Datasheet

36
LTC2400
TYPICAL APPLICATIONS
U
The 12.1k resistor should be a precision resistor such as
a Vishay S102 series, or must be temperature stabilized.
The excitation current is low enough for most sensors that
the self-heating effect is near the noise floor of the LTC2400.
The use of a bipolar amplifier configuration shown in
Figure 39 offers a potential resolution of 0.001°C
In order to achieve these results, the following effects
must be considered. Variation in the self-heating of the
RTD element due to air currents is the most difficult
challenge. If the RTD is mounted in a sealed glass enclo-
sure and painted black, the LTC2400 can detect the arrival
of a person in the room. This is also true of infrared
thermocouple sensors (thermopiles) that can also be used
directly with the LTC2400. A variation of this circuit with
two RTDs can detect small differential temperatures in
order to determine heat inflow or outflow from a process.
In order for this circuit to be practical, the ambient tem-
perature of the amplifier and resistors must be controlled
or the resistors must exhibit very low temperature coeffi-
cients. Precision resistor networks are always a good
alternative and are available from Vishay or Caddock.
Half-Bridge Strain Gauge
The circuit in Figure 40 is a ratiometric half-bridge circuit
with direct connection to the LTC2400. The use of two
thin-film strain gauges in a half-bridge configuration can
produce 2mV/V output and approximately 12-bit resolu-
tion. The 175Ω source impedance seen by the LTC2400
does not compromise operation.
The optional resistor shown can be up to 5k and will
provide surge and transient protection for the LTC2400
if the strain gauges are located some distance from the
LTC2400, or if the strain bearing member is not well
grounded and may be subject to ESD discharge. Thin-
film strain elements form coupling capacitance to the
strain bearing member to which they are bonded. If noise
–
+
LT1028
6
TO
LTC2400
3
2
300Ω
1k
5V
–5V
V
REF
R3*
9.09k
R4**
100Ω
R2*
9.09k
Pt RTD
100Ω
MUST BE 5ppm/°C OR BETTER,
AN ARRAY IS RECOMMENDED
MUST BE VERY STABLE <5ppm/°C
*
**
R1*
9.09k
0.1µF
2400 F38
FS
V
IN
SDO
SCK
CS
3
5V
2
350Ω
STRAIN
ELEMENT
R1
5k
OPTIONAL
350Ω
STRAIN
ELEMENT
1
4
5
6
7
8
V
REF
V
CC
0.1µF
GND
LTC2400
F
O
2400 F39
5V
10k
60Hz 50Hz
Figure 39. Extremely High Resolution RTD Interface Figure 40. Half-Bridge Connection for Strain Gauges