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參數(shù)資料
| 型號(hào)： | 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(hào)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(hào)調(diào)理（通用，多通道，工業(yè)信號(hào)調(diào)節(jié)的A / D轉(zhuǎn)換器） |
| 文件頁(yè)數(shù)： | 8/12頁(yè) |
| 文件大小： | 150K |
| 代理商： | AD280 |
AD280
–8–
REV. 0
PRELMNARY
TECHNCAL
DATA
The excitation current level can be set for any value up to 2 mA;
however, it is suggested that the lowest practical level be used to
minimize self-heating and general power dissipation. A good
value for most 100
platinum or nickel RTD types is 250
μ
A;
higher currents (up to 2mA) are usually used only for copper
RTDs. If the high level excitation feature will not be used, this
pin may be left open.
AUXILIARY DIGITAL INPUTS AND OUTPUTS
The AD280 provides for four general purpose digital inputs,
and four general purpose digital outputs. These inputs and
outputs are accessed via the serial interface, and can be used for
any desired purpose. Some of the applications for these I/O pins
include controlling external excitation circuits for specialized
transducers, or for activating or monitoring other external
devices. The general purpose outputs have a limited drive capa-
bility, sufficient for driving CMOS logic inputs; in some applica-
tions it may be necessary to buffer these outputs with external
devices.
REFERENCE INPUT AND OUTPUT
The AD280 contains an internal bandgap reference voltage
generator, the output of which is connected to Pin 16 (REFOUT).
The reference is nominally +2.5V. In most applications, it
should be connected to REFIN, Pin 17. For applications requir-
ing better stability than that provided by the internal reference,
the REFOUT pin may be left unconnected and the REFIN pin
may be driven from an external reference source such as the
AD685.
The REFOUT signal may also be useful to drive external cir-
cuitry in some applications. Up to 1 mA may be drawn from this
pin for external use.
PGA OUTPUT
The output of the Programmable Gain Amplifier is brought out
to this pin, primarily for the purposes of diagnostics and testing,
although there may be applications where it can be utilized. The
nominal output swing of this pin is
±
2.5V, with a typical linear
overrange capability to
±
3V.
EXTERNAL INTEGRATION CAPACITOR
The charge balancing A/D converter within the AD280 requires
an external integration capacitor, connected to CINT1 and
CINT2 (Pins 22 and 23). The nominal value of this capacitor is
4.7nF. The value is not especially critical, so loose tolerance
types (
±
20%) may be employed. The dielectric characteristics of
this capacitor have relatively little effect on the performance or
accuracy of this type of A/D converter, so expensive film capaci-
tors are unnecessary; inexpensive ceramic types (NPO or X7R
dielectrics) will suffice.
RESET INPUT
The RESET input to the AD280 is used to initialize the state of
the serial interface. When brought low, this signal will place the
serial interface into a “command” state, where it is ready to
accept a command. The RESET input also has the effect of
terminating an A/D conversion cycle in progress, and resetting
the A/D converter control circuitry. This input may be driven
from an external microcontroller supervisory circuit, or can be
connected to a simple RC reset circuit.
The RESET input is not absolutely required, and is provided
for convenience. If it is not used, the serial interface can be
forced into the command state by shifting at least four consecu-
tive bytes whose values are “0”s into the DI pin of the serial
interface. The internal configuration registers can only be initial-
ized through the interface, and are
not
cleared or reset by the
RESET pin.
DRDY (DATA READY) OUTPUT PIN
The DRDY pin indicates the state of the A/D converter. When
low, it indicates that an A/D conversion is in progress; when
high, it indicates that a conversion is complete, and that the data
from the most recent conversion is available over the serial inter-
face. This pin replicates a signal that is also available for inspec-
tion via the serial interface; it is brought out as a separate pin in
applications where it may be needed to trigger an interrupt.
ENHANCED PROTECTION
Although the AD280 is designed to provide protection to 10 volts
beyond the power supply, there are many cases where signifi-
The AD280 has been designed with this requirement in mind.
When operated in the high impedance mode (i.e., for input
signals limited to
±
3 volts), high normal-mode protection can be
achieved through the use of large value series input resistors.
The effect of the bias current present at the inputs to the AD280,
working against a high external impedance, can be negated by
balancing the inputs, i.e., always using a similarly valued resis-
tance in series with the GND SENSE input.
R1
R2
+
–
PTC
AD280
EXC
AIN
GND SENSE
Figure 5. Enhanced Input Normal-Mode Protection via
Large Value Input and Balancing Resistors and PTC
Devices
It should be noted, however, that when employing high value
series input resistors, it is advisable to avoid extra capacitance
directly at the input terminals. The PGA’s bias current is present
at the inputs only when the multiplexer is addressed for that
input; at other times, the input current falls to a negligible value,
consisting of just the leakage current of the multiplexer itself.
Relatively low values of capacitance at the input terminals will
therefore begin to slowly charge from the bias current when the
channel is selected, and discharge through the source impedance
of the input signal when deselected. This charging/discharging
behavior can cause large errors at very high gains.
The excitation sources may also be protected via the use of a
simple external circuit as shown in Figure 5. In this example, a
PTC (positive temperature coefficient) protection element is
used to protect the excitation source. When used together with a
high value external resistor to protect the input pin, complete
protection for a universal input (TC, RTD, mV or V) can be
afforded.
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