Datasheet

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SBOS278B − SEPTEMBER 2003 − REVISED JANUARY 2004
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13
To achieve a maximally-flat, 2nd-order Butterworth
frequency response, the feedback pole should be set to:
1
2pR
F
C
F
+
GBW
4pR
F
C
D
Ǹ
Bandwidth is calculated by:
f
*3dB
+
GBW
2pR
F
C
D
Ǹ
Hz
For even higher transimpedance bandwidth, the
high-speed CMOS OPA354 (100MHz GBW), OPA300
(180 MHz GBW), OPA355 (200MHz GBW), or OPA656,
OPA657 (400MHz GBW) may be used.
For single-supply applications, the +IN 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. (Refer to Figure 6.) This bias
voltage also appears across the photodiode, providing a
reverse bias for faster operation.
OPA725
V
OUT
10M
Ω
V+
R
F
C
F
(1)
<1pF
λ
NOTE: (1) C
F
is optional to prevent gain peaking.
It includes the stray capacitance of R
F
.
+V
Bias
Figure 6. Single-Supply Transimpedance
Amplifier
For additional information, refer to Application Bulletin
SBOA055, Compensate Transimpedance Amplifiers
Intuitively, available for download at www.ti.com.
OPTIMIZING THE TRANSIMPEDANCE
CIRCUIT
To achieve the best performance, components should be
selected according to the following guidelines:
1. For lowest noise, select R
F
to create the total required
gain. Using a lower value for R
F
and adding gain after
the transimpedance amplifier generally produces
poorer noise performance. The noise produced by R
F
increases with the square-root of R
F
, whereas the
signal increases linearly. Therefore, signal-to-noise
ratio is improved 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 its
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
F
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 Application Bulletins
Noise Analysis of FET Transimpedance Amplifiers
(SBOA060), and Noise Analysis for High-Speed Op Amps
(SBOA066), available for download at the TI web site.
(1)
(2)