ADM1033
http://onsemi.com
15
Table 12. CHANNEL SELECTOR
Bits 5:4 Channel Selector (Configuration 2)
00 Local Channel = Default
01 Remote 1 Channel
10 Reserved
11 Reserved
Removing Temperature Errors
As CPUs run faster and faster, it gets more difficult to
avoid high frequency clocks when routing the D+ and D−
traces around a system board. Even when the recommended
layout guidelines are followed, temperature errors attributed
to noise coupled onto the D+ and D− lines remain. High
frequency noise generally gives temperature measurements
that are consistently too high. The ADM1033 has Local and
Remote temperature offset registers at 0x16 and 0x17; one
for each channel. By completing a one-time calibration, the
user can determine the offset caused by the system board
noise and remove it using the offset registers. The registers
automatically add a twos compliment word to the remote
temperature measurements, ensuring correct readings in the
value registers.
Table 13. OFFSET REGISTERS
Registration Description Default
0x16 Local Offset 0x00
0x17 Remote 1 Offset 0x00
Table 14. OFFSET REGISTER VALUES
Code Offset Value
0 0000 000 0C (Default Value)
0 0000 001 0.125C
0 0000 111 0.875C
0 0001 111 1.875C
0 0111 111 7.875C
0 1111 111 15.875C
1 0000 000 −16C
1 1111 000 −0.875C
Layout Considerations
Digital boards can be electrically noisy environments. Try
to protect the analog inputs from noise, particularly when
measuring the very small voltages from a remote diode
sensor. Take the following precautions:
Place the ADM1033 as close as possible to the remote
sensing diode. A distance of 4 inches to 8 inches is
adequate, provided that the worst noise sources such as
clock generators, data/address buses, and CRTs are
avoided.
Route the D+ and D− tracks close together, in parallel,
with grounded guard tracks on each side. Provide a
ground plane under the tracks if possible.
Use wide tracks to minimize inductance and reduce
noise pickup. At least 5 mil track width and spacing are
recommended.
Figure 28. Arrangement of Signal Tracks
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
5 MIL
GND
D−
D+
GND
Try to minimize the number of copper/solder joints,
because they can cause thermocouple effects. Where
copper/solder joints are used, make sure that they are in
both the D+ and D− paths and at the same temperature.
Thermocouple effects are not a major problem because
1C corresponds to approximately 200 mV, a n d
thermocouple voltages are approximately 3 mV/C of
temperature difference. Unless there are two
thermocouples with a big temperature differential between
them, the voltages should be much less than 200 mV.
Place a 0.1 mF bypass capacitor close to the ADM1033.
If the distance to the remote sensor is more than 8 inches,
twisted pair cable is recommended. This works up to
about 6 feet to 12 feet.
For very long distances (up to 100 feet), use shielded
twisted pair such as Belden #8451 microphone cable.
Connect the twisted pair to D+ and D− and the shield to
GND, close to the ADM1033. Leave the remote end of
the shield unconnected to avoid ground loops.
Because the measurement technique uses switched
current sources, excessive cable and/or filter capacitance
can affect the measurement. When using long cables, the
filter capacitor C1 may be reduced or removed. In any case,
the total shunt capacitance should never exceed 1000 pF.
Noise Filtering
For temperature sensors operating in noisy environments,
common practice is to place a capacitor across the D+ and
D− pins to help combat the effects of noise. However, large
capacitances affect the accuracy of the temperature
measurement, leading to a recommended maximum
capacitor value of 1000 pF. While this capacitor reduces the
noise, it does not eliminate it, making it difficult to use the
sensor in a very noisy environment.
The ADM1033 has a major advantage over other devices
when it comes to eliminating the effects of noise on the
external sensor. The series resistance cancellation feature
allows a filter to be constructed between the external
temperature sensor and the part. The effect of any filter
resistance seen in series with the remote sensor is
automatically cancelled from the temperature.
