A/D Conversion Sequence

If a Start command is written (or generated automatical-

ly in the free-running autoconvert mode), all channels

are converted, and the results of all measurements are

available after the end of conversion. A BUSY status bit

in the status byte shows that the device is actually per-

forming 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-qual-

ity, diode-connected small-signal transistor. Accuracy

has been experimentally verified for all of the devices

listed in Table 1. The MAX1668/MAX1805 can also

directly measure the die temperature of CPUs and other

ICs having on-board temperature-sensing diodes.

The transistor must be a small-signal type, either NPN

or PNP, with a relatively high forward voltage; other-

wise, the A/D input voltage range can be violated. The

forward 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 100µA; 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-cur-

rent gain (+50 to +150, for example) indicate that the

manufacturer has good process controls and that the

devices have consistent VBE characteristics.

For heat-sink mounting, the 500-32BT02-000 thermal

sensor from Fenwal Electronics is a good choice. This

device consists of a diode-connected transistor, an

aluminum plate with screw hole, and twisted-pair cable

(Fenwal Inc., Milford, MA, 508-478-6000).

Thermal Mass and Self-Heating

Thermal mass can seriously degrade the MAX1668/

MAX1805’s effective accuracy. The thermal time con-

stant of the QSOP-16 package is about 140s in still air.

For the MAX1668/MAX1805 junction temperature to set-

tle 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, improves the situation. Take care to account

for thermal gradients between the heat source and the

sensor, and ensure that stray air currents across the

sensor package do not interfere with measurement

accuracy.

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 sinking maximum current

at the ALERT output. For example, with ALERT sinking

0.4V x 1mA. Package theta J-A is about 150°C/W, so

the resulting temperature rise is:

dT = 2.4mW x 150°C/W = 0.36°C

Even with these contrived circumstances, it is difficult

to introduce significant self-heating errors.

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 proper

external noise filtering are required for high-accuracy

remote measurements in electrically noisy environ-

ments.

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

capacitance. Higher capacitance than 3300pF intro-

duces errors due to the rise time of the switched cur-

rent source.

Nearly all noise sources tested cause additional error

measurements, typically by +1°C to +10°C, depending

on the frequency and amplitude (see Typical Operating

Characteristics).

PC Board Layout

1) Place the MAX1668/MAX1805 as close as practical

to the remote diode. In a noisy environment, such

Multichannel Remote/Local

Temperature Sensor

8

_______________________________________________________________________________________

CMPT3904

Central Semiconductor (USA)

MMBT3904

Motorola (USA)

MMBT3904

SST3904

Rohm Semiconductor (Japan)

KST3904-TF

Samsung (Korea)

FMMT3904CT-ND

Zetex (England)

MANUFACTURER

MODEL NUMBER

SMBT3904

Siemens (Germany)

Table 1. Remote-Sensor Transistor

Manufacturers

Note: Transistors must be diode connected (base shorted to

collector).

National Semiconductor (USA)