AN-1244

Photo-Diode Current-to-Voltage Converters Photo-Diode Current-to-Voltage Converters Converting the small output current of a photo-diode transducer to a fast responding voltage is often challenging. Here are some ways to use high-speed Current Feedback and Voltage Feedback op amps to do the job National Semiconductor Application Note 1244 Hooman Hashemi September 2002 Current Feedback Amplifier Solution Current Feedback Amplifiers (CFA) are especially suited to implement this function, as shown in Figure 1. With an effective internal buffer on the inverting node of the op amp, the output impedance RO (internal to U1, not shown) and the photo-diode’s output capacitance CIN (typically 10-200pF) introduce a zero in the noise gain at approximately 1/2π x (RO x CIN). In comparison, the zero produced by a Voltage Feedback op amp in a similar configuration [1/2π x (RIN||RF||RBIAS) x CIN] tends to be much lower in frequency and more troublesome. This being the case, CIN has less of an effect on reduction of the converter bandwidth, and achieving stability is easier when using a CFA. If CIN is sufficiently large, the closed loop phase shift will approach – 180˚ at the cross-over frequency (where open loop transimpedance gain crosses the noise gain function). As with Voltage Feedback Amplifiers, the closed loop amplifier can be compensated by adding a small capacitor (CF) across RF. In the case of Figure 1, using the CLC450 CFA, CF was experimentally determined to be around 2pF for about 10% overshoot in the step response. CF improves stability by counteracting the effect of the zero discussed in the paragraph above by introducing a low frequency pole (1/2π x RF x CF) and an inconsequential zero (1/2π x RO x CF). 20050001 FIGURE 1. Single-Supply Photo-Diode Amplifier Using CLC450 Current-Feedback Amplifier It is possible to change the required 2pF compensation capacitor to a more practical value, by adding RA and RB in a voltage divider, as shown in Figure 2. The new value of C'f is (1+RB/RA) x CF. This relationship holds true as long as RB