Linear Technology DC961B Demo Manual page 5

Quick test proceDure
Design Procedure
The following procedure is from the LT1994 data sheet.
The design schematic resistor and capacitor designators
have the following correspondence with the DC961 resis-
tor and capacitor designators:
Design Schematic DC961
R11 Z1
R21 R3
R31 R9
C11 C11
C21 C5
C22 C6
R12 Z2
R22 R7
R32 R8
DEMO MANUAL DC961B
Differential 2nd Order Butterworth Lowpass Filter
f ≤ 2.5MHz and Gain ≤ 8.8 or Gain ≤
3dB
Component Calculation:
R11 = R12, R21 = R22, R31 = R32, C21 = C22, C11 = 10
• C21, R1 = R11, R2 = R21, R3 = R31, C2 = C21 and C1
= C11
1. Calculate an absolute value for C2 (C2
specified –3dB frequency
5
4 • 10
(
C2 = C2
abs abs
f
3dB
2. Select a standard 5% capacitor value nearest the abso-
lute value for C2(C1 = 10 • C2)
3. Calculate R3, R2 and R1 using the standard 5% C2
value, the specified f
3dB
gain (G ):
N
R1, R2 and R3 equations (C2 in pF and f
(
1.121– (1.131– 0.127 • G
R3 =
(
+ 1
G
N
15
1.266 • 10
R2 =
2 2
R3 • C2 • f
3dB
2.5MHz
f
3dB
) using a
abs
)
IN pF AND f IN kHz
3dB
and the specified passband
in kHz)
3dB
)
8
) • 10
N
)
• C2 • f
3dB
R2
R1=
G
N
dc961bf
5