LT1880

9

The circuit of Figure 2 shows an extreme example of the

inverting case. The input voltage at the 1M resistor can

swing

±13.5V and the LT1880 will output an inverted,

divided-by-ten version of the input voltage. The input

accuracy is limited by the resistors to 0.2%. Output

referred, this error becomes 2.7mV. The 40

µV input offset

voltage contribution, plus the additional error due to input

bias current times the ~100k effective source impedance,

contribute only negligibly to error.

–

+

OP AMP INPUTS DO NOT MOVE,

BUT ARE FIXED AT DC BIAS

INPUT DOES NOT HAVE TO BE

RAIL-TO-RAIL

–

+

INPUTS MOVE BY AS MUCH AS

MORE

INPUT MAY NOT HAVE TO BE

RAIL-TO-RAIL

–

+

INPUTS MOVE AS MUCH AS

OUTPUT

INPUT MUST BE RAIL-TO-

RAIL FOR OVERALL CIRCUIT

RAIL-TO-RAIL PERFORMANCE

APPLICATIO S I FOR ATIO

1.5V

–1.5V

100k, 0.1%

1M, 0.1%

±1.35V

OUTPUT

SWING

±13.5V SWINGS

WELL OUTSIDE

SUPPLY RAILS

–

+

LT1880

Precision Photodiode Amplifier

Photodiode amplifiers usually employ JFET op amps be-

cause of their low bias current; however, when precision

is required, JFET op amps are generally inadequate due to

their relatively high input offset voltage and drift. The

LT1880 provides a high degree of precision with very low

cable to this demanding task. Figure 3 shows an LT1880

configured as a transimpedance photodiode amplifier.

OUT

5V

–5V

PHOTODIODE

(SEE TEXT)

LT1880

WORST-CASE

OUTPUT OFFSET

≤196µV AT 25°C

≤262µV 0°C TO 70°C

≤323µV –40°C TO 85°C

Figure 1. Some Op Amp Configurations Do Not Require

Rail-to Rail Inputs to Achieve Rail-to-Rail Outputs

Figure 2. Extreme Inverting Case: Circuit Operates Properly

with Input Voltage Swing Well Outside Op Amp Supply Rails.

Figure 3. Precision Photodiode Amplifier