ADM1021A
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8
To measure DV
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, and
then to a chopper-stabilized amplifier that performs the
functions of amplification and rectification of the waveform
to produce a dc voltage proportional to DV
BE
. This voltage
is measured by the ADC to give a temperature output in
8-bit, twos complement format. To reduce the effects of
noise further, digital filtering is performed by averaging the
results of 16 measurement cycles.
Signal conditioning and measurement of the internal
temperature sensor is performed in a similar manner.
Differences Between the ADM1021 and the ADM1021A
Although the ADM1021A is pin-for-pin compatible with
the ADM1021, there are some differences between the two
devices. Below is a summary of these differences and
reasons for the changes.
1. The ADM1021A forces a larger current through
the remote temperature sensing diode, typically
205 mA vs. 90 mA for the ADM1021. The primary
reason for this is to improve the noise immunity of
the part.
2. As a result of the greater remote sensor source
current, the operating current of the ADM1021A is
higher than that of the ADM1021, typically
205 mA vs. 160 mA.
3. The temperature measurement range of the
ADM1021A is 0C to 127C, compared with
−128C to +127C for the ADM1021. As a result,
the ADM1021 should be used if negative
temperature measurement is required.
4. The power-on reset values of the remote and local
temperature values are −128C in the ADM1021A
as compared to 0C in the ADM1021. As the part
is powered up converting (except when the part is
in standby mode, that is, Pin 15 is pulled low), the
part measures the actual values of remote and local
temperature and writes these to the registers.
5. The four MSBs of the revision register can be used
to identify the part. The ADM1021 revision register
reads 0x0x, and the ADM1021A reads 0x3x.
6. The power-on default value of the address pointer
register is undefined in the ADM1021A and is
equal to 0x00 in the ADM1021. As a result, a
value must be written to the address pointer
register before a read is performed in the
ADM1021A. The ADM1021 is capable of reading
back local temperature without writing to the
address pointer register, as it defaulted to the local
temperature measurement register at powerup.
7. Setting the mask bit (Bit 7 Config Reg) on the
ADM1021A masks current and future ALERTs.
On the ADM1021, the mask bit, masks only
ALERTs. Any current ALERT has to be cleared
using an ARA.
Temperature Data Format
One LSB of the ADC corresponds to 1C so the ADC can
theoretically measure from −128C to +127C, although the
device does not measure temperatures below 0C; therefore,
the actual range is 0C to 127C. The temperature data
format is shown in Table 5.
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.
Table 5. TEMPERATURE DATA FORMAT
Temperature (5C) Digital Output
0 0 000 0000
1 0 000 0001
10 0 000 1010
25 0 001 1001
50 0 011 0010
75 0 100 1011
100 0 110 0100
125 0 111 1101
127 0 111 1111
Registers
The ADM1021A contains nine registers that are used to
store the results of remote and local temperature
measurements, and high and low temperature limits, and to
configure and control the device. A description of these
registers follows, and further details are given in Table 6 to
Table 8. It should be noted that the ADM1021A’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, produces an invalid result. Register
addresses above 0x0F are reserved for future use or used for
factory test purposes and should not be written to.
Address Pointer Register
The address pointer register does not have and does not
require an address, because 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.
Value Registers
The ADM1021A has two registers to store the results of
local and remote temperature measurements. These registers
are written to by the ADC and can only be read over the
SMBus.
Status Register
Bit 7 of the status register indicates when it is high that the
ADC is busy converting. Bit 5 to Bit 3 are flags that indicate
the results of the limit comparisons.
