Datasheet AD825 (Analog Devices) - 11

link to page 11 link to page 11
Data Sheet AD825 SECOND-ORDER LOW-PASS FILTER +5V C1
A second-order Butterworth low-pass filter can be implemented
24pF C3 0.1
µ
F
using the AD825 as shown in Figure 33. The extremely low bias
R1 R2 9.31k
Ω
9.31k
Ω
V
currents of the AD825 allow the use of large resistor values and,
IN C2 AD825 VOUT
consequently, smal capacitor values without concern for
6pF
developing large offset errors. Low current noise is another factor in permitting the use of large resistors without having to
C4
worry about the resultant voltage noise.
0.1
µ
F
With the values shown, the corner frequency wil be 1 MHz.
–5V
00876-E-031 The equations for component selection are shown below. Note Figure 33. Second-Order Butterworth Low-Pass Filter that the noninverting input (and the inverting input) has an input capacitance of 6 pF. As a result, the calculated value of C1
0
(12 pF) is reduced to 6 pF.
–10
1.414 C1 =
–20
2π fCUTOFFR1
–30
0.707
–40
C2 ( farads) = 2π f
–50
CUTOFF R1 R1 = R2 = User Selected (Typically 10 Ω k to 100 Ω k )
–60 –70
A plot of the filter frequency response is shown in Figure 34;
HIGH FREQUENCY REJECTION (dB) –80
better than 40 dB of high frequency rejection is provided.
10k 100k 1M 10M 100M FREQUENCY (Hz)
00876-E-032 Figure 34. Frequency Response of Second-Order Butterworth Filter Rev. G | Page 11 of 12 Document Outline Features Applications Connection Diagrams General Description Table of Contents Revision History Specifications Absolute Maximum Ratings Pin Configurations ESD Caution Typical Performance Characteristics Driving Capacitive Loads Theory of Operation Input Consideration Grounding and Bypassing Second-Order Low-Pass Filter Outline Dimensions Ordering Guide