OPA340 datasheet(18/46 Pages) TI2

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OPA340, OPA2340, OPA4340

SBOS073C – SEPTEMBER 1997 – REVISED AUGUST 2016

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Typical Applications (continued)

8.2.2 Two-Pole, Low-Pass Filter

If even more attenuation is required, a multiple pole filter is required. The Sallen-Key filter may be used for this

task, as shown in Figure 31. For best results, the amplifier must have effective bandwidth that is at least 10 times

higher than the filter cutoff frequency. Failure to follow this guideline results in a phase shift of the amplifier,

which in turn leads to lower precision of the filter bandwidth. Additionally, to minimize the loading effect between

multiple RC pairs on overall the filter cutoff frequency, choose R = 10 × R1 and C2 = C1/10; see Figure 32.

Figure 31. Two-Pole, Lowpass Filter

Equation 3 through Equation 5 show calculations for corner frequency and gain:

(3)

(4)

(5)

8.2.2.1 Detailed Design Procedure

Use these design values:

•

C1 = 10 nF and C2 = 1 nF

•

R1 = 1 kΩ and R2= 10 kΩ

•

RG = 10 kΩ

•

RF = 90 kΩ

Figure 32 shows the Sallen-Key filter second-order response for different RC values: for R and C values above,

= 15.9 kHz.

To further limit the bandwidth, a larger RC value must be used: increasing C values 100 times, such as

C1 = 1 µF and C2 = 0.1 µF, with unchanged resistors, results in the second-order rolloff at

= 159 Hz. See Figure 32.

OPA340 Datasheet(PDF) 18 Page - Texas Instruments