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| OPA2810 Datasheet, PDF (19/26 Pages) Texas Instruments – OPA2810 Dual High-Performance, Low-Power, Wide Supply Range, Rail-to-Rail Input/Output FET-Input Operational Amplifier | |||
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OPA2810
SBOS789 â AUGUST 2017
8.2 Typical Application
The high GBP and low input voltage and current noise for the OPA2810 make it an ideal wideband
transimpedance amplifier for moderate to high transimpedance gains.
VBIAS
CD
20 pF
Supply Decoupling not
+5 V
shown
OPA2810
+
RS
50
CPCB
0.3 pF
-5 V
RF
100 k
Oscilloscope
with 50
Inputs
CF + CPCB
1.03 pF
Figure 6. Wideband, High-Sensitivity, Transimpedance Amplifier
8.2.1 Design Requirements
Design a high-bandwidth, high-gain transimpedance amplifier with the design requirements shown in Table 3.
Table 3. Design Requirements
TARGET BANDWIDTH (MHz) TRANSIMPEDANCE GAIN (KΩ) PHOTODIODE CAPACITANCE (pF)
>2
100
20
8.2.2 Detailed Design Procedure
Designs that require high bandwidth from a large area detector with relatively high transimpedance gain benefit
from the low input voltage noise of the OPA2810. This input voltage noise is peaked up over frequency by the
diode source capacitance, and can, in many cases, become the limiting factor to input sensitivity. The key
elements to the design are the expected diode capacitance (CD) with the reverse bias voltage (VBIAS) applied the
desired transimpedance gain, RF, and the GBP for the OPA2810 (70 MHz). Figure 6 shows a transimpedance
circuit with the parameters as described in Table 3. With these three variables set (and including the parasitic
input capacitance for the OPA2810 and the PCB added to CD), the feedback capacitor value (CF) may be set to
control the frequency response. To achieve a maximally-flat second-order Butterworth frequency response, set
the feedback pole to:
1
GBP
=
2pRFCF 4pRFCD
(3)
The input capacitance of the amplifier is the sum of its common-mode and differential capacitance (2.5+0.5) pF.
The parasitic capacitance from the photo-diode package and the PCB is approximately 0.3 pF. This results in a
total input capacitance, CD = 23.3 pF. From Equation 3, set the feedback pole at 1.55 MHz. Setting the pole at
1.55 MHz requires a total feedback capacitance of 1.03 pF
The approximate â3-dB bandwidth of the transimpedance amplifier circuit is given by:
f-3dB = GBP / (2pRFCD ) Hz
(4)
Equation 4 estimates a closed-loop bandwidth of 2.19 MHz.
Copyright © 2017, Texas Instruments Incorporated
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