LT1880 APPLICATIONS INFORMATION VREF + VIN + VIN + RG VIN – – – RF RF RG VREF INVERTING: AV = –RF/RG NONINVERTING: AV = 1 +RF/RG NONINVERTING: AV = +1 OP AMP INPUTS DO NOT MOVE, INPUTS MOVE BY AS MUCH AS INPUTS MOVE AS MUCH AS BUT ARE FIXED AT DC BIAS VIN, BUT THE OUTPUT MOVES OUTPUT POINT VREF MORE INPUT MUST BE INPUT DOES NOT HAVE TO BE INPUT MAY NOT HAVE TO BE RAIL-TO-RAIL FOR OVERALL RAIL-TO-RAIL RAIL-TO-RAIL CIRCUIT RAIL-TO-RAIL PERFORMANCE 1880 F01 Figure 1. Some Op Amp Confi gurations Do Not Require Rail-to Rail Inputs to Achieve Rail-to-Rail Outputs The circuit of Figure 2 shows an extreme example of the Precision Photodiode Amplifier inverting case. The input voltage at the 1M resistor can Photodiode amplifiers usually employ JFET op amps be- swing ±13.5V and the LT1880 will output an inverted, cause of their low bias current; however, when precision divided-by-ten version of the input voltage. The input is required, JFET op amps are generally inadequate due to accuracy is limited by the resistors to 0.2%. Output their relatively high input offset voltage and drift. The referred, this error becomes 2.7mV. The 40μV input offset LT1880 provides a high degree of precision with very low voltage contribution, plus the additional error due to input bias current (I bias current times the ~100k effective source impedance, B = 150pA typical) and is therefore appli- cable to this demanding task. Figure 3 shows an LT1880 contribute only negligibly to error. configured as a transimpedance photodiode amplifier. 1.5V ±1.35V ±13.5V SWINGS OUTPUT CF WELL OUTSIDE SWING SUPPLY RAILS WORST-CASE + OUTPUT OFFSET RF 51.1k ≤196μV AT 25°C LT1880 ≤262μV 0°C TO 70°C ≤323μV –40°C TO 85°C 5V VIN – 1M, 0.1% – PHOTODIODE (SEE TEXT) 100k, 0.1% LT1880 OUT CD + –1.5V 1880 F02 –5V Figure 2. Extreme Inverting Case: Circuit Operates Properly with 1880 F02 Input Voltage Swing Well Outside Op Amp Supply Rails.Figure 3. Precision Photodiode Amplifi er 1880fa 9
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