LTC1064CS datasheet(8/16 Pages) LINER

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LTC1064

APPLICATIONS INFORMATION

Offset Nulling

Lowpass filters may have too much DC offset for some

users. A servo circuit may be used to actively null the

offsets of the LTC1064 or any LTC switched-capacitor

filter. The circuit shown in Figure 4 will null offsets to better

than 300

µV. This circuit takes seconds to settle because of

the integrator pole frequency.

Noise

All the noise performance mentioned excludes the clock

feedthrough. Noise measurements will degrade if the

already described grounding bypassing and buffering

techniques are not practiced. The graph Wideband Noise

vs Q in the Typical Performance Characteristics section is

a very good representation of the noise performance of

this device.

Figure 3. Buffering the Output of a 4th Order Bandpass Realization

Figure 4. Servo Amplifier

PRIMARY MODES

Mode 1

In Mode 1, the ratio of the external clock frequency to the

center frequency of each 2nd order section is internally

fixed at 50:1 or 100:1. Figure 5 illustrates Mode 1 provid-

ing 2nd order notch, lowpass and bandpass outputs.

Mode 1 can be used to make high order Butterworth

lowpass filters; it can also be used to make low Q notches

and for cascading 2nd order bandpass functions tuned at

the same center frequency with unity gain. Mode 1 is faster

than Mode 3. Note that Mode 1 can only be implemented

with three of the four LTC1064 sections because Section

D has no externally available summing node. Section D,

however, can be internally connected in Mode 1 upon

special request.

ODES OF OPERATIO

R21

R11

R31

LTC1064

0.1

µF

1064 F03

10k

R32

R22

R12

0.1

µF

V+ TRACE FOR FILTER

0.1

µF

VIN

–

10k

®318

LT1007

LT1056

NEGATIVE

SUPPLY

POSITIVE

SUPPLY

SEPARATE V+ POWER SUPPLY TRACE FOR BUFFER

µF

0.1

µF

–

∫

1064 F05

+ Σ

AGND

1/4 LTC1064

–

VIN

fO =

; f n = fO; HOLP = –

; HOBP = –

; HON1 = –

; Q =

fCLK

100(50)

Figure 5. Mode 1: 2nd Order Filter Providing Notch,

Bandpass and Lowpass

0.1

µF

0.1

µF

1064 F04

TO FILTER

FIRST SUMMING

NODE

C1 = C2 = LOW LEAKAGE FILM

(I.E. POLYPROPYLENE)

R1 = R2 = METAL FILM 1%

FROM

FILTER OUTPUT

100k

–

LT1012

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