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參數(shù)資料
| 型號： | AD280 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Universal Multichannel Industrial Signal Conditioning ADC(通用,多通道,工業(yè)信號調(diào)節(jié)A/D轉(zhuǎn)換器) |
| 中文描述： | 工業(yè)通用多通道ADC的信號調(diào)理（通用，多通道，工業(yè)信號調(diào)節(jié)的A / D轉(zhuǎn)換器） |
| 文件頁數(shù)： | 6/12頁 |
| 文件大小： | 150K |
| 代理商： | AD280 |
AD280
–6–
REV. 0
PRELMNARY
the AD22100 or AD590.
The multiplexer also connects to an internal PTAT sensor,
which may be used to sense and track the internal temperature
of the die. While not practical for cold junction calibration pur-
poses, this signal might be used for diagnostic purposes in some
designs.
TECHNCAL
Under program control, the inputs can be reconfigured as at-
tenuated inputs. When enabled (by activating SW1 as shown in
Figure 2), the input impedance drops to approximately 100 k
,
and the linear input range therefore expands to
±
10 volts via a
4:1 attenuator. Using this provision, the AD280 can be used to
measure signals of up to
±
10 volts without additional external
components. A specialized ESD protection circuit at each signal
input pin will clamp at approximately
±
11 volts; protection for
input voltages higher than this level can be achieved via the use
of external series input resistors.
Table I. Multiplexer Control Bits
Input
PGA2
PGA1
PGA0
AIN0
AIN1
AIN2
AIN3
GND SENSE
PTAT
CJC
VREF
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
The input multiplexer also provides inputs dedicated to measur-
ing both analog ground and the reference voltage; these inputs
are provided to facilitate the use of closed loop autocalibration
algorithms. The ground sense input has an internal series resis-
tor, in order to balance the effects of bias current at the inputs.
The multiplexer also provides an input designed for use in cold
junction compensation for thermocouple applications. This
input has a programmable excitation source, providing appro-
priate excitation for a 10 k
interchangeable thermistor. The
excitation may be deactivated under digital control, allowing the
input to be used for semiconductor temperature sensors such as
OSCILLATOR
The AD280 requires a clock signal for operating the internal
charge balancing A/D converter, and for operating the optional
internal charge pump used in single supply applications. The
clock frequency affects the resolution of the A/D conversion
process vs. the integration time; higher frequencies result in
higher resolution. An internal oscillator allows for the connec-
tion of a crystal, and optionally allows for an externally derived
clock.
The oscillator is a parallel resonant design known as a “Pierce”
oscillator, and is similar to those found on most microcontrollers.
Two external shunt capacitors are required to complete the
circuit, as shown in Figure 2. The value of the capacitors and
the tolerance of the crystal affect clock speed accuracy, although
very precise clock speed is not required in most applications.
For crystals in the 10MHz to 12 MHz range, 22pF capacitors
are usually appropriate.
FUNCTIONAL DESCRIPTION
The AD280 is a multifunction front end building block IC
intended for applications in the industrial instrumentation and
data acquisition fields. It contains an input multiplexer, PGA,
A/D converter, excitation sources and serial interface, all opti-
mized for the measurement of RTDs, thermocouples, volt and
millivolt signals commonly found in industrial control environ-
ments. When used with a microcontroller and the appropriate
closed loop algorithms and firmware, the AD280 can support up
to four thermocouple (or millivolt or voltage) inputs, or up to
2 RTDs operating in either the 3-wire or 4-wire mode. The
AD280 contains additional provisions for cold junction compen-
sation of thermocouples, open lead detection and input protec-
tion from normal-mode faults.
Figure 1 illustrates an example application, utilizing the AD280
along with a microcontroller to create a two-chip, four-channel
industrial signal conditioner with serial communications.
CH 0
CH 1
CH 2
CH 3
EXC0
AIN0
EXC1
EXC2
AIN1
AIN2
AIN3
EXC3
REFIN
REFOUT
GND SENSE
CE
SK
DIN
DO
XTALOUT
AD280
4700pF
3.3
m
F
16V
3.3
m
F
16V
1
5DC
TxD
RxD
MICRO-
CONTROLLER
(80C51,
68HC05, ETC.)
1
2
1
2
1
2
1
2
Figure 1. An Example Application
INPUT MULTIPLEXER
The AD280 contains an input multiplexer primarily designed to
support four input channels, along with additional channels for
measurement of ground, reference, a CJC sensor and a built-in
PTAT signal. Each of the input channels (AIN0 through AIN3)
constitutes a high input impedance (>20 M
) input for signals
of up to
±
3 volts, with series limiting resistance and an active
clamping structure. The input terminals are specifically de-
signed to allow for input overloads of up to
±
11 volts without
affecting the operation of any other channel.
AINx
MUX
75k
V
25k
V
SW1
Figure 2. Input Attenuator Structure
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