OPA145 Datasheet, PDF (26/40 Pages) Texas Instruments – High-Precision, Low-Noise, Rail-to-Rail Output, 5.5-MHz JFET Operational Amplifiers

OPA145, OPA2145, OPA4145
SBOS427 – JUNE 2017
9 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
The OPA145, OPA2145, and OPA4145 are unity-gain stable operational amplifiers with low noise, low input bias
current, and low input offset voltage. Applications with noisy or high-impedance power supplies require
decoupling capacitors placed close to the device pins. In most cases, 0.1-μF capacitors are adequate. Designers
can easily use the rail-to-rail output swing and input range that includes V– to take advantage of the low-noise
characteristics of JFET amplifiers while also interfacing to modern, single-supply, precision data converters.
9.2 Typical Application
Input
R4
2.94 k
R1
590
R3
499
C2
39 nF
C5
1 nF
±
+
OPA145
Output
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Figure 45. 25-kHz Low-pass Filter
9.2.1 Design Requirements
Low-pass filters are commonly employed in signal processing applications to reduce noise and prevent aliasing.
The OPAx145 devices are ideally suited to construct high-speed, high-precision active filters. Figure 45 shows a
second-order, low-pass filter commonly encountered in signal processing applications.
Use the following parameters for this design example:
• Gain = 5 V/V (inverting gain)
• Low-pass cutoff frequency = 25 kHz
• Second-order Chebyshev filter response with 3-dB gain peaking in the passband
9.2.2 Detailed Design Procedure
The infinite-gain multiple-feedback circuit for a low-pass network function is shown in Figure 45. Use Equation 1
to calculate the voltage transfer function.
Output _s
1 R₁R₃C₂C₅
Input
s² s C₂ 1 R₁ 1 R₃ 1 R₄ 1 R₃R₄C₂C₅
(1)
This circuit produces a signal inversion. For this circuit, the gain at DC and the low-pass cutoff frequency are
calculated by Equation 2:
Gain ^R4
R₁
f_C
1
2S
1 R₃R₄C₂C₅
(2)
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