11

LTC1402

LT1630: Dual 30MHz Rail-to-Rail Voltage FB Amplifier.

2.7V to

520ns settling to 0.5LSB for a 4V swing. THD and noise

applications (to 1/3 Nyquist) where rail-to-rail perfor-

mance is desired. Quad version is available as LT1631.

LT1632: Dual 45MHz Rail-to-Rail Voltage FB Amplifier.

2.7V to

400ns settling to 0.5LSB for a 4V swing. It is suitable for

applications with a single 5V supply. THD and noise are

applications where rail-to-rail performance is desired.

Quad version is available as LT1633.

LT1813: Dual 100MHz 750V/

µs 3mA Voltage Feedback

Amplifier. 5V to

±5V supplies. Distortion is –86dB to

100kHz and – 77dB to 1MHz with

500

Ω). Excellent part for fast AC applications with ±5V

supplies.

INPUT FILTERING AND SOURCE IMPEDANCE

The noise and the distortion of the input amplifier and

other circuitry must be considered since they will add to

the LTC1402 noise and distortion. The small-signal band-

width of the sample-and-hold circuit is 80MHz. Any noise

or distortion products that are present at the analog inputs

will be summed over this entire bandwidth. Noisy input

circuitry should be filtered prior to the analog inputs to

minimize noise. A simple 1-pole RC filter is sufficient for

many applications. For example, Figure 2 shows a 68pF

to limit the input bandwidth to 47MHz. The 68pF capacitor

also acts as a charge reservoir for the input sample-and-

hold and isolates the ADC input from sampling glitch-

sensitive circuitry.

High quality capacitors and resistors should be used since

these components can add distortion. NPO and silver mica

type dielectric capacitors have excellent linearity.

Carbon surface mount resistors can generate distortion

from self heating and from damage that may occur during

soldering. Metal film surface mount resistors are much

less susceptible to both problems. When high amplitude

unwanted signals are close in frequency to the desired signal

frequency, a multiple pole filter is required. Figure 3 shows

a simple implementation using an LTC1560-1 fifth order

elliptic continuous-time 1MHz filter.

LTC1402

+

–

AGND1

AGND2

GAIN

1402 F02

3

4

2

6

5

7

10

µF

68pF

51

Ω

ANALOG

INPUT

Figure 2. RC Input Filter

LTC1560-1

1

2

3

4

8

7

6

5

0.1

µF

5V

0.1

µF

–5V

LTC1402

+

–

AGND1

AGND2

GAIN

1402 F03

3

4

2

6

5

7

10

µF

Figure 3. 1MHz Fifth Order Elliptic Lowpass Filter

BIPOLAR AND UNIPOLAR INPUT RANGES

The

±2V bipolar input range of the LTC1402 is optimized

for low noise and low distortion. Most op amps also

perform best over this same range, allowing direct cou-

pling to the analog inputs and eliminating the need for

special translation circuitry. The inputs of the LTC1402

may also be driven fully differential in bipolar mode with

a single supply. Each input should not swing more than

supply amplifiers.

The 0V to 4V range is ideal for single ended input use with

single supply applications.