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AD7418ARUZ Datasheet(PDF) 15 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) 15 Page - Analog Devices

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REV. G
AD7416/AD7417/AD7418
–15–
With a VDD = 3 V, for each 400
µs cycle, the AD7416 spends
40
µs (or 10% of the time) in conversion mode. It spends 360 µs
(or 90% of time) in partial power-down mode. Thus, the aver-
age power dissipated by the AD7416/AD7417/AD7418 is
30 1 1
09
12
mW
×+
×
=
..
.
mW
mW
Mode 2
For applications where temperature measurements are required at
a slower rate, e.g., every second, power consumption of the part
can be reduced by writing to the part to go to a full power-down
between reads. The current consumption in full power-down is
typically 0.2
µA and full power-down is initiated when D0 = 1
in the Configuration Register. When a measurement is required,
a write operation can be performed to power-up the part. The
part then performs a conversion and is returned to power-down.
The temperature value can be read in the full power-down
because the I
2C bus is continuously active.
The power dissipation in this mode depends on the rate at which
reads take place. Taking the requirements for a temperature
measurement every 100 ms as an example, the optimum power
dissipation is achieved by placing the part in full power-down,
waking it up every 100 ms, letting it operate for 400
µs and
putting it into full power-down again. In this case, the average
power consumption is calculated as follows. The part spends
40
µs (or 0.04% of time) converting with 3 mW dissipation and
a 99.96 ms (99.96% of time) in full shutdown with 60 nW dissipa-
tion. Thus, the average power dissipation is
30 004
60
0 9996
1 2
mW
nW
W
×+
×
=
..
.
µ
The fastest throughput rate at which the AD7416/AD7417/
AD7418 can be operated is 2.5 kHz (i.e., a read every 400
µs
conversion period). Since TOTI and THYST are 2-byte reads, the
read time with the I
2C operating at 100 kbit/s would be 270
µs.
If temperature reads are called too often, reads will overlap
with conversions, aborting them continuously, which results in
invalid readings.
CONVERT START MODE
The AD7417/AD7418 has an extra mode, set by writing to the
MSB of the Config2 Register.
CONVST Pin Mode
By setting the
CONVST Mode Bit to 1, conversions are initiated
only by using the
CONVST pin. In this method of operation,
CONVST is normally low.
The rising edge of
CONVST starts the power-up time. This
power-up time is 4
µs. If the CONVST high time is longer than
4
µs, a conversion is initiated on the falling edge of CONVST
and the track-and-hold also enters its hold mode at this time. If
the
CONVST high time is less than 4
µs, an internal timer,
initiated by the rising edge of
CONVST holds off the track-and-
hold and the initiation of conversion until the timer times out
(4
µs after the rising edge of CONVST, which corresponds with
the power-up time).
CONVST input remains low at the end of
conversion, thus causing the part to enter its power-down mode.
In this method of operation,
CONVST is normally low with a
high going pulse controlling the power-up and conversion starts.
The
CONVST pin should not be pulsed when reading from or
writing to the port.
Figure 17 shows the recommended minimum times for the
CONVST pulse when the temperature channel is selected.
Figure 18 shows the minimum times an analog input channel
is selected.
APPLICATIONS INFORMATION
SUPPLY DECOUPLING
The AD7416/AD7417/AD7418 should be decoupled with a
0.1
µF ceramic capacitor between V
DD and GND. This is
particularly important if the part is mounted remote from the
power supply.
POWER-ON-RESET
To ensure proper power-on-reset, make sure that the supply
voltage on the VDD pin is at 0 V. Refer to application note
AN-588 for more information. A failed power-on-reset can
prevent the default values from being loaded into the AD7416/
AD7417/AD7418 registers. If the correct values are not loaded
into the registers, then the device will not start operating. The
output from the value registers will be a constant value.
To get the device operating again, the registers will have to be
loaded with their default values via the I
2C bus. Therefore, in
the event of an inadequate power-on-reset and for all three
devices, the following registers should be loaded with their
default values:
Configuration Register 1—Default Value = 00h
Configuration Register 2—Default Value = 00h
THYST Setpoint Register—Default Value = 4B00h
TOTI Setpoint Register—Default Value = 5500h
MOUNTING THE AD7416
The AD7416/AD7417/AD7418 can be used for surface or air
temperature sensing applications. If the device is cemented to a
surface with thermally conductive adhesive, the die temperature
will be within about 0.2
°C of the surface temperature, thanks to
the device’s low power consumption. Care should be taken to
insulate the back and leads of the device from the air, if the
ambient air temperature is different from the surface tempera-
ture being measured.
The ground pin provides the best thermal path to the die, so the
temperature of the die will be close to that of the printed circuit
ground track. Care should be taken to ensure that this is in good
thermal contact with the surface being measured.
As with any IC, the AD7416/AD7417/AD7418 and its associ-
ated wiring and circuits must be kept free from moisture to
prevent leakage and corrosion, particularly in cold conditions
where condensation is more likely to occur. Water resistant
varnishes and conformal coatings can be used for protection.
The small size of the AD7416 package allows it to be mounted
inside sealed metal probes that provide a safe environment for
the device.


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