Datasheet

LT1228
17
1228fd
TYPICAL APPLICATIONS
Filters
LT1228 • TA16
–
+
–
+
g
m
CFA
V
OUT
8
3
2
R
F
1k
R2
120Ω
5
1
6
R3
120Ω
C
330pF
f
C
= × × ×
R3A
1k
I
SET
R
G
1k
R2A
1k
V
IN
LOWPASS
INPUT
V
IN
HIGHPASS
INPUT
f
C
= 10
9
I
SET
FOR THE VALUES SHOWN
10
2π
I
SET
C
R
F
+ 1
R
G
R2
R2
+ R2A
Single Pole Low/High/Allpass Filter
Allpass Filter Phase Response
FREQUENCY (Hz)
10k
PHASE SHIFT (DEGREES)
–90
–45
0
100k 1M 10M
LT1228 • TA17
–135
–180
1mA SET CURRENT
100µA 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 10µA. This limits
the filters to about a 100:1 tuning range.
The Single Pole circuit realizes a single pole filter with a
corner frequency (f
C
) proportional to the set current. The
values shown give a 100kHz corner frequency for 100µA
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.