LTC1064CS datasheet(9/16 Pages) LINER

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LTC1064

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.

–

∫

+ Σ

AGND

1/4 LTC1064

–

VIN

1064 F06

1064 F06 Eq

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:

fO =

; Q =

; H OHP = –

;

fCLK

100

MODE 3 (100:1):

√

HOBP = –

; HOLP = –

fO =

; Q =

;

fCLK

MODE 3 (50:1):

√

16R4

1.005

√

–

R3 =

; THEN CALCULATE R1 TO SET

THE DESIRED GAIN.

1.005

√R2

16R4

HOHP = –

; HOBP = –

; H OLP = –

16R4

1 –

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.

–

∫

+ Σ

AGND

–

VIN

1064 F07

1/4 LTC1064

1064 F07 Eq

fO =

; f n = fO; Q =

;

fCLK

100(50)

fCLK

R5 + R6

√

R5 + R6

√

HON1(f→ 0) = HON2 f→

= –

; HOLP = –

;

HOBP = –

; R5

R6 ≤ 5k

()

R5 + R6

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.

ODES OF OPERATIO

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

Bandpass and Lowpass

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

Bandpass and Lowpass

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