ADC10461CIWM/NOPB National Semiconductor, ADC10461CIWM/NOPB Datasheet - Page 14

• Manufacturer Part Number: ADC10461CIWM/NOPB
• Description: IC ADC 10BIT 600 NS 20-SOIC
• Manufacturer: National Semiconductor

Specifications of ADC10461CIWM/NOPB

• Number Of Bits: 10
• Sampling Rate (per Second): 800k
• Data Interface: Parallel
• Number Of Converters: 3
• Power Dissipation (max): 235mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (0.300", 7.50mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *ADC10461CIWM; *ADC10461CIWM/NOPB; ADC10461CIWM

www.national.com
Applications Information
4.0 INHERENT SAMPLE-AND-HOLD
Because the ADC10461, ADC10462, and ADC10464
sample the input signal once during each conversion, they
are capable of measuring relatively fast input signals without
the help of an external sample-hold. In a non-sampling
successive-approximation A/D converter, regardless of
speed, the input signal must be stable to better than
LSB during each conversion cycle or significant errors will
result. Consequently, even for many relatively slow input
signals, the signals must be externally sampled and held
constant during each conversion if a SAR with no internal
sample-and-hold is used.
Because they incorporate a direct sample/hold control input,
the ADC10461, ADC10462, and ADC10464 are suitable for
use in DSP-based systems. The S/H input allows synchro-
nization of the A/D converter to the DSP system’s sampling
rate
ADC10464s.
5.0 POWER SUPPLY CONSIDERATIONS
The ADC10461, ADC10462, and ADC10464 are designed to
operate from a +5V (nominal) power supply. There are two
supply pins, AV
external bypass capacitors for the analog and digital portions
of the circuit. To guarantee 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. De-
pending upon the circuit board layout and other system
considerations, more bypassing may be necessary.
The ADC10461 has a single ground pin, and the ADC10462
and ADC10464 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.
6.0 LAYOUT AND GROUNDING
In order to ensure fast, accurate conversions from the
ADC10461, ADC10462, and ADC10464, 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.
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 convert-
and
to
CC
other
and DV
ADC10461s,
CC
. These pins allow separate
ADC10462s,
(Continued)
±
and
1/2
14
er’s performance with AC input signals. The important speci-
fications 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 char-
acteristics such as signal-to-noise ratio (SNR) and total har-
monic 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 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
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 ADC10461 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
In applications that require faster conversion times, the
Speed Adjust pin (pin 14 on the ADC10462, pin 17 on the
ADC10464) 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
current 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
P-P
, 100 kHz