Effect of Series Resistance
Series resistance in a sense diode contributes addition-
al errors. For nominal diode currents of 10µA and
100µA, change in the measured voltage is:
ΔV
M
= R
S
(100µA - 10µA) = 90µA x R
S
Since 1°C corresponds to 198.6µV, series resistance
contributes a temperature offset of:
Assume that the diode being measured has a series
resistance of 3Ω. The series resistance contributes an
offset of:
The effects of the ideality factor and series resistance
are additive. If the diode has an ideality factor of 1.002
and series resistance of 3Ω, the total offset can be cal-
culated by adding error due to series resistance with
error due to ideality factor:
1.36°C - 2.13°C = -0.77°C
for a diode temperature of +85°C.
In this example, the effect of the series resistance and
the ideality factor partially cancel each other.
For best accuracy, the discrete transistor should be a
small-signal device with its collector connected to GND
and base connected to DXN. Table 12 lists examples of
discrete transistors that are appropriate for use with the
MAX6639.
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 PCB carefully with proper external noise fil-
tering for high-accuracy remote measurements in elec-
trically 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 approximately 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 MAX6639 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.
.
