LT6350 datasheet(17/28 Pages) AD

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LT6350

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applicaTions inForMaTion

The capacitance serves to provide the bulk of the charge

during the sampling process, while the two resistors at the

outputs of the LT6350 are used to dampen and attenuate

any charge injected by the ADC. The RC filter can also be

used to the additional benefit of band limiting broadband

output noise. See the Noise Considerations section for

more information.

The selection of the RC time constant depends on the ADC;

but generally, longer time constants will improve SNR at

the expense of longer settling time. Excessive settling time

can introduce gain errors and can cause distortion if the

filter components are not perfectly linear. Note also that

too small of a resistor will not properly dampen the load

transient of the sampling process, prolonging the time

required for settling. 16-bit applications typically require

a minimum settling time of eleven RC time constants of

a first order filter.

Note that the filter’s series resistance also serves to

decouple the LT6350 outputs from load capacitance. The

outputs of the LT6350 are designed to drive a maximum

of 40pF to ground or 20pF differentially; higher values

of filter capacitor should always be decoupled with filter

resistors of at least 25Ω.

High quality resistors and capacitors should be used in

the RC filter since these components can contribute to

distortion. For lowest distortion, choose capacitors with

a high quality dielectric, such as a C0G multilayer ceramic

capacitor. Metal film surface mount resistors are more

linear than carbon types.

SETTING THE OUTPUT COMMON MODE VOLTAGE

The output common mode voltage is set by the voltage

applied to pin +IN2 in accordance with Equation (3). The

usable output common mode range is determined by the

input common mode range of the inverting op amp and

is from V– + 1.5V to V+.

In single supply applications, the optimal common mode

input range to the ADC is often determined by the ADC’s

reference. If the ADC has an output pin for setting the input

common mode voltage, it can be directly tied to the +IN2

pin, as long as it is capable of providing the input current

into +IN2 as listed in the Electrical Characteristics Table.

Alternatively, +IN2 may be driven by an external precision

reference such as the LT1790.

For lowest offset, the +IN2 pin should see 499Ω of driving

resistance in all applications (see Offset Considerations).

If the driving resistance is nominally less than 499Ω,

additional resistance can be added to make up the

difference. The resistor noise bandwidth can be reduced

by bypassing the +IN2 pin to the ground plane with a

chip ceramic capacitor of at least 0.1µF (see the Typical

Application on the front page). The bypass capacitance

also helps prevent AC signals on this pin from being

inadvertently converted to differential signals.

SHDN

If the SHDN pin (Pin 7), is pulled low within 300mV of the

negative supply rail, the LT6350 will power down. The pin

is connected through a diode to an internal current source

of 20µA. When pulled below the shutdown threshold, the

20µA current will flow from the pin. If the pin is left open

or pulled high (above V– + 2V), the part will enter normal

active operation, and the current into the pin will be very

small due to the reverse-biased diode.

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