Rainbow Electronics MAX1299 User 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