Datasheet

LT1223
10
1223fb
have about 2.5mV thermal tail on a 10V step. Unfortu-
nately, reducing the input signal and increasing the gain
always results in a thermal tail of about the same amount
for a given output step. For this reason we show separate
graphs of 10mV and 1mV non-inverting amplifier settling
times. Just as the bandwidth of the LT1223 is fairly
constant for various closed-loop gains, the settling time
remains constant as well.
Adjustable Gain Amplifier
To make a variable gain amplifier with the LT1223, vary the
value of R
G
. The implementation of R
G
can be a pot, a light
controlled resistor, a FET, or any other low capacitance
variable resistor. The value of R
F
should not be varied to
change the gain. If R
F
is changed, then the bandwidth will
be reduced at maximum gain and the circuit will oscillate
when R
F
is very small.
Adjustable Bandwidth Amplifier
Because the resistance at the inverting input determines
the bandwidth of the LT1223, an adjustable bandwidth
circuit can be made easily. The gain is set as before with
R
F
and R
G
; the bandwidth is maximum when the variable
resistor is at a minimum.
Accurate Bandwidth Limiting the LT1223
It is very common to limit the bandwidth of an op amp by
putting a small capacitor in parallel with R
F
. DO NOT PUT
A SMALL CAPACITOR FROM THE INVERTING INPUT OF
A CURRENT FEEDBACK AMPLIFIER TO ANYWHERE ELSE,
ESPECIALLY NOT TO THE OUTPUT. The capacitor on the
inverting input will cause peaking or oscillations. If you
need to limit the bandwidth of a current feedback amplifier,
use a resistor and capacitor at the noninverting input (R1
and C1). This technique will also cancel (to a degree) the
peaking caused by stray capacitance at the inverting input.
Unfortunately, this will not limit the output noise the way
it does for the op amp.
Current Feedback Amplifier Integrator
Since we remember that the inverting input wants to see
a resistor, we can add one to the standard integrator
circuit. This generates a new summing node where we can
apply capacitive feedback. The LT1223 integrator has
excellent large signal capability and accurate phase shift at
high frequencies.
APPLICATIO S I FOR ATIO
WUU
U
LT1223 • TA03
+
–
IN
V
OUT
V
LT1223
R
F
R
G
LT1223 • TA04
+
–
IN
V
OUT
V
LT1223
R
F
5k
R
G
LT1223 • TA05
+
–
IN
V
OUT
V
LT1223
R
F
R
G
R1
R1 = 300
C1
Ω
C1 = 100pF
BW = 5MHz
LT1223 • TA06
+
–
IN
V
OUT
V
LT1223
R
F
1k
C1
OUT
V
IN
V
1
sC1R1
=
R1