AD7294 Analog Devices, AD7294 Datasheet - Page 24

• Manufacturer Part Number: AD7294
• Description: 12-Bit Monitor and Control System with Multichannel ADC, DACs, Temperature Sensor, and Current Sense
• Manufacturer: Analog Devices

Specifications of AD7294

• Resolution (bits): 12bit
• # Chan: 9
• Sample Rate: 200kSPS
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (Vref) p-p,2 V p-p,Uni (Vref),Uni (Vref) x 2
• Adc Architecture: SAR
• Pkg Type: CSP

AD7294
increased about the preset (V
I
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 AV
to the R
TEMPERATURE SENSOR
The
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.
The temperature sensor module 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.
TRANSISTORS
SENSE
T1
SENSING
REMOTE
AD7294
OVERRANGE pin is set to a high logic level enabling
T2
SENSE
Figure 45. Internal and Remote Temperature Sensors
or V
contains one local and two remote temperature
D2+
D1+
16 × I
D1–
D2–
PP
pin.
4 × I
AD7294
MUX
I
REF
× 1.2)/12.5. If this occurs, the
BIAS DIODE
f
I-BIAS
C
V
= 65kHz
DD
LPF
AD7294
MUX
is based on the
REGISTERS
SENSOR
DD
TEMP
CAP
TO ADC
LIMIT
is applied
ALERT
Rev. H | Page 24 of 48
Remote Sensing Diode
The
2N3904 and 2N3906. If an alternative transistor is used, the
AD7294
are adhered to.
Ideality Factor
The ideality factor, n
deviation of the thermal diode from ideal behavior. The
AD7294
equation to calculate the error introduced at a Temperature T
(°C) when using a transistor whose n
To factor this in, the user can write the ∆T value to the offset
register. The
from, the temperature measurement.
Base Emitter Voltage
The
emitter voltage is greater than 0.25 V at 8 µA at the highest
operating temperature, and less than 0.95 V at 128 µA for the
lowest operating temperature.
Base Resistance
The base resistance should be less than 100 Ω.
h
A transistor with small variation in h
150) should be used. Small variation in h
control of the V
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.
FE
Variation
AD7294
AD7294
ΔT = (n
Figure 46. Measuring Temperature Using an NPN Transistor
Figure 47. Measuring Temperature Using a PNP Transistor
operates as specified provided the following conditions
is trimmed for an n
2N3906
f
2N3904
AD7294
is designed to work with discrete transistors,
operates as specified provided that the base-
− 1.008) × (273.15 K + T)
PNP
NPN
BE
characteristics.
f
, of the transistor is a measure of the
automatically adds it to, or subtracts it
10pF
10pF
f
value of 1.008. Use the following
f
FE
D+
D–
does not equal 1.008:
D+
D–
AD7294
(approximately 50 to
AD7294
FE
indicates tight
Data Sheet