ADM1024 View Datasheet(PDF) - Analog Devices
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ADM1024 Datasheet PDF : 32 Pages
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ADM1024
(R2 has no effect as the input voltage at the device pin is 2.5 V
when VIN = plus full scale).
Offset voltages other than 2.5 V can be used, but the calculation
becomes more complicated.
TEMPERATURE MEASUREMENT SYSTEM
Internal Temperature Measurement
The ADM1024 contains an on-chip band gap temperature sensor,
whose output is digitized by the on-chip ADC. The temperature
data is stored in the Temperature Value Register (address 27h)
and the LSB from Bits 6 and 7 of the Temperature Configuration
Register (address 4Bh). As both positive and negative tempera-
tures can be measured, the temperature data is stored in twos
complement format, as shown in Table IV. Theoretically, the
temperature sensor and ADC can measure temperatures from
–128°C to +127°C with a resolution of 1°C, although tempera-
tures below –40°C and above +125°C are outside the operating
temperature range of the device.
External Temperature Measurement
The ADM1024 can measure the temperature of two external
diode sensors or diode connected transistors, connected to
Pins 13 and 14 or 17 and 18.
Pins 13 and 14 are a dedicated temperature input channel.
Pins 17 and 18 can be configured to measure a diode sensor by
setting Bit 2 of the Channel Mode Register to 1.
The forward voltage of a diode or diode connected transistor,
operated at a constant current, exhibits a negative temperature
coefficient of about –2 mV/°C. Unfortunately, the absolute
value of VBE varies from device to device, and individual calibra-
tion is required to null this out, so the technique is unsuitable
for mass-production.
The technique used in the ADM1024 is to measure the change
in VBE when the device is operated at two different currents.
This is given by:
∆VBE = KT/q × ln(N )
where:
K is Boltzmann’s constant.
q is the charge on the carrier.
T is the absolute temperature in Kelvins.
N is the ratio of the two currents.
Figure 7 shows the input signal conditioning used to measure the
output of an external temperature sensor. This figure shows the
external sensor as a substrate transistor provided for temperature
monitoring on some microprocessors, but it could equally well
be a discrete transistor.
I
D+
REMOTE
SENSING
TRANSISTOR D–
N ؋ I IBIAS
VDD
LOW-PASS
FILTER
fC = 65kHz
BIAS
DIODE
VOUT+
TO
ADC
VOUT–
Figure 7. Signal Conditioning for External Diode
Temperature Sensors
If a discrete transistor is used, the collector will not be grounded
and should be linked to the base. If a PNP transistor is used, the
base is connected to the D– input and the emitter to the D+ input.
If an NPN transistor is used, the emitter is connected to the D–
input and the base to the D+ input.
To prevent ground noise from interfering with the measurement,
the more negative terminal of the sensor is not referenced to
ground, but is biased above ground by an internal diode at the
D– input. As the sensor is operating in a noisy environment, C1
is provided as a noise filter. See the section on layout consider-
ations for more information on C1.
To measure ⌬VBE, the sensor is switched between operating
currents of I and N × I. The resulting waveform is passed
through a 65 kHz low-pass filter to remove noise, then to a
chopper stabilized amplifier that performs the functions of
amplification and rectification of the waveform to produce a dc
voltage proportional to ⌬VBE. This voltage is measured by the
ADC to give a temperature output in 8-bit twos complement
format. To further reduce the effects of noise, digital filtering is
performed by averaging the results of 16 measurement cycles.
An external temperature measurement takes nominally 9.6 ms.
The results of external temperature measurements are stored in
8-bit, twos complement format, as illustrated in Table IV.
Table IV. Temperature Data Format
Temperature
–128°C
–125°C
–100°C
–75°C
–50°C
–25°C
0°C
+0.5°C
+10°C
+25°C
+50°C
+75°C
+100°C
+125°C
+127°C
Digital Output
1000 0000
1000 0011
1001 1100
1011 0101
1100 1110
1110 0111
0000 0000
0000 0000
0000 1010
0001 1001
0011 0010
0100 1011
0110 0100
0111 1101
0111 1111
REV. B
–13–
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