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G771 Datasheet(PDF) 6 Page - Global Mixed-mode Technology Inc

Part No. G771
Description  Two Remote Temperature Sensors with SMBus Serial Interface and System Reset Circuit
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Maker  GMT [Global Mixed-mode Technology Inc]
Homepage  http://www.gmt.com.tw
Logo 

 6 page
background image
Ver 0.2 Preliminary
Dec 11, 2001
TEL: 886-3-5788833
http://www.gmt.com.tw
6
G771
Global Mixed-mode Technology Inc.
A/D Conversion Sequence
If a Start command is written (or generated automati-
cally in the free-running auto-convert mode), both two
channels are converted, and the results of both meas-
urements are available after the end of conversion. A
BUSY status bit in the status byte shows that the de-
vice is actually performing a new conversion; however,
even if the ADC is busy, the results of the previous
conversion are always available.
Remote-Diode Selection
Temperature
accuracy
depends
on
having
a
good-quality, diode-connected small-signal transistor.
Accuracy has been experimentally verified for all of
the devices listed in Table 1. The G771 can also di-
rectly measure the die temperature of CPUs and other
integrated
circuits
having
on-board
temperature-
sensing diodes. The transistor must be a small-signal
type with a relatively high forward voltage; otherwise,
the A/D input voltage range can be violated. The for-
ward voltage must be greater than 0.25V at 10µA;
check to ensure this is true at the highest expected
temperature. The forward voltage must be less than
0.95V at 200A; check to ensure this is true at the low-
est expected temperature. Large power transistors
don't work at all. Also, ensure that the base resistance
is less than 100
Ω. Tight specifications for forward
current gain (+50 to +150, for example) indicate that
the manufacturer has good process controls and that
the devices have consistent VBE characteristics.
Thermal Mass and Self-Heating
Thermal mass can seriously degrade the G771's ef-
fective accuracy. The thermal time constant of the
SSOP-16 package is about 140sec in still air. For the
G771 junction temperature to settle to within +1°C
after a sudden +100°C change requires about five
time constants or 12 minutes. The use of smaller
packages for remote sensors, such as SOT23s, im-
proves the situation. Take care to account for thermal
gradients between the heat source and the sen-
sor ,and ensure that stray air current across the sen-
sor package do not interfere with measurement accu-
racy.
Table 1. Remote-Sensor Transistor Manufacturers
MANUFACTURER
MODEL NUMBER
Philips
PMBS 3904
Motorola (USA)
MMBT3904
National Semiconductor (USA)
MMBT3904
Note:Transistors must be diode-connected (base short
-ed to collector).
ADC Noise Filtering
The ADC is an integrating type with inherently good
noise rejection, especially of low-frequency signals
such as 60Hz/120Hz power-supply hum. Micro-power
operation places constraints on high-frequency noise
rejection; therefore, careful PC board layout and
proper external noise filtering are required for high-
accuracy remote measurements in electrically noisy
environments.
High-frequency EMI is best filtered at DXP and DXN
with an external 2200pF capacitor. This value can be
increased to about 3300pF(max), including cable ca-
pacitance. Higher capacitance than 3300pF introduces
errors due to the rise time of the switched current
source.
Nearly all noise sources tested cause the ADC meas-
urements to be higher than the actual temperature,
typically by +1°C to 10°C, depending on the frequency
and amplitude (see Typical Operating Characteristics).
PC Board Layout
Place the G771 as close as practical to the remote
diode. In a noisy environment, such as a computer
motherboard, this distance can be 4 in. to 8 in. (typical)
or more as long as the worst noise sources (such as
CRTs, clock generators, memory buses, and ISA/PCI
buses) are avoided.
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.
Route the DXP and DXN traces in parallel and in close
proximity to each other, away from any high-voltage
traces such as +12VDC. Leakage currents from PC
board contamination must be dealt with carefully,
since a 20M
Ω leakage path from DXP to ground
causes about +1°C error.
Route the 2 pairs of DXP1-DXN and DXP2-DXN
traces independently (Figure 2a). Connect the com-
mon DXN as close as possible to the DXN pin on IC
(Figure 2a).
Connect guard traces to GND on either side of the
DXP-DXN traces (Figure 2b). With guard traces in place,
routing near high-voltage traces is no longer an issue.
Route through as few vias and crossunders as possi-
ble to minimize copper/solder thermocouple effects.
When introducing a thermocouple, make sure that
both the DXP and the DXN paths have matching
thermocouples. In general, PC board- induced ther-
mocouples are not a serious problem, A copper-solder
thermocouple exhibits 3µV/°C, and it takes about
200µV of voltage error at DXP-DXN to cause a +1°C
measurement error. So, most parasitic thermocouple
errors are swamped out.
Use wide traces. Narrow ones are more inductive and
tend to pick up radiated noise. The 10 mil widths and
spacing recommended on Figure 2 aren't absolutely
necessary (as they offer only a minor improvement in
leakage and noise), but try to use them where practical.




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