OPA2354-Q1 Datasheet, PDF (14/23 Pages) Texas Instruments – 250 MHz, RAIL-TO-RAIL I/O, CMOS OPERATIONAL AMPLIFIERS

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.
C_F
< 1pF
(prevents gain peaking)
R_F
10MΩ
+V
λ
C_D OPA354
V_OUT
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.
Ǹ ¹
2pR_FC_F
+
GBP
4pR_FC_D
(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.
Ǹ f_*3dB +
GBP
2pR_FC
D
Hz
(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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