Global Mixed-mode Technology Inc.
Self-heating does not significantly affect measurement
accuracy. Remote-sensor self-heating due to the diode
current source is negligible. For the local diode, the
worst-case error occurs when auto-converting at the
fastest rate and simultaneously sinking maximum cur-
rent at the ALERT output. For example, at an 8Hz rate
and with
ALERT
sinking 1mA, the typical power dissi-
pation is Vcc x 450µA plus 0.4V x 1mA. Package theta
J-A is about 150°C /W, so with Vcc = 5V and no copper
PC board heat-sinking, the resulting temperature rise is:
dT = 2.7mW x 150°C /W = 0.4°C
Even with these contrived circumstances, it is difficult
to introduce significant self-heating errors.
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.
G767
Route the DXP and DXN traces in parallel and in close
proximity to each other, away from any high-voltage
traces such as +12V
DC
. 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.
Connect guard traces to GND on either side of the
DXP-DXN traces (Figure 2). With guard traces in place,
routing near high-voltage traces is no longer an issue.
Route through as few vias and crossunders as
possible to minimize copper/solder thermocouple ef-
fects.
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 practi-
cal.
Keep in mind that copper can’t be used as an EMI
shield, and only ferrous materials such as steel work
will. Placing a copper ground plane between the
DXP-DXN traces and traces carrying high-frequency
noise signals does not help reduce EMI.
Table 1. Remote-Sensor Transistor Manufacturers
MANUFACTURER
Philips
Motorola(USA)
National Semiconductor(USA)
MODEL NUMBER
PMBS3904
MMBT3904
MMBT3904
Note:Transistors must be diode-connected (base
shorted 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. Micropower
operation places constraints on high-frequency noise
rejection; therefore, careful PC board layout and prop-
er 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 G767 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.
PC Board Layout Checklist
Place the G767 close to a remote diode.
Keep traces away from high voltages (+12V bus).
Keep traces away from fast data buses and CRTs.
Use recommended trace widths and spacing.
Place a ground plane under the traces
Use guard traces flanking DXP and DXN and con
necting to GND.
Place the noise filter and the 0.1µF Vcc bypass
capacitors close to the G767.
Add a 200
Ω
resistor in series with Vcc for best
noise filtering (see Typical Operating Circuit).
Ver: 2.5
Dec 14, 2004
TEL: 886-3-5788833
http://www.gmt.com.tw
6
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
