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LTC1852/LTC1853

18523fa

LT1809/LT1810: Single and Dual 180MHz Rail-to-Rail

Voltage Feedback Ampliﬁer. Single 3V to ±15V supplies.

20mA supply current. Lowest distortion.

LT1812/LT1813: Single and Dual 100MHz Voltage Feed-

back Ampliﬁer. Single 5V to ±5V supplies. 3.6mA supply

current. Low noise and 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 LTC1852/LTC1853 noise and distortion. 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 instance, a 200Ω source resistor

and a 1000pF capacitor to ground on the input will limit

the input bandwidth to 800kHz.The 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 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.

REFERENCE

The LTC1852/LTC1853 includes an on-chip, temperature

compensated, curvature corrected, bandgap reference

that is factory trimmed to 2.500V and has a very ﬂexible

3-pin interface. REFOUT is the 2.5V bandgap output, REFIN

is the input to the reference buffer and REFCOMP is the

reference buffer output. The input span is determined by

the voltage appearing on the REFCOMP pin as shown in

Table 2. The reference buffer has a gain of 1.6384 and

is factory trimmed by forcing an external 2.500V on the

REFIN pin and trimming REFCOMP to 4.096V. The 3-pin

interface allows for three pin-strappable Reference modes

as well as two additional external Reference modes. For

voltages on the REFIN pin ranging from 1V to 2.6V, the

output voltage on REFCOMP will equal 1.6384 times the

voltage on the REFIN pin. In this mode, the REFIN pin can

be tied to REFOUT to use the internal 2.5V reference to get

4.096V on REFCOMP or driven with an external reference

or DAC. If REFIN is tied low, the internal 2.5V reference

divided by 2 (1.25V) is connected internally to the input

of the reference buffer resulting in 2.048V on REFCOMP.

If REFIN is tied high, the reference buffer is disabled and

REFCOMP can be tied to REFOUT to achieve a 2.5V span

or driven with an external reference or DAC. Table 3 sum-

marizes the Reference modes.

Table 3. Reference Mode Table

MODE

REFIN

REFCOMP

REFIN Tied Low

0V Input

2.048V Output

REFIN is Buffer Input

1v to 2.6 Input

1.6384V to 4.26V Output

(1.6384 • REFIN)

REFIN Tied High

5V Input

Input, 19.2kΩ to Ground

Full Scale and Offset

In applications where absolute accuracy is important,

offset and full-scale errors can be adjusted to zero dur-

ing a calibration sequence. Offset error must be adjusted

before full-scale error. Zero offset is achieved by adjust-

ing the offset applied to the “–” input. For single-ended

inputs, this offset should be applied to the COM pin. For

differential inputs, the “–” input is dictated by the MUX

address. For zero offset error, apply 0.5LSB (actual volt-

age will vary with input span selected) to the “+” input

and adjust the offset at the “–” input until the output code

ﬂickers between 0000 0000 0000 and 0000 0000 0001

for the LTC1853 and between 00 0000 0000 and 00 0000

0001 for the LTC1852.

As mentioned earlier, the internal reference is factory

trimmed to 2.500V. To make sure that the reference buffer

gain is not compensating for trim errors in the reference,

REFCOMP is trimmed to 4.096V with an extremely accurate

external 2.5V reference applied to REFIN. Likewise, to make

sure that the full-scale gain trim is not compensating for

errors in the reference buffer gain, the input full-scale gain

is trimmed with an extremely accurate 4.096V reference

applied to REFCOMP (REFIN = 5V to disable the reference

buffer). This allows the use of either a 2.5V reference applied

to REFIN or a 4.096V reference applied to REFCOMP to

achieve accurate results. Full-scale errors can be trimmed

to zero by adjusting the appropriate reference voltage. For

unipolar inputs, an input voltage of FS – 1.5LSBs should

be applied to the “+” input and the appropriate reference

APPLICATIONS INFORMATION

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