lt1801is8-trpbf Linear Technology Corporation, lt1801is8-trpbf Datasheet - Page 16

• Manufacturer Part Number: lt1801is8-trpbf
• Description: Lt1802 - Quad 80mhz, 25v/?s Low Power Rail-to-rail Input And Output Precision Op Amps
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT1801/LT1802
Capacitive Load
The LT1801/LT1802 are optimized for high bandwidth,
low power and precision applications. They can drive a
capacitive load of about 75pF in a unity-gain configuration,
and more for higher gain. When driving a larger capacitive
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 amplifier when driving capacitive
load with a specified series resistor.
TYPICAL APPLICATIO S
Single 3V Supply, 1MHz, 4th Order Butterworth Filter
The circuit shown on the first 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 filter powered from a 3V supply. The amplifiers
are configured in the inverting mode for the lowest distor-
tion and the output can swing rail-to-rail for maximum
dynamic range. Also on the first page of this data sheet, the
graph displays the frequency response of the filter.
Stopband attenuation is greater than 100dB at 50MHz.
With a 2.25V
monic distortion products of less than – 85dBc. Worst
case output offset voltage is less than 6mV.
16
0.1 Ω
I
0A TO 1A
L
P-P
V
f
UNCERTAINTY DUE TO V
–3dB
Figure 2. Fast 1A Current Sense
OUT
52.3 Ω
52.3 Ω
, 250kHz input signal, the filter has har-
= 2 • I
= 4MHz
U
L
U
+
–
1/2 LT1801
OS,
3V
I
18012 F02
B
U
< 4mA
W
1k
V
0V TO 2V
OUT
U
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
probably oscillate. The pole is formed at 12.7MHz that will
reduce phase margin by 57 degrees when the crossover
frequency of the amplifier is around 20MHz. A capacitor of
5pF or higher connected across the feedback resistor will
eliminate any ringing or oscillation.
Fast 1A Current Sense Amplifier
A simple, fast current sense amplifier in Figure 2 is suitable
for quickly responding to out-of-range currents. The cir-
cuit amplifies 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
voltage is 60mV, corresponding to 30mA. The large-signal
response of the circuit is shown in Figure 3.
500mV/DIV
Figure 3. Current Sense Amplifier Large-Signal Response
0V
OS
V
and I
S
= 3V
B
is less than 4mA. The minimum output
50ns/DIV
18012 F03
18012fb