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AD7416ARZ Datasheet(Fiches technique) 12 Page - Analog Devices

Numéro de pièce AD7416ARZ
Description  10-Bit Digital Temperature Sensor (AD7416) and Four Single-Channel ADCs
Télécharger  24 Pages
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Fabricant  AD [Analog Devices]
Site Internet  http://www.analog.com
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AD7416/AD7417/AD7418
Rev. I | Page 12 of 24
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.
REFIN
SW1
26kΩ
2.5V
24kΩ
1.2V
1.2V
EXTERNAL
REFERENCE
DETECT
BUFFER
Figure 11. 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
()
N
q
KT
V
BE
n
1
/ ×
=
Δ
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.
SENSING
TRANSISTOR
TO ADC
SENSING
TRANSISTOR
VOUT+
VDD
VOUT–
IN × I
Figure 12. Temperature Measurement Technique
Figure 12 shows the method the AD7416/AD7417/AD7418 use
to measure the device temperature. To measure ΔVBE, the
sensor (substrate transistor) is switched between operating
currents of I and N × I. The resulting waveform is passed through
a chopper-stabilized amplifier that performs the functions of
amplification and rectification of the waveform to produce a dc
voltage proportional to ΔVBE. This voltage is measured by the ADC
to give a temperature output in 10-bit twos complement form.
The temperature resolution of the ADC is 0.25°C, which corres-
ponds to 1 LSB of the ADC. The ADC can theoretically measure a
temperature span of 255°C; the guaranteed temperature range is
−40°C to +125°C. The result of the conversion is stored in the
temperature value register (0x00) as a 16-bit word. The 10 MSBs
of this word store the temperature measurement (see Table 9
and Table 10).
The temperature conversion formulas using the 10 MSBs of the
temperature value register are
Positive Temperature = ADC Code/4
(1)
Negative Temperature = (ADC Code − 512)/4
(2)
The MSB is removed from ADC Code in Equation 2.
INTERNAL REGISTER STRUCTURE
The AD7417/AD7418 have seven internal registers, as shown in
Figure 13. Six of these are data registers and one is an address
pointer register. The AD7416 has five internal registers (the
ADC and Config2 registers are not applicable to the AD7416).
TEMPERATURE
VALUE
REGISTER
(READ-ONLY
ADDRESS 0x00)
CONFIGURATION
REGISTER
(READ/WRITE
ADDRESS 0x01)
THYST SETPOINT
REGISTER
(READ/WRITE
ADDRESS 0x02)
TOTI SETPOINT
REGISTER
(READ/WRITE
ADDRESS 0x03)
ADC VALUE
REGISTER
(READ-ONLY
ADDRESS 0x04)
CONFIG2
REGISTER
(READ/WRITE
ADDRESS 0x05)
SDA
DATA
SCL
ADDRESS POINTER
REGISTER
(SELCTS DATA REGISTER
FOR READ/WRITE)
ADDRESS
SERIAL BUS INTERFACE
Figure 13. AD7417/AD7418 Register Structure
Address Pointer Register
The address pointer register is an 8-bit register that stores an
address that points to one of the six data registers. The first data
byte of every serial write operation to the AD7416/AD7417/
AD7418 is the address of one of the data registers, which is
stored in the address pointer register, and selects the data
register to which subsequent data bytes are written. Only the
three LSBs of the address pointer register are used to select a
data register.
Table 7. Address Pointer Register
P71
P61
P51
P41
P31
P2
P1
P0
0
0
0
0
0
Register select
1 P3 to P7 must be set to 0.




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