LT1880

10

The transimpedance gain is set to 51.1k

response time is not an issue, or it may be selected for

maximally flat response and highest possible bandwidth

Total output offset is below 262

µV, worst-case, over

temperature (0

°C–70°C). With a 5V output swing, this

guarantees a minimum 86dB dynamic range over

temperature (0

°C–70°C), and a full-scale photodiode

current of 98

µA.

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 1k

Ω 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 1.25k

Ω of resis-

tance set by R1 and R2. Imprecise components R4 and C1

ensure circuit stability, which would otherwise be exces-

sively 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 1k

Ω 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

RISE TIME

100mV OUTPUT STEP

5V

R3

150k, 1%

LT1634ACS8

-1.25

R1

1.24K

0.1%

R2

10

Ω

1%

C1

0.1

µF

1k

Ω

AT 0

°C

RTD*

R5

180

Ω, 5%

R4

1k, 5%

–50

°C TO 600°C

+

–

5V

*OMEGA F3141 1k

Ω, 0.1% PLATINUM RTD

(800) 826-6342

LT1880

APPLICATIO S I FOR ATIO

Figure 5. Single Supply Current Source for Platinum RTD