OPA354-Q1_15 Datasheet, PDF (19/36 Pages) Texas Instruments – OPAx354-Q1 250-MHz, Rail-to-Rail I/O, CMOS Operational Amplifiers

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OPA354-Q1, OPA2354-Q1, OPA4354-Q1
SBOS492B – JUNE 2009 – REVISED DECEMBER 2014
8.2.1.2 Detailed Design Procedure
To achieve a maximally-flat, second-order Butterworth frequency response, the feedback pole should be set to:
1
GBW
=
2 ´ p ´ R_(FB) ´ C_(FB) 4 ´ p ´ R_(FB) ´ C_(D)
(3)
Use Equation 4 to calculate the bandwidth.
ƒ_{(–3 dB)} =
GBW
2 ´ p ´ R_(FB) ´ C_(D)
(4)
For other transimpedance bandwidths, consider the high-speed CMOS OPA380 (90-MHz GBW), OPA354 (100-
MHz GBW), OPA300 (180-MHz GBW), OPA355 (200-MHz GBW), or OPA656 and OPA657 (400-MHz GBW).
For single-supply applications, the +INx input can be biased with a positive DC voltage to allow the output to
reach true zero when the photodiode is not exposed to any light, and respond without the added delay that
results from coming out of the negative rail; this configuration is shown in Figure 38. This bias voltage also
appears across the photodiode, providing a reverse bias for faster operation.
0.5 pF
100 k
SFH213
1 F
±
OPAx354-Q1
+
13.7 k
280
5V
VOUT
Figure 38. Single-Supply Transimpedance Amplifier
For additional information, refer to the application bulletin from TI, Compensate Transimpedance Amplifiers
Intuitively (SBOA055).
8.2.1.2.1 Optimizing The Transimpedance Circuit
To achieve the best performance, components should be selected according to the following guidelines:
1. For lowest noise, select R_(FB) to create the total required gain. Using a lower value for R_(FB) and adding gain
after the transimpedance amplifier generally produces poorer noise performance. The noise produced by
R_(FB) increases with the square-root of R_(FB), whereas the signal increases linearly. Therefore, signal-to-noise
ratio improves when all the required gain is placed in the transimpedance stage.
2. Minimize photodiode capacitance and stray capacitance at the summing junction (inverting input). This
capacitance causes the voltage noise of the op amp to be amplified (increasing amplification at high
frequency). Using a low-noise voltage source to reverse-bias a photodiode can significantly reduce the
capacitance. Smaller photodiodes have lower capacitance. Use optics to concentrate light on a small
photodiode.
3. Noise increases with increased bandwidth. Limit the circuit bandwidth to only that required. Use a capacitor
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