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AD7418ARUZ Datasheet(PDF) 9 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) 9 Page - Analog Devices

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REV. G
AD7416/AD7417/AD7418
–9–
TYPICAL CONNECTION DIAGRAM
Figure 2 shows a typical connection diagram for the AD7417.
Using the A0, A1, and A2 pins allows the user to select from up
to eight AD7417s on the same serial bus, if desired. An external
2.5 V reference can be connected at the REFIN pin. If an exter-
nal reference is used, a 10
µF capacitor should be connected
between REFIN and GND. SDA and SCL form the 2-wire I
2C
compatible interface. For applications where power consump-
tion is of concern, the automatic power-down at the end of a
conversion should be used to improve power performance. See
Operating Modes section of this data sheet.
VDD
AIN1
GND
REFIN
SUPPLY
2.7V TO
5.5V
0.1 F
10 F
10 F FOR
EXTERNAL
REFERENCE
OPTIONAL
EXTERNAL
REFERENCE
AD780/
REF-192
0V TO 2.5V
INPUT
CONVST
SDA
AIN2
AIN3
AIN4
OTI
SCL
2-WIRE
SERIAL
INTERFACE
AD7417
A0
A1
A2
C/ P
Figure 2. Typical Connection Diagram
ANALOG INPUTS
Figure 3 shows an equivalent circuit of the analog input struc-
ture of the AD7417 and AD7418. The two diodes, D1 and D2,
provide ESD protection for the analog inputs. Care must be
taken to ensure that the analog input signal never exceeds the
supply rails by more than 200 mV. This will cause these diodes
to become forward-biased and start conducting current into the
substrate. The maximum current these diodes can conduct
without causing irreversible damage to the part is 20 mA. The
capacitor C2 in Figure 3 is typically about 4 pF and can prima-
rily be attributed to pin capacitance. The resistor R1 is a lumped
component made up of the on resistance of a multiplexer and a
switch. This resistor is typically about 1 k
Ω. The capacitor C1 is
the ADC sampling capacitor and has a capacitance of 3 pF.
AIN
D1
C1
3pF
VDD
D2
C2
4pF
VBALANCE
CONVERT PHASE – SWITCH OPEN
TRACK PHASE – SWITCH CLOSED
R1
1k
Figure 3. Equivalent Analog Input Circuit
ON-CHIP REFERENCE
The AD7416/AD7417/AD7418 has an on-chip 1.2 V band gap
reference that is gained up by a switched capacitor amplifier to
give an output of 2.5 V. The amplifier is only powered up at the
start of the conversion phase and is powered down at the end of
the conversion. The on-chip reference is selected by connecting
the REFIN pin to analog ground. This causes SW1 (see Figure 4)
to open and the reference amplifier to power up during a conver-
sion. Therefore, the on-chip reference is not available externally.
An external 2.5 V reference can be connected to the REFIN pin.
This has the effect of shutting down the on-chip reference circuitry.
1.2V
REFIN
SW1
2.5V
EXTERNAL
REFERENCE
DETECT
BUFFER
1.2V
26k
24k
Figure 4. On-Chip Reference
TEMPERATURE MEASUREMENT
A common method of measuring temperature is to exploit the
negative temperature coefficient of a diode, or the base-emitter
voltage of a transistor, operated at a constant current. Unfortu-
nately, this technique requires calibration to null out the effect
of the absolute value of VBE, which varies from device to device.
The technique used in the AD7416/AD7417/AD7418 is to
measure the current change in VBE when the device is operated
at two different currents.
This is given by
∆VKT q
n N
BE
()
/1
where:
K is Boltzmann’s constant.
q is the charge on the electron (1.6
× 10-19 Coulombs).
T is the absolute temperature in Kelvins.
N is the ratio of the two currents.
I
N
I
VDD
SENSING
TRANSISTOR
SENSING
TRANSISTOR
VOUT
TO ADC
VOUT
Figure 5. Temperature Measurement Technique


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