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ADM1023
Theory of Operation
Functional Description
The ADM1023 contains a two-channel analog-to-digital
converter (ADC) with special input-signal conditioning to
enable operation with remote and on-chip diode temperature
sensors. When the ADM1023 is operating normally, the
ADC operates in a free-running mode. The analog input
multiplexer alternately selects either the on-chip
temperature sensor to measure its local temperature or the
remote temperature sensor. These signals are digitized by
the ADC, and the results are stored in the local and remote
temperature value registers. Only the eight most significant
bits (MSBs) of the local temperature value are stored as an
8-bit binary word. The remote temperature value is stored as
an 11 ? bit binary word in two registers. The eight MSBs are
stored in the remote temperature value high byte register at
Address 0x01. The three least significant bits (LSBs) are
stored, left justified, in the remote temperature value low
byte register at Address 0x10.
Error sources such as PCB track resistance and clock noise
can introduce offset errors into measurements on the remote
channel. To achieve the specified accuracy on this channel,
these offsets must be removed, and two offset registers are
provided for this purpose at Address 0x11 and Address
0x12.
An offset value may automatically be added to or
subtracted from the measurement by writing an 11-bit, twos
complement value to Register 0x11 (high byte) and Register
0x12 (low byte, left-justified).
The offset registers default to 0 at powerup and have no
effect if nothing is written to them.
The measurement results are compared with local and
remote, high and low temperature limits, stored in six
on-chip limit registers. As with the measured value, the local
temperature limits are stored as 8-bit values and the remote
temperature limits as 11-bit values. Out-of-limit
comparisons generate flags that are stored in the status
register, and one or more out-of-limit results cause the
ALERT output to pull low.
Registers can be programmed, and the device controlled
and configured, via the serial system management bus
(SMBus). The contents of any register can also be read back
via the SMBus.
Control and configuration functions consist of:
? Switching the Device between Normal Operation and
Standby Mode
? Masking or Enabling the ALERT Output
? Selecting the Conversion Rate
On initial powerup, the remote and local temperature
values default to ? 128 ? C. The device normally powers up
converting, making a measure of local and remote
temperature. These values are then stored before making a
comparison with the stored l imits. However, if the part is
powered up in standby mode (STBY pin pulled low), no new
values are written to the register before a comparison is
made. As a result, both RLOW and LLOW are tripped in the
status register, thus generating an ALERT output. This may
be cleared in one of two ways:
? Change both the local and remote lower limits to
–128 ? C and read the status register (which in turn
clears the ALERT output).
? Take the part out of standby and read the status register
(which in turn clears the ALERT output). This works
only when the measured values are within the limit
values.
D+
I
N y I
I BIAS
V DD
V OUT+
REMOTE
SENSING
TRANSISTOR
C1 1
D–
BIAS
DIODE
LOW ? PASS FILTER
f C = 65kHz
TO ADC
V OUT–
1 CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.
C1 = 1000pF MAX.
Figure 13. Input Signal Conditioning
Measurement Method
A simple method of measuring temperature is to exploit
the negative temperature coefficient of a diode, or the base
emitter voltage of a transistor, operating at constant current.
Thus, the temperature may be obtained from a direct
measurement of V BE where:
This technique, however, requires calibration to nullify
the effect of the absolute value of V BE , which varies from
device to device.
The technique used in the ADM1023 is to measure the
change in V BE when the device is operated at two different
collector currents.
q
V BE + nKT
1n
I C
I S
(eq. 1)
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