LT1802CS datasheet(16/20 Pages) LINER

16 / 20 page

LT1801/LT1802

18012fb

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 2.25VP-P, 250kHz input signal, the filter has har-

monic distortion products of less than – 85dBc. Worst

case output offset voltage is less than 6mV.

1/2 LT1801

0.1

Ω

0A TO 1A

VOUT

0V TO 2V

VOUT = 2 • IL

f–3dB = 4MHz

UNCERTAINTY DUE TO VOS, IB < 4mA

18012 F02

52.3

Ω

52.3

Ω

Figure 2. Fast 1A Current Sense

500mV/DIV

VS = 3V

50ns/DIV

18012 F03

Figure 3. Current Sense Amplifier Large-Signal Response

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 0.1

Ω 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 VOS and IB 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

capacitive load of about 75pF in a unity-gain configuration,

and more for higher gain. When driving a larger capacitive

load, a resistor of 10

Ω 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.

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.

APPLICATIO S I FOR ATIO

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