AD7416-7418 Analog Devices, AD7416-7418 Datasheet - Page 14

• Manufacturer Part Number: AD7416-7418
• Description: 10-Bit Digital Temperature Sensor (AD7416) and Single/Four-Channel ADC (AD7417/AD7418)
• Manufacturer: Analog Devices

AD7416/AD7417/AD7418
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
The power dissipation in this mode depends on the rate at
which reads take place. Taking the requirements for a tempera-
ture 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 s
and putting it into full power-down again. In this case the aver-
age power consumption is calculated as follows. The part spends
40 s (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 dissi-
pation. Thus the average power dissipation is:
The fastest throughput rate at which the AD7416/AD7417/
AD7418 can be operated is 2.5 kHz (i.e., a read every 400 s
conversion period). Since T
the read time with the I
270 s. 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 Mode
By setting the CONVST mode bit to 1, conversions are initiated
only by using the CONVST pin. When the AD7417/AD7418 is
in this CONVST mode, there are two possible methods of op-
eration, depending on the status of the CONVST line at the end
of conversion.
If the CONVST line is high at the end of a conversion, the
AD7417/AD7418 remains powered up at the end of a conver-
sion and the next conversion is initiated and the track/hold goes
into hold mode on the next falling edge of CONVST. The part
never powers down provided the D0 bit of the configuration
register remains at 0. In this method of operation, CONVST is
normally high with a low-going pulse controlling the track/hold
and conversion starts.
The second method is where the CONVST input is low at the
end of conversion. When this happens, the part automatically
enters its power-down mode at the end of conversion. The next
rising edge of CONVST starts the power-up time. This power-
up time is 4 s. If the CONVST high time is longer than 4 s,
a conversion is initiated on the falling edge of CONVST and
the track/hold also enters its hold mode at this time. If the
CONVST high time is less than 4 s, an internal timer, initiated
by the rising edge of CONVST holds of the track/hold and the
initiation of conversion until timer times out (4 s after the
rising edge of CONVST, which corresponds with the power-up
time). 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. When performing temperature conversions,
a minimum of 40 s should be left between any two CONVST
pulses to allow temperature conversions to complete before
starting another.
3 mW
0.004 + 60 nW
2
C operating at 100 kbit/s would be
2
C bus is continuously active.
OTI
and T
0.9996 = 1.2 W
HYST
are two byte reads,
–14–
APPLICATIONS INFORMATION
SUPPLY DECOUPLING
The AD7416/AD7417/AD7418 should be decoupled with a
0.1 F ceramic capacitor between +V
particularly important if the part is mounted remote from the
power supply.
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, which provide a safe environment for
the device.
FAN CONTROLLER
Figure 13 shows a simple fan controller that will switch on a
cooling fan when the temperature exceeds 80 C, and switch it
off again when the temperature falls below 75 C. The AD7416
can be used stand-alone in this application, or with a serial bus
interface if different trip temperatures are required. If the AD7416
is used with a bus interface, the sense of OTI can be set to ac-
tive high, Q1 and R1 can be omitted, and OTI connected di-
rectly to the gate of Q2, with R2 as the pull-up resistor.
Figure 13. AD7416 Used as a Fan Controller
3V TO 5.5V
AD7416
+V
S
R1
10k
R2
10k
Q1
2N3904
OR SIMILAR
S
and GND. This is
+12V
Q2
LOGIC LEVEL
MOSFET RATED
TO SUIT FAN
CURRENT
REV. B