The transistor must be a small-signal type with a rela-
tively high forward voltage; otherwise, the ADC input
voltage range can be violated. The forward voltage at
the highest expected temperature must be greater than
0.25V at 10μA, and at the lowest expected temperature,
the forward voltage must be less than 0.95V at 100μA.
Large-power transistors must not be used. Also, ensure
that the base resistance is less than 100Ω. Tight speci-
fications for forward current gain (50 < fl < 150, for
example) indicate that the manufacturer has good
process controls and that the devices have consistent
V
BE
characteristics.
ADC Noise Filtering
The integrating ADC has inherently good noise rejec-
tion, especially of low-frequency signals such as
60Hz/120Hz power-supply hum. Micropower operation
places constraints on high-frequency noise rejection.
Lay out the printed-circuit board (PCB) carefully with
proper external noise filtering for high-accuracy remote
measurements in electrically noisy environments.
Filter high-frequency electromagnetic interference
(EMI) at DXP and DXN with an external 2200pF capaci-
tor connected between the two inputs. This capacitor
can be increased to about 3300pF (max), including
cable capacitance. A capacitance higher than 3300pF
introduces errors due to the rise time of the switched-
current source.
Twisted Pairs and Shielded Cables
For remote-sensor distances longer than 8in, or in par-
ticularly noisy environments, a twisted pair is recom-
mended. Its practical length is 6ft to 12ft (typ) before
noise becomes a problem, as tested in a noisy elec-
tronics laboratory. For longer distances, the best solu-
tion is a shielded twisted pair like that used for audio
microphones. For example, Belden #8451 works well
for distances up to 100ft in a noisy environment.
Connect the twisted pair to DXP and DXN and the
shield to ground, and leave the shield’s remote end
unterminated. Excess capacitance at DXN or DXP limits
practical remote-sensor distances (see the
Typical
Operating Characteristics
).
For very long cable runs, the cable’s parasitic capaci-
tance often provides noise filtering, so the recommend-
ed 2200pF capacitor can often be removed or reduced
in value. Cable resistance also affects remote-sensor
accuracy. A 1Ω series resistance introduces about
+1/2°C error.
PCB Layout Checklist
1) Place the MAX6640 as close as practical to the
remote diode. In a noisy environment, such as a
computer motherboard, this distance can be 4in to
8in, or more, as long as the worst noise sources
(such as CRTs, clock generators, memory buses,
and ISA/PCI buses) are avoided.
2) Do not route the DXP/DXN lines next to the deflection
coils of a CRT. Also, do not route the traces across a
fast memory bus, which can easily introduce +30°C
error, even with good filtering. Otherwise, most noise
sources are fairly benign.
3) Route the DXP and DXN traces parallel and close to
each other, away from any high-voltage traces such
as +12VDC. Avoid leakage currents from PCB cont-
amination. A 20MΩ leakage path from DXP ground
causes approximately +1°C error.
4) Connect guard traces to GND on either side of the
DXP/DXN traces. With guard traces, placing routing
near high-voltage traces is no longer an issue.
5) Route as few vias and crossunders as possible to
minimize copper/solder thermocouple effects.
6) When introducing a thermocouple, make sure that
both the DXP and the DXN paths have matching
thermocouples. In general, PCB-induced thermo-
couples are not a serious problem. A copper solder
thermocouple exhibits 3μV/°C, and it takes approxi-
mately 200μV of voltage error at DXP/DXN to cause
a +1°C measurement error, so most parasitic ther-
mocouple errors are swamped out.
7) Use wide traces. Narrow traces are more inductive
and tend to pick up radiated noise. The 10-mil widths
and spacings recommended are not absolutely nec-
essary (as they offer only a minor improvement in
leakage and noise), but use them where practical.
8) Placing an electrically clean copper ground plane
between the DXP/DXN traces and traces carrying
high-frequency noise signals helps reduce EMI.
MAX6640
2-Channel Temperature Monitor with Dual
Automatic PWM Fan-Speed Controller
______________________________________________________________________________________ 19
