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ADC12H030 Datasheet(PDF) 35 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. ADC12H030
Description  Self-Calibrating 12-Bit Plus Sign Serial I/O A/D Converters with MUX and Sample/Hold
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com

ADC12H030 Datasheet(HTML) 35 Page - National Semiconductor (TI)

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Application Hints (Continued)
The ADC12030/2/4/8’s performance can be maximized
through proper grounding techniques. These include the use
of separate analog and digital ground planes. The digital
ground plane is placed under all components that handle
digital signals, while the analog ground plane is placed under
all components that handle analog signals. The digital and
analog ground planes are connected together at only one
point, either the power supply ground or at the pins of the
ADC. This greatly reduces the occurence of ground loops
and noise.
Shown in Figure 18 is the ideal ground plane layout for the
ADC12038 along with ideal placement of the bypass capaci-
tors. The circuit board layout shown in Figure 18 uses three
bypass capacitors: 0.01 µF (C1) and 0.1 µF (C2) surface
mount capacitors and 10 µF (C3) tantalum capacitor.
The ADC12030/2/4/8’s performance is optimized by routing
the analog input/output and reference signal conductors as
far as possible from the conductors that carry the clock
signals to the CCLK and SCLK pins. Ground traces parallel
to the clock signal traces can be used on printed circuit
boards to reduce clock signal interference on the analog
input/output pins.
A calibration cycle needs to be started after the power sup-
plies, reference, and clock have been given enough time to
stabilize after initial turn-on. During the calibration cycle,
correction values are determined for the offset voltage of the
sampled data comparator and any linearity and gain errors.
These values are stored in internal RAM and used during an
analog-to-digital conversion to bring the overall full-scale,
offset, and linearity errors down to the specified limits.
Full-scale error typically changes ±0.4 LSB over tempera-
ture and linearity error changes even less; therefore it should
be necessary to go through the calibration cycle only once
after power up if the Power Supply Voltage and the ambient
temperature do not change significantly (see the curves in
the Typical Performance Characteristics).
To correct for any change in the zero (offset) error of the A/D,
the auto-zero cycle can be used. It may be necessary to do
an auto-zero cycle whenever the ambient temperature or the
power supply voltage change significantly. (See the curves
titled “Zero Error Change vs Ambient Temperature” and
“Zero Error Change vs Supply Voltage” in the Typical Perfor-
mance Characteristics.)
Many applications require the A/D converter to digitize AC
signals, but the standard DC integral and differential nonlin-
earity specifications will not accurately predict the A/D con-
verter’s performance with AC input signals. The important
specifications for AC applications reflect the converter’s abil-
ity to digitize AC signals without significant spectral errors
and without adding noise to the digitized signal. Dynamic
characteristics such as signal-to-noise (S/N), signal-tonoise
+ distortion ratio (S/(N + D)), effective bits, full power band-
width, aperture time and aperture jitter are quantitative mea-
sures of the A/D converter’s 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. S/(N + D)
and S/N are calculated from the resulting FFT data, and a
spectral plot may also be obtained. Typical values for S/N
FIGURE 18. Ideal Ground Plane

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