increased about the preset (VREF × 1.2)/12.5. " />
參數(shù)資料
型號(hào)： AD7294BCPZ
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 18/49頁(yè)
文件大?。?/td> 0K
描述： IC ADC 12BIT I2C/SRL 1M 56LFCSP
標(biāo)準(zhǔn)包裝： 1
類(lèi)型： ADC，DAC
分辨率（位）： 12 b
采樣率（每秒）： 22.22k
數(shù)據(jù)接口： I²C，串行
電壓電源： 模擬和數(shù)字
電源電壓： 4.4 V ~ 5.5 V
工作溫度： -40°C ~ 105°C
安裝類(lèi)型： 表面貼裝
封裝/外殼： 56-VFQFN 裸露焊盤(pán)，CSP
供應(yīng)商設(shè)備封裝： 56-LFCSP-VQ（8x8）
包裝： 托盤(pán)
AD7294
Data Sheet
Rev. H | Page 24 of 48
increased about the preset (VREF × 1.2)/12.5. If this occurs, the
ISENSE OVERRANGE pin is set to a high logic level enabling
appropriate action to be taken to prevent any damage to the
external circuitry.
The setpoint threshold level is fixed internally in the AD7294,
and the current sense amplifier saturates above this level. The
comparator also triggers if a voltage of less than AVDD is applied
to the RSENSE or VPP pin.
TEMPERATURE SENSOR
The AD7294 contains one local and two remote temperature
sensors. The temperature sensors continuously monitor the
three temperature inputs and new readings are automatically
available every 5 ms.
The on-chip, band gap temperature sensor measures the temper-
ature of the system. Diodes are used in conjunction with the two
remote temperature sensors to monitor the temperature of other
critical board components.
LIMIT
REGISTERS
TEMP
SENSOR
T1
T2
ALERT
D2+
D2–
D1+
D1–
AD7294
CAP
REMOTE
SENSING
TRANSISTORS
16 × I
I
I-BIAS
MUX
VDD
BIAS DIODE
TO ADC
4 × I
fC = 65kHz
LPF
05747-
032
Figure 45. Internal and Remote Temperature Sensors
The temperature sensor module on the AD7294 is based on the
three current principle (see Figure 45), where three currents are
passed through a diode and the forward voltage drop is measured
at each diode, allowing the temperature to be calculated free of
errors caused by series resistance.
Each input integrates, in turn, over a period of several hundred
microseconds. This takes place continuously in the background,
leaving the user free to perform conversions on the other channels.
When integration is complete, a signal passes to the control logic
to initiate a conversion automatically. If the ADC is in command
mode, the temperature conversion is performed as soon as the
next conversion is completed. In autocycle mode, the conversion
is inserted into an appropriate place in the current sequence; see
the Register Setting section for further details. If the ADC is
idle, the conversion takes place immediately.
Three registers store the result of the last conversion on each
temperature channel; these can be read at any time. In addition,
in command mode, one or both of the two external channel
registers can be read out as part of the output sequence.
Remote Sensing Diode
The AD7294 is designed to work with discrete transistors,
2N3904 and 2N3906. If an alternative transistor is used, the
AD7294 operates as specified provided the following conditions
are adhered to.
Ideality Factor
The ideality factor, nf, of the transistor is a measure of the
deviation of the thermal diode from ideal behavior. The
AD7294 is trimmed for an nf value of 1.008. Use the following
equation to calculate the error introduced at a Temperature T
(°C) when using a transistor whose nf does not equal 1.008:
ΔT = (nf 1.008) × (273.15 K + T)
To factor this in, the user can write the T value to the offset
register. The AD7294 automatically adds it to, or subtracts it
from, the temperature measurement.
Base Emitter Voltage
The AD7294 operates as specified provided that the base-
emitter voltage is greater than 0.25 V at 8 A at the highest
operating temperature, and less than 0.95 V at 128 A for the
lowest operating temperature.
Base Resistance
The base resistance should be less than 100 .
hFE Variation
A transistor with small variation in hFE (approximately 50 to
150) should be used. Small variation in hFE indicates tight
control of the VBE characteristics.
For RF applications, the use of high Q capacitors functioning as a
filter protects the integrity of the measurement. These capacitors,
such as Johanson Technology 10 pF high Q capacitors: Reference
Code 500R07S100JV4T, should be connected between the base and
the emitter, as close to the external device as possible. However,
large capacitances affect the accuracy of the temperature measure-
ment; thus, the recommended maximum capacitor value is 100 pF.
In most cases, a capacitor is not required; the selection of any
capacitor is dependent on the noise frequency level.
05747-
099
2N3904
NPN
AD7294
D+
D–
10pF
Figure 46. Measuring Temperature Using an NPN Transistor
05747-
100
10pF
2N3906
PNP
AD7294
D+
D–
Figure 47. Measuring Temperature Using a PNP Transistor
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