LTC1164

9

1164fa

3. 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 LTC1164 or any LTC switched capacitor

filter. The circuit shown in Figure 5 will null offsets to better

than 300µV. This circuit takes seconds to settle because of

the integrator pole frequency.

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

4. 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 Wideband Noise vs Q

curve shown in the Typical Performance Characteristics

Section is a very good representation of the noise

performance of this device.

Figure 5. 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 6 illustrates Mode 1 provid-

ing 2nd order notch, lowpass, and bandpass outputs.

Mode 1 can be used to make high order Butterworth

Iowpass 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 3 of the 4 LTC1164 sections because section D has no

externally available summing node. Section D, however,

can be internally connected in Mode 1 upon special

request.

Figure 6. Mode 1: 2nd Order Filter Providing Notch,

Bandpass, Lowpass

APPLICATIO S I FOR ATIO

LTC1164 • AI03

R3

100k

TO FILTER

FIRST SUMMING

NODE

C2

0.1µF

C1

0.1µF

R1

1M

FROM

FILTER

OUTPUT

R2

1M

C1 = C2 = LOW LEAKAGE FILM (I.E. POLYPROPYLENE)

R1 = R2 = METAL FILM 1%

–

+

LT1012

ODES OF OPERATIO

LTC1164 • MOO01

R3

R2

NS

BP

LP

R1

AGND

1/4 LTC1164

Q =

100(50)

–

+

+

–

Σ

∫

∫

R2

R1

R2

R1

R3

R2

R3

R1

LTC1164 • AI02

0.1µF

R11

R21

R3

R22

R12

R32

1k

LTC1164

NEGATIVE

SUPPLY

POSITIVE

SUPPLY

4

7

7

19

–

+

0.1µF

0.1µF

0.1µF