LT1228CS8 datasheet(17/20 Pages) LINER

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LT1228

APPLICATI

TYPICAL

Filters

Single Pole Low/High/Allpass Filter

LT1228 • TA16

–

CFA

VOUT

120

Ω

120

Ω

330pF

fC =

R3A

ISET

R2A

VIN

LOWPASS

INPUT

VIN

HIGHPASS

INPUT

fC = 109 ISET FOR THE VALUES SHOWN

ISET

RF + 1

R2 + R2A

Allpass Filter Phase Response

FREQUENCY (Hz)

10k

–90

–45

100k

10M

LT1228 • TA17

–135

–180

1mA SET CURRENT

100

µA SET CURRENT

Using the variable transconductance of the LT1228 to

make variable filters is easy and predictable. The most

straight forward way is to make an integrator by putting a

capacitor at the output of the transconductance amp and

buffering it with the current feedback amplifier. Because

the input bias current of the current feedback amplifier

must be supplied by the transconductance amplifier, the

set current should not be operated below 10

µA.Thislimits

the filters to about a 100:1 tuning range.

The Single Pole circuit realizes a single pole filter with a

corner frequency (fC) proportional to the set current. The

values shown give a 100kHz corner frequency for 100

µA

set current. The circuit has two inputs, a lowpass filter

input and a highpass filter input. To make a lowpass filter,

ground the highpass input and drive the lowpass input.

Conversely for a highpass filter, ground the lowpass input

and drive the highpass input. If both inputs are driven, the

result is an allpass filter or phase shifter. The allpass has

flat amplitude response and 0

° phase shift at low frequen-

cies, going to –180

° at high frequencies. The allpass filter

has –90

° phase shift at the corner frequency.

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