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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)信號調節(jié)A/D轉換器) |
| 中文描述： | 工業(yè)通用多通道ADC的信號調理（通用，多通道，工業(yè)信號調節(jié)的A / D轉換器） |
| 文件頁數(shù)： | 7/12頁 |
| 文件大?。?/td> | 150K |
| 代理商： | AD280 |
AD280
–7–
REV. 0
TECHNCAL
DATA
The magnitude of the excitation current used for RTD sensors
is set via a resistor connected between the I
SET
pin and analog
ground. This resistor sets the high level excitation magnitude for
both EXC0 and EXC1, but has no effect on the open circuit
detection current outputs of any of the EXC pins. The formula
for determining the approximate excitation current is:
I
EXC
= (2.5/
R
ISET
)
×
4
The temperature coefficient of the excitation current is directly
affected by the TC of this resistor, so it is suggested that a high
stability resistor (10 ppm/
°
C or less) be used. The total drift of
the excitation source will not only include the drift of this resis-
tor, but the reference drift as well.
XTAL1
XTAL2
XTALOUT
AD280
3
4
2
22pF (TYP)
22pF (TYP)
TO
m
CONTROLLER
XTAL1
XTAL2
XTALOUT
AD280
3
4
2
TO
m
CONTROLLER
FROM EXTERNAL
CLOCK
Figure 3. Oscillator Connections
To drive the AD280 from an externally derived clock, XTAL1
should be grounded, and the external clock should be applied to
the XTAL2 pin.
The XTALOUT pin provides a buffered replica of the clock.
This signal can be used to drive the clock input of a micro-
controller, as shown in the applications diagram (Figure 1).
GROUND SENSE INPUT
The ground sense input is an analog input to the front end
multiplexer, specifically designated for use in sensing (and
thereby eliminating) ground offset errors in an AD280 applica-
tion. While normally connected to AGND (Pin 20), this input
can be used for “Kelvin sensing” at the reference terminal in-
puts of an application. Like the CJC input, this input is not
designed for use as a field terminal input, and lacks the input
protection features found in AIN0 through AIN3. In most ap-
plications, the voltage on the ground sense input will be periodi-
cally sampled, and the resulting data used to compensate the
input signal data for offset.
COLD JUNCTION COMPENSATION INPUT
The AD280 provides an input explicitly for connection to a cold
junction compensation temperature sensor. This input is a stan-
dard input to the front end multiplexer, but with the addition of
a programmable excitation designed for use with thermistor
temperature sensors.
When enabled, the input is connected to a 34.4k
resistor
(factory trimmed for value) which, in turn, is connected to the
reference voltage. This kind of excitation is ideal for use with
10k
nominal precalibrated thermistors such as the Betatherm
10KA3. The 34.4k
resistor serves to help linearize the ther-
mistor voltage, although further digital linearization is required
for most applications. When disabled, the excitation circuit is
effectively disconnected from the input.
This input may also be used with a wide variety of silicon tem-
perature sensors, such as the AD590 or AD22100 series compo-
nents, using the REFOUT pin as a source of bias. Furthermore,
if no temperature sensor is required, this input may be used as
an additional analog input for any desired purpose. This input
does NOT exhibit the extended input voltage protection fea-
tures found in AIN0 through AIN3.
EXCITATION CURRENT OUTPUTS
The AD280 contains four excitation output pins programmable
for excitation level. These pins are separate from the analog
input pins to allow for the use of protection devices in envi-
ronments with the possibility of high level normal-mode fault
conditions.
Table II. Excitation Control Bits
Output
MSB
LSB
OFF
+25 nA
–25 nA
RTD
0
0
1
1
0
1
0
1
All four excitation outputs can be individually programmed (via
the serial interface) to supply either +25nA or –25nA for use in
open circuit detection. All four may also be individually dis-
abled. EXC0 and EXC1 may also be programmed for a higher
excitation current, the magnitude of which is determined by the
value of the I
SET
resistor attached to Pin 7.
These excitation sources are implemented as high impedance
±
2.5 volts with respect to
analog ground. In applications that do not require extended
normal-mode voltage protection, they may simply be connected
to their corresponding input signal pins (i.e., EXC0 connects to
AIN0, EXC1 to AIN1, etc.) and will not interfere with signal
measurement.
BETATHERM
10k
THERMISTOR
MUX
34.4k
V
+V
REF
(SWITCH
ENABLED)
MUX
34.4k
V
+V
REF
(SWITCH
DISABLED)
REF
1k
V
AD590
MUX
34.4k
V
+V
REF
(SWITCH
DISABLED)
REF
OUT
AD
22100
Figure 4. CJC Sensor Connections
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