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OPA2381AIDGKT Datasheet(PDF) 11 Page - Texas Instruments |
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OPA2381AIDGKT Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 24 page  OPA381 OPA2381 SBOS313B − AUGUST 2004 − REVISED NOVEMBER 2004 www.ti.com 11 TRANSIMPEDANCE BANDWIDTH AND NOISE Limiting the gain set by RF can decrease the noise occurring at the output of the transimpedance circuit. However, all required gain should occur in the transimpedance stage, since adding gain after the transimpedance amplifier generally produces poorer noise performance. The noise spectral density produced by RF increases with the square-root of RF, whereas the signal increases linearly. Therefore, signal-to-noise ratio is improved when all the required gain is placed in the transimpedance stage. Total noise increases with increased bandwidth. Limit the circuit bandwidth to only that required. Use a capacitor, CF, across the feedback resistor, RF, to limit bandwidth (even if not required for stability), if total output noise is a concern. Figure 6a shows the transimpedance circuit without any feedback capacitor. The resulting transimpedance gain of this circuit is shown in Figure 7. The –3dB point is approximately 3MHz. Adding a 16pF feedback capacitor (Figure 6b) will limit the bandwidth and result in a –3dB point at approximately 200kHz (seen in Figure 7). Output noise will be further reduced by adding a filter (RFILTER and CFILTER) to create a second pole (Figure 6c). This second pole is placed within the feedback loop to maintain the amplifier’s low output impedance. (If the pole was placed outside the feedback loop, an additional buffer would be required and would inadvertently increase noise and dc error). Using RDIODE to represent the equivalent diode resistance, and CTOT for equivalent diode capacitance plus OPA381 input capacitance, the noise zero, fZ, is calculated by: f Z + R DIODE ) RF 2 pR DIODERF CTOT ) CF CFILTER = 3.9nF RFILTER = 102k Ω CF = 22pF OPA381 RF = 50kΩ OPA381 OPA381 RF = 50kΩ RF = 50kΩ C F = 16pF V OUT V BIAS (b) λ C STRAY = 0.2pF V OUT V BIAS (a) λ C STRAY =0.2pF V OUT V BIAS (c) λ C STRAY =0.2pF Figure 6. Transimpedance Circuit Configurations with Varying Total and Integrated Noise Gain (3) |
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