Rainbow Electronics MAX1299 - Manual (Page 17)

The transistor must be a small-signal type with a base
resistance less than 100
Ω. 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
be 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; 1 Ω
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-)
0 0 0 0 AIN0 AIN5
0 0 0 1 AIN1 AIN5
0 0 1 0 AIN2 AIN5
0 0 1 1 AIN3 AIN5
0 1 0 0 AIN4 AIN5
0 1 0 1 — —
0 1 1 0 AIN5 AIN5
0 1 1 1 Internal diode anode* Internal diode cathode
1 0 0 0 AIN0 AIN1
1 0 0 1 AIN2 AIN3
1 0 1 0 — —
1 0 1 1 V
DD
/4 GND
1 1 0 0 External diode 1 anode* (AIN0)
External diode 1 cathode
(AIN1)
1 1 0 1 External diode 2 anode* (AIN2)
External diode 2 cathode
(AIN3)
1 1 1 0 — —
1 1 1 1 — —
*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