Rainbow-electronics MAX1299 Manual de usuario descargar pdf (Pagina 17)

The transistor must be a small-signal type with a base

resistance less than 100Ω. Tight specifications for for-

ward current gain (+50 to +150, for example) indicate

that the manufacturer has good process controls and

that the devices have consistent V

characteristics.

(See Table 6 for recommended devices.)

For heatsink mounting, the 500-32BT02-000 thermal

sensor from Fenwal Electronics is a good choice. This

device consists of a diode-connected transistor, an alu-

minum plate with screw hole, and twisted-pair cable

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

Twisted-Pair and Shielded Cables

For remote-sensor distances greater than 8 inches, or

in particularly noisy environments, use a twisted pair. A

practical length is 6 to 12 feet. For longer distances, the

best solution is a shielded twisted pair such as that

used for audio microphones. For example, the Belden

8451 works well for distances up to 100 feet in a noisy

environment. Connect the shield to SHO.

Cable resistances affect remote-sensor accuracy; 1Ω

series resistance introduces +0.004°C error.

Remote Diode Shielding

Temperature measurements will reflect significant error

if a portion of the bias current supplied to the diode

anode is allowed to flow through parallel paths to

ground. If the diode-connected transistor is mounted

on a PC board, suppress error-producing “leakage”

current by surrounding the collector/base leads with a

metal trace that is connected to the SHO shield output

(Figure 8).

MAX1298/MAX1299

12-Bit Serial-Output Temperature Sensors

with 5-Channel ADC

______________________________________________________________________________________ 17

SEL3 SEL2 SEL1 SEL0 POSITIVE INPUT (IN+) NEGATIVE INPUT (IN-)

0000 AIN0 AIN5

0001 AIN1 AIN5

0010 AIN2 AIN5

0011 AIN3 AIN5

0100 AIN4 AIN5

0101 — —

0110 AIN5 AIN5

0111 Internal diode anode* Internal diode cathode

1000 AIN0 AIN1

1001 AIN2 AIN3

1010 — —

1011

GND

1100 External diode 1 anode* (AIN0)

External diode 1 cathode

(AIN1)

1101 External diode 2 anode* (AIN2)

External diode 2 cathode

(AIN3)

1110 — —

1111 — —

*Temperature-measurement mode

Table 5. Input Selection

Table 6. Remote-Sensor Transistor

Manufacturer

MANUFACTURER MODEL NUMBER

Central Semiconductor

(USA)

CMPT3904

Fairchild Semiconductor

(USA)

MMBT3904

Motorola (USA) MMBT3904

Rohm Semiconductor

(Japan)

SST3904

Siemens (Germany) SMB3904

Zetex (England) FMMT3904CT-ND

1 2 ... 12 13 14 15 16 17 18 19 20

MAX1298/MAX1299 1

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