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AD7418ARUZ Datasheet(PDF) 14 Page - Analog Devices

Part No. AD7418ARUZ
Description  10-Bit Digital Temperature Sensor (AD7416) and Single/Four-Channel ADC (AD7417/AD7418)
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7418ARUZ Datasheet(HTML) 14 Page - Analog Devices

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REV. G
–14–
AD7416/AD7417/AD7418
Please note that when reading back from the ADT7416/
ADT7417/ADT7418, no more than three bytes of data must be
read back. A STOP command must be inserted at the end of
the read communication. If a STOP command is not inserted
by the master and the ADT7416/ADT7417/ADT7418 receives
more SCL cycles than the maximum needed for three bytes of
data, then the I
2C interface on the ADT7416/ADT7417/
ADT7418 will pull the SDA line low and prevent it from going
high again. To recover the ADT7416/ADT7417/ADT7418
interface the part must be powered off and on again. Reference
the application note, AN-686, on the Analog Devices website
for more information on I
2C interfaces.
OTI OUTPUT
The OTI output has two operating modes, which are selected
by Bit D1 of the Configuration Register. In the comparator
mode, (D1 = 0), the OTI output becomes active when the
temperature exceeds TOTI and remains active until the tempera-
ture falls below THYST. This mode allows the AD7416/AD7417/
AD7418 to be used as a thermostat, for example, to control the
operation of a cooling fan.
TOTI
THYST
OTI OUTPUT
COMPARATOR
MODE
OTI OUTPUT
INTERRUPT
MODE
READ* READ* READ* READ* READ* READ* READ*
IN INTERRUPT MODE, A READ OPERATION OR SHUTDOWN RESETS THE OTI
OUTPUT; OTHERWISE THE OTI OUTPUT REMAINS ACTIVE INDEFINITELY, ONCE
TRIGGERED.
*
Figure 13. Operation of OTI Output (Shown Active Low)
The open-drain configuration of OTI allows the OTI outputs of
several AD7416/AD7417/AD7418s to be wire-ANDed together
when in active low mode.
The OTI output is used to indicate that an out-of-limit tem-
perature excursion has occurred. OTI is an open-drain output
that can be programmed to be active low by setting Bit D2 of
the Configuration Register to 0 or active high by setting Bit D2
of the Configuration Register to 1.
In the interrupt mode (D1 = 1), the OTI output becomes active
when the temperature exceeds TOTI and remains active even if
the temperature falls below THYST, until it is reset by a read opera-
tion. Once OTI has become active by the temperature exceeding
TOTI, and has then been reset, it will remain inactive even if the
temperature remains, or subsequently rises again, above TOTI. It
will not become active again until the temperature falls below
THYST. It will then remain active until reset by a read opera-
tion. Once OTI has become active by the temperature falling
below THYST and then reset, it will remain inactive even if the
temperature remains, or subsequently falls again, below THYST.
OTI is also reset when the AD7416/AD7417/AD7418 is placed
in shutdown mode by setting Bit D0 of the Configuration Regis-
ter to 1.
The OTI output requires an external pull-up resistor. This can
be connected to a voltage different from VDD (for example, to
allow interfacing between 5 V and 3.3 V systems) provided that
the maximum voltage rating of the OTI output is not exceeded.
The value of the pull-up resistor depends on the application but
should be as large as possible to avoid excessive sink currents at
the OTI output, which can heat the chip and affect the tempera-
ture reading. The maximum value of the pull-up resistor that
will meet the output high current specification of the OTI out-
put is 30 k
Ω, but higher values may be used if a lower output
current is required. For most applications, a value of 10 k
Ω will
prove suitable.
FAULT QUEUE
To avoid false triggering of the AD7416/AD7417/AD7418 in
noisy environments, a fault queue counter is provided that can
be programmed by Bits D3 and D4 of the Configuration Regis-
ter (see Table V) to count 1, 2, 4, or 6 fault events before OTI
becomes active. In order to trigger OTI, the faults must occur
consecutively. For example, if the fault queue is set to 4, then
four consecutive temperature measurements greater than TOTI
(or less than THYST) must occur. Any reading that breaks the
sequence will reset the fault queue counter, so if there are three
readings greater than TOTI followed by a reading less than TOTI,
the fault queue counter will be reset without triggering OTI.
POWER-ON DEFAULTS
The AD7416/AD7417/AD7418 always powers up with the
following defaults.
Address pointer pointing to Temperature Value Register com-
parator mode
TOTI = 80
°C
THYST = 75
°C
OTI Active LOW
Fault Queue = 1
These default settings allow the AD7416/AD7417/AD7418 to
be used as a standalone thermostat without any connection to a
serial bus.
OPERATING MODES
The AD7416/AD7417/AD7418 has two possible modes of
operation depending on the value of D0 in the Configuration
Register.
Mode 1
Normal operation of the AD7416/AD7417/AD7418 occurs when
D0 = 0. In this active mode, a conversion takes place every
400
µs. Once the conversion has taken place, the part partially
powers down, consuming typically 350
µA of the current until
the next conversion occurs.
Two situations can arise in this mode on the request of a tempera-
ture read. If a read occurs during a conversion, the conversion
aborts and a new one starts on the stop/repeat start condition.
The temperature value that is read is that of the previous com-
pleted conversion. The next conversion will typically occur
400
µs after the new conversion has begun.
If a read is called between conversions, a conversion is initiated
on the stop/repeat start condition. After this conversion, the part
returns to performing a conversion every 400
µs.


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