OPA2320-Q1_15 Datasheet, PDF (18/31 Pages) Texas Instruments – Precision, 20-MHz, 0.9-pA, Low-Noise, RRIO,CMOS Operational Amplifier

OPA2320-Q1
SLOS884 – SEPTEMBER 2014
www.ti.com
ƒ_{(–3 dB)} =
GBW
2 ´ p ´ R_(FB) ´ C_(D)
(3)
For even higher transimpedance bandwidth, 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 37. This bias voltage also
appears across the photodiode, providing a reverse bias for faster operation.
(1)
C_(FB)
< 1pF
R_(FB)
10 MΩ
V_(V+)
l
OPA320-Q1
V_O
+V_(BIAS)
(1) C_(FB) is optional to prevent gain peaking. C_(FB) includes the stray capacitance of R_(FB).
Figure 37. 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
across the R_(FB) to limit bandwidth, even if not required for stability.
4. Circuit board leakage can degrade the performance of an otherwise well-designed amplifier. Clean the circuit
board carefully. A circuit board guard trace that encircles the summing junction and is driven at the same
voltage can help control leakage.
For additional information, refer to the following application bulletins from TI: Noise Analysis of FET
Transimpedance Amplifiers (SBOA060), and Noise Analysis for High-Speed Op Amps (SBOA066).
8.2.1.3 Application Curves
Wide gain bandwidth as shown in Figure 38 and low input voltage noise as shown in Figure 39 make the
OPA2320-Q1 device an ideal wideband photodiode transimpedance amplifier.
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