Datasheet

=
)
(
[IB1
( )
n
2
(
)
RREFIBIB 212 +
( )
n
ADC_DOUT
2
=
2
VREF
( )
GainVIN
=
( )
22
(
>
@
)
Gain
RTD - RCOMP
( )
n
2
ADC_DOUT
ADC_DOUT
RTD - RCOMP
Gain]
RREF
VIN0 = IB1 (RLINE1 + RTD) + (IB1 + IB2) (RLINE3 + RREF)
VIN1 = IB2 (RLINE2 + RCOMP) + (IB1 + IB2) (RLINE3 + RREF)
If RLINE1 = RLINE2, then:
VIN = (VIN0 - VIN1) = IB1 (RTD - RCOMP)
RLINE1
RLINE2
RREF
IB1
VIN7/VREFN2
IB2
RLINE3
VIN6/VREFP2
VIN0
VIN1
VA
RTD
PT-100
RCOMP
= 0:
IB1 =
1 mA
IB2 =
1 mA
+
0.1 PF
+
VIO
LMP90100
XOUT
XIN/CLK
SCLK
CSB
SDO
SDI
drdyb = D6
Microcontroller
3V 3V
12 pF
12 pF
D5
3.57
MHz
1 PF
0.1 PF 1 PF
LMP90100
SNAS510P –JANUARY 2011–REVISED MARCH 2013 www.ti.com
Figure 70. Topology #1: 3-wire RTD Using 2 Current Sources
Figure 70 shows the first topology for a 3-wire resistive temperature detector (RTD) application. Topology #1
uses two excitation current sources, IB1 and IB2, to create a differential voltage across VIN0 and VIN1. As a
result of using both IB1 and IB2, only one channel (VIN0-VIN1) needs to be measured. As shown in Equation 2,
the equation for this channel is IB1 x (RTD – RCOMP) assuming that RLINE1 = RLINE2.
Equation 2 — VIN Equation for Topology #1 (16)
The PT-100 changes linearly from 100 Ohm at 0°C to 146.07 Ohm at 120°C. If desired, choose a suitable
compensating resistor (RCOMP) so that VIN can be virtually 0V at any desirable temperature. For example, if
RCOMP = 100 Ohm, then at 0°C, VIN = 0V and thus a higher gain can be used.
The advantage of this circuit is its ratiometric configuration, where VREF = (IB1 + IB2) x (RREF). Equation 3
shows that a ratiometric configuration eliminates IB1 and IB2 from the output equation, thus increasing the
overall performance.
Equation 3 — ADC_DOUT Showing IB1 & IB2 Elimination (17)
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