MAX6636UP9A+T【PDF 16/18 页】IC PREC TEMP MONITOR 7CH 20TSSOP

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ADC Noise Filtering

The integrating ADC has good noise rejection for low-

frequency signals, such as power-supply hum. In envi-

ronments with significant high-frequency EMI, connect

an external 2200pF capacitor between DXP_ and

DXN_. Larger capacitor values can be used for added

filtering, but do not exceed 3300pF because it can

introduce errors due to the rise time of the switched

current source. High-frequency noise reduction is

needed for high-accuracy remote measurements.

Noise can be reduced with careful PCB layout as dis-

cussed in the PCB Layout section.

Slave Address

The 7-bit slave address of the MAX6636UP9A+ is

1001 101 (binary).

PCB Layout

Follow these guidelines to reduce the measurement

error when measuring remote temperature:

1) Place the MAX6636 as close as is practical to the

remote diode. In noisy environments, such as a

computer motherboard, this distance 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 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癈 error, even with good filtering.

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

close proximity to each other. Each parallel pair of

traces should go to a remote diode. Route these

traces away from any higher voltage traces, such as

+12VDC. Leakage currents from PCB contamination

must be dealt with carefully since a 20M?leakage

path from DXP to ground causes about +1癈 error. If

high-voltage traces are unavoidable, connect guard

traces to GND on either side of the DXP-DXN traces

(Figure 5).

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

sible to minimize copper/solder thermocouple

effects.

5) Use wide traces when practical. 5mil to 10mil traces

are typical. Be aware of the effect of trace resistance

on temperature readings when using long, narrow

traces.

6) When the power supply is noisy, add a resistor (up

to 47? in series with V

7-Channel Precision Temperature Monitor

16 ______________________________________________________________________________________

510 mils

MINIMUM

510 mils

GND

DXP

DXN

GND

Figure 5. Recommended DXP-DXN PCB Traces. The two outer

guard traces are recommended if high-voltage traces will be

near the DXN and DXP traces.

MANUFACTURER

MODEL NO.

Central Semiconductor (USA)

CMPT3904

Rohm Semiconductor (USA)

SST3904

Samsung (Korea)

KST3904-TF

Siemens (Germany)

SMBT3904

Zetex (England)

FMMT3904CT-ND

Table 10. Remote-Sensors Transistor

Manufacturer

Note: Discrete transistors must be diode connected (base

shorted to collector).

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