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參數(shù)資料
| 型號： | TSC2200IRHB |
| 英文描述： | PDA ANALOG INTERFACE CIRCUIT |
| 中文描述： | 掌上電腦模擬接口電路 |
| 文件頁數(shù)： | 35/40頁 |
| 文件大小： | 645K |
| 代理商： | TSC2200IRHB |
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35
www.ti.com
TSC2200
SBAS191F
LAYOUT
The following layout suggestions should provide optimum
performance from the TSC2200. However, many portable
applications have conflicting requirements concerning power,
cost, size, and weight. In general, most portable devices
have fairly
“
clean
”
power and grounds because most of the
internal components are very low power. This situation would
mean less bypassing for the converter
’
s power and less
concern regarding grounding. Still, each situation is unique
and the following suggestions should be reviewed carefully.
For optimum performance, care should be taken with the
physical layout of the TSC2200 circuitry. The basic SAR
architecture is sensitive to glitches or sudden changes on the
power supply, reference, ground connections, and digital
inputs that occur just prior to latching the output of the analog
comparator. Therefore, during any single conversion for an
‘
n-bit
’
SAR converter, there are n
‘
windows
’
in which large
external transient voltages can easily affect the conversion
result. Such glitches might originate from switching power
supplies, nearby digital logic, and high power devices. The
degree of error in the digital output depends on the reference
voltage, layout, and the exact timing of the external event.
The error can change if the external event changes in time
with respect to the SCL input.
With this in mind, power to the TSC2200 should be clean and
well bypassed. A 0.1
μ
F ceramic bypass capacitor should be
placed as close to the device as possible. A 1
μ
F to 10
μ
F
capacitor may also be needed if the impedance of the
connection between +V
DD
and the power supply is HIGH.
A bypass capacitor is generally not needed because the
reference is buffered by an internal op amp. If an external
reference voltage originates from an op amp, make sure that
it can drive any bypass capacitor that is used without oscil-
lation.
The TSC2200 architecture offers no inherent rejection of
noise or voltage variation in regards to using an external
reference input. This is of particular concern when the
reference input is tied to the power supply. Any noise and
ripple from the supply will appear directly in the digital
results. While high-frequency noise can be filtered out,
voltage variation due to line frequency (50Hz or 60Hz) can
be difficult to remove.
The GND pin should be connected to a clean ground point.
In many cases, this will be the
“
analog
”
ground. Avoid
connections that are too near the grounding point of a
microcontroller or digital signal processor. If needed, run a
ground trace directly from the converter to the power-supply
entry or battery-connection point. The ideal layout will in-
clude an analog ground plane dedicated to the converter and
associated analog circuitry.
In the specific case of use with a resistive touch screen, care
should be taken with the connection between the converter
and the touch screen. Since resistive touch screens have
fairly low resistance, the interconnection should be as short
and robust as possible. Loose connections can be a source
of error when the contact resistance changes with flexing or
vibrations.
As indicated previously, noise can be a major source of error
in touch screen applications (e.g., applications that require a
back-lit LCD panel). This EMI noise can be coupled through
the LCD panel to the touch screen and cause
“
flickering
”
of
the converted data. Several things can be done to reduce
this error, such as utilizing a touch screen with a bottom-side
metal layer connected to ground. This will couple the major-
ity of noise to ground. Additionally, filtering capacitors, from
Y+, Y
–
, X+, and X
–
to ground, can also help. Note, however,
that the use of these capacitors will increase screen settling
time and require longer panel voltage stabilization times, as
well as increased precharge and sense times for the
PENIRQ
circuitry of the TSC2200.
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