LT1064 Просмотр технического описания (PDF) - Linear Technology

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LT1064

Low Noise, Fast, Quad Universal Filter Building Block

Linear Technology 

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ODES OF OPERATIO

Mode 3

Mode 3 is the second of the primary modes. In Mode 3, the

ratio of the external clock frequency to the center fre-

quency of each 2nd order section can be adjusted above or

below 50:1 or 100:1. Side D of the LTC1064 can only be

connected in Mode 3. Figure 6 illustrates Mode 3, the

classical state variable configuration, providing highpass,

bandpass and lowpass 2nd order filter functions. Mode 3

is slower than Mode 1. Mode 3 can be used to make high

order all-pole bandpass, lowpass, highpass and notch

filters.

When the internal clock-to-center frequency ratio is set at

50:1, the design equations for Q and bandpass gain are

different from the 100:1 case. This was done to provide

speed without penalizing the noise performance.

R1

VIN

R4

R3

R2

–

+

AGND

CC

HP

S

+

–

Σ

∫

1/4 LTC1064

BP

LP

∫

1064 F06

MODE 3 (100:1):

MODE 3 (50:1):

√ √ fO

=

fCLK

100

R2

R4

;

Q

=

R3

R2

R2

R4

;

H

OHP

=

–

R2

R1

;

HOBP

=

–

R3

R1

;

HOLP

=

–

R4

R1

√ √ fO

=

fCLK

50

R2

R2 ; Q = 1.005 R4 ;

R4

R2

R3

–

R2

16R4

R3

HOHP

=

–

R2

R1

;

HOBP

=–

1

R1

– R3

;

H

OLP

=–

R4

R1

16R4

NOTE: THE 50:1 EQUATIONS FOR MODE 3 ARE DIFFERENT FROM THE EQUATIONS

FOR MODE 3 OPERATIONS OF THE LTC1059, LTC1060 AND LTC1061. START WITH

fO, CALCULATE R2/R4, SET R4; FROM THE Q VALUE, CALCULATE R3:

R3 =

R2

; THEN CALCULATE R1 TO SET

√ 1.005 R2 + R2 THE DESIRED GAIN.

Q R4 16R4

1064 F06 Eq

Figure 6. Mode 3: 2nd Order Filter Providing Highpass,

Bandpass and Lowpass

LTC1064

SECONDARY MODES

Mode 1b

Mode 1b is derived from Mode 1. In Mode 1b, Figure 7, two

additional resistors R5 and R6 are added to alternate the

amount of voltage fed back from the lowpass output into

the input of the SA (or SB or SC) switched-capacitor

summer. This allows the filter’s clock-to-center frequency

ratio to be adjusted beyond 50:1 or 100:1. Mode 1b

maintains the speed advantages of Mode 1.

R1

VIN

R3

R2

–

+

AGND

R6

R5

N

S

+–

Σ∫

1/4 LTC1064

BP

LP

∫

1064 F07

√ √ fO

=

fCLK

100(50)

R6 ;

R5 + R6

f

n

=

fO;

Q

=

R3

R2

R6 ;

R5 + R6

( ) HON1(f→ 0) = HON2

f→

fCLK

2

=

–

R2

R1

;

HOLP =

–

R2

R1

R6

;

R5 + R6

HOBP =

–

R3

R1

;

R5R6

≤

5k

1064 F07 Eq

Figure 7. Mode 1b: 2nd Order Filter Providing Notch,

Bandpass and Lowpass

Mode 2

Mode 2 is a combination of Mode 1 and Mode 3, as shown

in Figure 8. With Mode 2, the clock-to-center frequency

ratio fCLK/fO is always less than 50:1 or 100:1. The

advantage of Mode 2 is that it provides less sensitivity to

resistor tolerances than does Mode 3. As in Mode 1, Mode

2 has a notch output which depends on the clock fre-

quency and the notch frequency is therefore less than the

center frequency fO.

When the internal clock-to-center frequency ratio is set at

50:1, the design equations for Q and bandpass gain are

different from the 100:1 case.

9

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