Datasheet

OPA354
V
OUT
R
F
10M
Ω
C
F
< 1pF
(prevents gain peaking)
+V
λ
C
D
1
2pR
F
C
F
+
GBP
4pR
F
C
D
Ǹ
f
*3dB
+
GBP
2pR
F
C
D
Ǹ
Hz
OPA354-Q1
OPA2354-Q1
SBOS492A –JUNE 2009–REVISED AUGUST 2009 ....................................................................................................................................................... www.ti.com
Wideband Transimpedance Amplifier
Wide bandwidth, low input bias current, and low input voltage and current noise make the OPA354 an ideal
wideband photodiode transimpedance amplifier for low-voltage single-supply applications. 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, as shown in Figure 8, are the expected diode capacitance
(including the parasitic input common-mode and differential-mode input capacitance (2 + 2) pF for the OPA354),
the desired transimpedance gain (R
F
), and the gain-bandwidth product (GBW) for the OPA354 (100 MHz). With
these three variables set, the feedback capacitor value (C
F
) may be set to control the frequency response.
Figure 8. Transimpedance Amplifier
To achieve a maximally flat second-order Butterworth frequency response, the feedback pole should be set as
shown in Equation 1.
(1)
Typical surface-mount resistors have a parasitic capacitance of around 0.2 pF that must be deducted from the
calculated feedback capacitance value.
Bandwidth is calculated as shown in Equation 2.
(2)
For even higher transimpedance bandwidth, the high-speed CMOS OPA355 (200-MHz GBW) or the OPA655
(400-MHz GBW) may be used.
PCB Layout
Good high-frequency printed circuit board (PCB) layout techniques should be employed for the OPA354.
Generous use of ground planes, short and direct signal traces, and a suitable bypass capacitor located at the V+
pin assures clean stable operation. Large areas of copper also provides a means of dissipating heat that is
generated in normal operation.
Sockets are not recommended for use with any high-speed amplifier.
A 10-nF ceramic bypass capacitor is the minimum recommended value; adding a 1-μF or larger tantalum
capacitor in parallel can be beneficial when driving a low-resistance load. Providing adequate bypass
capacitance is essential to achieving very low harmonic and intermodulation distortion.
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