LT1801/LT1802
16
18012fc
APPLICATIONS INFORMATION
TYPICAL APPLICATIONS
Single 3V Supply, 1MHz, 4th Order Butterworth Filter
The circuit shown on the fi rst page of this data sheet
makes use of the low voltage operation and the wide
bandwidth of the LT1801 to create a DC accurate 1MHz
4th order lowpass fi lter powered from a 3V supply. The
amplifi ers are confi gured in the inverting mode for the
lowest distortion and the output can swing rail-to-rail for
maximum dynamic range. Also on the fi rst page of this
data sheet, the graph displays the frequency response of
the fi lter. Stopband attenuation is greater than 100dB at
50MHz. With a 2.25V
P-P
, 250kHz input signal, the fi lter
has harmonic distortion products of less than –85dBc.
Worst case output offset voltage is less than 6mV.
+
–
1/2 LT1801
0.1Ω
I
L
0A TO 1A
V
OUT
0V TO 2V
V
OUT
= 2 • I
L
f
–3dB
= 4MHz
UNCERTAINTY DUE TO V
OS,
I
B
< 4mA
3V
1k
18012 F02
52.3Ω
52.3Ω
Figure 2. Fast 1A Current Sense
500mV/DIV
0V
V
S
= 3V 50ns/DIV 18012 F03
Figure 3. Current Sense Amplifi er Large-Signal Response
Fast 1A Current Sense Amplifi er
A simple, fast current sense amplifi er in Figure 2 is suitable
for quickly responding to out-of-range currents. The circuit
amplifi es the voltage across the 0.1Ω sense resistor by
a gain of 20, resulting in a conversion gain of 2V/A. The
–3dB bandwidth of the circuit is 4MHz, and the uncertainty
due to V
OS
and I
B
is less than 4mA. The minimum output
voltage is 60mV, corresponding to 30mA. The large-signal
response of the circuit is shown in Figure 3.
Capacitive Load
The LT1801/LT1802 are optimized for high bandwidth,
low power and precision applications. They can drive a
c a p a c i t i v e l o a d o f a b o u t 7 5 p F i n a u n i t y - g a i n c o n fi g u r a t i o n ,
and more for higher gain. When driving a larger capaci-
tive load, a resistor of 10Ω to 50Ω should be connected
between the output and the capacitive load to avoid ringing
or oscillation. The feedback should still be taken from the
output so that the resistor will isolate the capacitive load
to ensure stability. Graphs on capacitive loads indicate the
transient response of the amplifi er when driving capacitive
load with a specifi ed series resistor.
Feedback Components
When feedback resistors are used to set up gain, care must
be taken to ensure that the pole formed by the feedback
resistors and the total capacitance at the inverting input
does not degrade stability. For instance, the LT1801/LT1802
in a noninverting gain of 2, setup with two 5k resistors
and a capacitance of 5pF (part plus PC board) will prob-
ably oscillate. The pole is formed at 12.7MHz that will
reduce phase margin by 57 degrees when the crossover
frequency of the amplifi er is around 20MHz. A capacitor
of 5pF or higher connected across the feedback resistor
will eliminate any ringing or oscillation.
