11

LTC1406

LT1229/LT1230: Dual and Quad 100MHz Current Feed-

back Amplifiers.

±2V to ±15V supplies. Low noise. 6mA

supply current each amplifier.

LT1259/LT1260: Dual and Triple 130MHz Current Feed-

back Amplifiers.

±2V to ±14V supplies. 5mA supply cur-

rent. Low distortion. Low noise.

LT1363: 70MHz Voltage Feedback Amplifier.

±2.5V to

±15V supplies. 7.5mA supply current. Low distortion.

LT1364/LT1365: Dual and Quad 70MHz Voltage Feedback

Amplifiers.

±2.5V to ±15V supplies. 7.5mA supply current

per amplifier. Low distortion.

Input Filtering

The noise and the distortion of the input amplifier and

other circuitry must be considered since they will add to

the LTC1406 noise and distortion. The small-signal band-

width of the sample-and-hold circuit is 250MHz. 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 9 shows a 220pF

limit the input bandwidth to 9.6MHz. The 220pF capacitor

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

hold and isolates the ADC input from sampling glitch sen-

sitive circuitry. Larger value capacitors may be substituted

to further limit the input bandwidth. High quality capaci-

tors 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 also 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.

Input/Output Characteristics

Figure 10 shows the ideal input/output characteristics for

the LTC1406. The code transitions occur midway between

successive integer LSB values (i.e., – FS + 0.5LSB, – FS +

1.5LSB, – FS + 2.5LSB...FS – 1.5LSB, FS – 0.5LSB). The

output is straight binary with 1LSB = FS – (– FS)/256 = 2V/

256 = 7.8125mV. The OF/UF bit indicates that the input has

exceeded full scale and can be used to detect an overrange

or underrange condition. A logic high output on the OF/UF

pin with an output code of 0000 0000 indicates the input

is less than the negative full scale. A logic high output on

the OF/UF pin with an output code of 1111 1111 indicates

that the input is greater than the positive full scale. A logic

low output on the OF/UF pin indicates the input is within

the full-scale range of the converter.

In applications where absolute accuracy is important, off-

set and full-scale errors can be adjusted to zero. Offset

error must be adjusted before full-scale error. Zero offset

For zero offset error, apply a voltage equal to the input

Figure 10. Transfer Characteristics

Figure 9. RC Input Filter

+

ANALOG INPUT

1.5V TO 3.5V

2.5V

220pF

75

Ω

1406 F09

–

LTC1406

INPUT VOLTAGE (V)

0

–1

LSB

1406 F10

1111 1111

OF/UF BIT

1111 1110

1111 1101

1000 0001

1000 0000

0111 1111

0111 1110

0000 0000

0000 0001

0000 0010

1

LSB

FS – 1LSB

–FS