AD7417 AD [Analog Devices], AD7417 Datasheet - Page 9

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

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 REF
nal reference is used, a 10 µF capacitor should be connected
between REF
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.
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 kΩ. The capacitor C1 is
the ADC sampling capacitor and has a capacitance of 3 pF.
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
REV. G
A
IN
REFERENCE
EXTERNAL
OPTIONAL
4pF
C2
SUPPLY
2.7V TO
Figure 3. Equivalent Analog Input Circuit
0V TO 2.5V
5.5V
Figure 2. Typical Connection Diagram
IN
REF-192
INPUT
AD780/
V
and GND. SDA and SCL form the 2-wire I
DD
D1
D2
10 F
CONVERT PHASE – SWITCH OPEN
TRACK PHASE – SWITCH CLOSED
GND
A
A
A
A
IN4
IN1
IN2
IN3
0.1 F
1k
R1
AD7417
REF
V
DD
CONVST
IN
10 F FOR
EXTERNAL
REFERENCE
3pF
C1
SDA
SCL
OTI
INTERFACE
A0
A1
A2
SERIAL
2-WIRE
IN
V
BALANCE
pin. If an exter-
C/ P
2
C
–9–
start of the conversion phase and is powered down at the end of
the conversion. The on-chip reference is selected by connecting
the REF
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 REF
This has the effect of shutting down the on-chip reference circuitry.
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 V
The technique used in the AD7416/AD7417/AD7418 is to
measure the current change in V
at two different currents.
This is given by
where:
K is Boltzmann’s constant.
q is the charge on the electron (1.6 × 10
T is the absolute temperature in Kelvins.
N is the ratio of the two currents.
TRANSISTOR
SENSING
Figure 5. Temperature Measurement Technique
IN
1.2V
pin to analog ground. This causes SW1 (see Figure 4)
I
Figure 4. On-Chip Reference
+
∆V
AD7416/AD7417/AD7418
TRANSISTOR
BE
SENSING
BE
SW1
=
, which varies from device to device.
N
REF
KT q
I
26k
IN
24k
BE
/
1.2V
when the device is operated
×
V
1
DD
n N
2.5V
-19
( )
+
Coulombs).
EXTERNAL
REFERENCE
DETECT
BUFFER
IN
TO ADC
V
V
pin.
OUT
OUT