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OPA380AIDG4 Datasheet(PDF) 11 Page - Texas Instruments |
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OPA380AIDG4 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 22 page  OPA380 OPA2380 SBOS291G − NOVEMBER 2003 − REVISED SEPTEMBER 2007 www.ti.com 11 BIASING PHOTODIODES IN SINGLE-SUPPLY CIRCUITS The +IN input can be biased with a positive DC voltage to offset the output voltage and allow the amplifier output to indicate a true zero photodiode measurement when the photodiode is not exposed to any light. It will also prevent the added delay that results from coming out of the negative rail. This bias voltage appears across the photodiode, providing a reverse bias for faster operation. An RC filter placed at this bias point will reduce noise, as shown in Figure 4. This bias voltage can also serve as an offset bias point for an ADC with range that does not include ground. OPA380 V OUT 100k Ω V+ R F 10M Ω C F (1) <1pF 0.1 µF λ NOTE: (1) C F is optional to prevent gain peaking. It includes the stray capacitance of R F. +V Bias Figure 4. Filtered Reverse Bias Voltage TRANSIMPEDANCE AMPLIFIER Wide bandwidth, low input bias current, and low input voltage and current noise make the OPA380 an ideal wideband photodiode transimpedance amplifier. Low-voltage noise is important because photodiode capacitance causes the effective noise gain of the circuit to increase at high frequency. The key elements to a transimpedance design are shown in Figure 5: the total input capacitance (CTOT), consisting of the photodiode capacitance (CDIODE) plus the parasitic common-mode and differential-mode input capacitance (3pF + 1.1pF for the OPA380); the desired transimpedance gain (RF); the Gain Bandwidth Product (GBW) for the OPA380 (90MHz). With these three variables set, the feedback capacitor value (CF) can be set to control the frequency response. CSTRAY is the stray capacitance of RF, which is 0.2pF for a typical surface-mount resistor. To achieve a maximally flat, 2nd-order, Butterworth frequency response, the feedback pole should be set to: 1 2 pR F CF ) C STRAY + GBW 4 pR FCTOT Bandwidth is calculated by: f*3dB + GBW 2 pR FCTOT Hz These equations will result in maximum transimpedance bandwidth. For even higher transimpedance bandwidth, the high-speed CMOS OPA300 (SBOS271 (180MHz GBW)), or the OPA656 (SBOS196 (230MHz GBW)) may be used. For additional information, refer to Application Bulletin AB−050 (SBOA055), Compensate Transimpedance Amplifiers Intuitively, available for download at www.ti.com. C TOT (3) OPA380 VOUT −5V 10M Ω +5V R F C F (1) C STRAY (2) λ NOTE: (1) C F is optional to prevent gain peaking. (2) C STRAY is the stray capacitance of RF (typically, 0.2pF for a surface−mount resistor). (3) C TOT is the photodiode capacitance plus OPA380 input capacitance. R P (optional pulldown resistor) Figure 5. Transimpedance Amplifier (1) (2) |
Similar Part No. - OPA380AIDG4 |
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Similar Description - OPA380AIDG4 |
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