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OPA140_15 Datasheet, PDF (24/44 Pages) Texas Instruments – High-Precision, Low-Noise, Rail-to-Rail Output,11-MHz JFET Op Amp
OPA140, OPA2140, OPA4140
SBOS498B – JULY 2010 – REVISED NOVEMBER 2015
8 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.
8.1 Application Information
The OPA140, OPA2140, and OPA4140 are unity-gain stable, operational amplifiers with very low noise, input
bias current, and 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.
8.2 Typical Application
Input
R4
2.94 k
R1
590
R3
499
C2
39 nF
C5
1 nF
±
+
OPA140
Output
Figure 40. 25-kHz Low-pass Filter
8.2.1 Design Requirements
Lowpass filters are commonly employed in signal processing applications to reduce noise and prevent aliasing.
The OPAx140 are ideally suited to construct high-speed, high-precision active filters. Figure 40 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
8.2.2 Detailed Design Procedure
The infinite-gain multiple-feedback circuit for a low-pass network function is shown in. 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 lowpass cutoff frequency are
calculated by Equation 2:
Gain R4
R1
fC
1
2S
1 R3R4C2C5
(2)
24
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