LT1880
10
1880fa
APPLICATIONS INFORMATION
The transimpedance gain is set to 51.1kΩ by R
F
. The
feedback capacitor, C
F
, may be as large as desired where
response time is not an issue, or it may be selected for
maximally flat response and highest possible bandwidth
given a photodiode capacitance C
D
. Figure 4 shows a
chart of C
F
and rise time versus C
D
for maximally flat
response. Total output offset is below 262μV, worst-case,
over temperature (0°C to 70°C). With a 5V output swing,
this guarantees a minimum 86dB dynamic range over
temperature (0°C to 70°C), and a full-scale photodiode
current of 98μA.
Single-Supply Current Source for Platinum RTD
The precision, low bias current input stage of the LT1880
makes it ideal for precision integrators and current
sources. Figure 5 shows the LT1880 providing a simple
precision current source for a remote 1kΩ RTD on a 4-wire
connection. The LT1634 reference places 1.25V at the
noninverting input of the LT1880, which then maintains
its inverting input at the same voltage by driving 1mA
of current through the RTD and the total 1.25kΩ of
resistance set by R1 and R2. Imprecise components R4
and C1 ensure circuit stability, which would otherwise be
excessively dependant on the cable characteristics. R5 is
also noncritical and is included to improve ESD immunity
and decouple any cable capacitance from the LT1880’s
output. The 4-wire cable allows Kelvin sensing of the RTD
voltage while excluding the cable IR drops from the voltage
reading. With 1mA excitation, a 1kΩ RTD will have 1V
across it at 0°C, and +3.85mV/°C temperature response.
This voltage can be easily read in myriad ways, with the
best method depending on the temperature region to be
emphasized and the particular ADC that will be reading
the voltage.
0.1 1 10 100 1000
100
10
1
0.1
C
D
(pF)
C
F
RISE TIME
RISE TIME (μs), C
F
(pF)
100mV OUTPUT STEP
1880 F04
–
+
5V
R3
150k, 1%
LT1634ACS8
-1.25
R1
1.24K
0.1%
R2
10Ω
1%
C1
0.1μF
1kΩ
AT 0°C
RTD*
R5
180Ω, 5%
R4
1k, 5%
V
OUT
= 1.00V AT 0°C + 3.85mV/°C
–50°C TO 600°C
+
–
5V
*OMEGA F3141 1kΩ, 0.1% PLATINUM RTD (800) 826-6342
LT1880
1880 F05
Figure 4. Feedback C
F
and Rise Time vs Photodiode C
D
Figure 5. Single Supply Current Source for Platinum RTD
