8 Application and Implementation

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 OPAx145 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.

8.2 Typical Application

+

±

OPA145

Output

Input

R1

590

R4

2.94 k

R3

499

C2

39 nF

C5

1 nF

Copyright © 2017, Texas Instruments Incorporated

Figure 8-1. 25-kHz Low-Pass Filter

8.2.1 Design Requirements

Low-pass filters are commonly employed in signal processing applications to reduce noise and prevent aliasing.

The OPAx145 are designed to construct high-speed, high-precision active filters. Figure 8-1 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 Figure 8-1. Use Equation 2

to calculate the voltage transfer function.

1 3

2

5

2

2

1

3

4

3

4

2

5

1 R R C C

Output

s

Input

s

s C

1 R

1 R

1 R

1 R R C C

(2)

This circuit produces a signal inversion. For this circuit, the gain at dc and the low-pass cutoff frequency are

calculated by Equation 3:

4

1

C

3

4

2

5

R

Gain

R

1

f

1 R R C C

(3)

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OPA145,, OPA2145

SBOS427E – JUNE 2017 – REVISED OCTOBER 2020

Copyright © 2020 Texas Instruments Incorporated

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Product Folder Links: OPA145, OPA2145