Rainbow Electronics MAX6649 User Manual

Page 13

ADC Noise Filtering

The integrating ADC used has good noise rejection for
low-frequency signals such as 60Hz/120Hz power-sup-
ply hum. In noisy environments, high-frequency noise
reduction is needed for high-accuracy remote mea-
surements. The noise can be reduced with careful PC
board layout and proper external noise filtering.

High-frequency EMI is best filtered at DXP and DXN with
an external 2200pF capacitor. Larger capacitor values
can be used for added filtering, but do not exceed
3300pF because larger values can introduce errors due
to the rise time of the switched current source.

PC Board Layout

Follow these guidelines to reduce the measurement
error of the temperature sensors:

1) Place the MAX6646/MAX6647/MAX6649 as close as

is practical to the remote diode. In noisy environ-
ments, such as a computer motherboard, this dis-
tance can be 4in to 8in (typ). This length can be
increased if the worst noise sources are avoided.
Noise sources include CRTs, clock generators,
memory buses, and ISA/PCI buses.

2) Do not route the DXP-DXN lines next to the deflec-

tion coils of a CRT. Also, do not route the traces
across fast digital signals, which can easily intro-
duce 30°C error, even with good filtering.

3) Route the DXP and DXN traces in parallel and in

close proximity to each other, away from any higher
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. If high-voltage traces
are unavoidable, connect guard traces to GND on
either side of the DXP-DXN traces (Figure 4).

4) Route through as few vias and crossunders as possi-

ble to minimize copper/solder thermocouple effects.

5) When introducing a thermocouple, make sure that

both the DXP and the DXN paths have matching
thermocouples. A copper-solder thermocouple
exhibits 3µV/°C, and takes about 200µV of voltage
error at DXP-DXN to cause a 1°C measurement
error. Adding a few thermocouples causes a negligi-
ble error.

6) Use wide traces. Narrow traces are more inductive

and tend to pick up radiated noise. The 10mil widths
and spacing recommended in Figure 4 are not
absolutely necessary, as they offer only a minor
improvement in leakage and noise over narrow
traces. Use wider traces when practical.

7) Add a 200

Ω resistor in series with V

for best noise

filtering (see Typical Operating Circuit).

8) Copper cannot be used as an EMI shield; only fer-

rous materials such as steel work well. Placing a
copper ground plane between the DXP-DXN traces
and traces carrying high-frequency noise signals
does not help reduce EMI.

Twisted-Pair and Shielded Cables

Use a twisted-pair cable to connect the remote sensor
for remote-sensor distance longer than 8in, or in very
noisy environments. Twisted-pair cable lengths can be
between 6ft and 12ft before noise introduces excessive
errors. For longer distances, the best solution is a
shielded twisted pair like that used for audio micro-
phones. For example, Belden 8451 works well for dis-
tances up to 100ft in a noisy environment. At the
device, connect the twisted pair to DXP and DXN and
the shield to GND. Leave the shield unconnected at the
remote sensor.

For very long cable runs, the cable’s parasitic capaci-
tance often provides noise filtering, so the 2200pF
capacitor can often be removed or reduced in value.
Cable resistance also affects remote-sensor accuracy.
For every 1

Ω of series resistance, the error is approxi-

mately 0.5°C.

Thermal Mass and Self-Heating

When sensing local temperature, these devices are
intended to measure the temperature of the PC board
to which the devices are soldered. The leads provide a
good thermal path between the PC board traces and
the die. Thermal conductivity between the die and the
ambient air is poor by comparison, making air tempera-
ture measurements impractical. Because the thermal
mass of the PC board is far greater than that of the
MAX6646/MAX6647/MAX6649, the device follows tem-
perature changes on the PC board with little or no per-
ceivable delay.

MAX6646/MAX6647/MAX6649

+145°C Precision SMBus-Compatible Remote/

Local Sensors with Overtemperature Alarms

______________________________________________________________________________________

MINIMUM

10 MILS

GND

DXN

DXP

GND

Figure 4. Recommended DXP-DXN PC Traces