFORMANCE
Many applications require the A/D converter to digitize AC signals, but conventional DC integral and differential
nonlinearity specifications don't accurately predict the A/D converter's performance with AC input signals. The
important specifications for AC applications reflect the converter's ability to digitize AC signals without significant
spectral errors and without adding noise to the digitized signal. Dynamic 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 using 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 Typical Performance Characteristics. The large peak
is the fundamental frequency, and the noise and distortion components (if any are present) are visible above and
below the fundamental 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 fundamental frequency to the rms value at all other
frequencies, excluding any harmonic distortion components. Typical values 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 ratio, 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 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:
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