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ADM1021 Datasheet(PDF) 6 Page - Analog Devices

Part No. ADM1021
Description  Low Cost Microprocessor System Temperature Monitor
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADM1021 Datasheet(HTML) 6 Page - Analog Devices

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ADM1021
–6–
REV. 0
Figure 12 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 tem-
perature monitoring on some microprocessors, but it could
equally well be a discrete transistor. If a discrete transistor is
used, the collector will not be grounded and should be linked to
the base. To prevent ground noise interfering with the measure-
ment, 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. If the sensor is operating in a noisy environment,
C1 may optionally be added as a noise filter. Its value is typi-
cally 2200 pF, but should be no more than 3000 pF. See the
section on layout considerations for more information on C1.
To measure
∆V
be, 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, thence to a chopper-
stabilized amplifier that performs the functions of amplification
and rectification of the waveform to produce a dc voltage pro-
portional to
∆V
be. 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.
Signal conditioning and measurement of the internal tempera-
ture sensor is performed in a similar manner.
TEMPERATURE DATA FORMAT
One LSB of the ADC corresponds to 1
°C, so the ADC can
theoretically measure from –128
°C to +127°C, although the
practical lowest value is limited to –65
°C due to device maxi-
mum ratings. The temperature data format is shown in Table I.
The results of the local and remote temperature measurements
are stored in the local and remote temperature value registers,
and are compared with limits programmed into the local and
remote high and low limit registers.
C1*
D+
D–
REMOTE
SENSING
TRANSISTOR
IN
I
IBIAS
VDD
VOUT+
TO ADC
VOUT–
BIAS
DIODE
LOWPASS FILTER
fC = 65kHz
CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.
C1 = 2.2nF TYPICAL, 3nF MAX.
*
Figure 12. Input Signal Conditioning
Table I. Temperature Data Format
Temperature
Digital Output
–128
°C
1 000 0000
–125
°C
1 000 0011
–100
°C
1 001 1100
–75
°C
1 011 0101
–50
°C
1 100 1110
–25
°C
1 110 0111
–1
°C
1 111 1111
0
°C
0 000 0000
+1
°C
0 000 0001
+10
°C
0 000 1010
+25
°C
0 001 1001
+50
°C
0 011 0010
+75
°C
0 100 1011
+100
°C
0 110 0100
+125
°C
0 111 1101
+127
°C
0 111 1111
REGISTERS
The ADM1021 contains nine registers that are used to store the
results of remote and local temperature measurements, high and
low temperature limits, and to configure and control the device.
A description of these registers follows, and further details are
given in Tables II to IV. It should be noted that the ADM1021’s
registers are dual port, and have different addresses for read and
write operations. Attempting to write to a read address, or to
read from a write address, will produce an invalid result. Regis-
ter addresses above 0Fh are reserved for future use or used for
factory test purposes and should not be written to.
Address Pointer Register
The Address Pointer Register itself does not have, nor does it
require, an address, as it is the register to which the first data
byte of every Write operation is written automatically. This data
byte is an address pointer that sets up one of the other registers
for the second byte of the Write operation, or for a subsequent
read operation.


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