LT6237 datasheet(19/24 Pages) LINER

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LT6236/LT6237

623637f

Interfacing to ADCs

When driving an ADC, a single-pole, passive RC filter

should be used between the outputs of the LT6236/LT6237

and the inputs of the ADC. The sampling process of ADCs

creates a charge transient from the switching of the ADC

sampling capacitor. This momentarily “shorts” the output

of the amplifier as charge is transferred between amplifier

and sampling capacitor. The amplifier must recover and

settle from this load transient before the acquisition period

has ended for a valid representation of the input signal.

The RC network between the outputs of the driver and

the inputs of the ADC decouples the sampling transient

of the ADC. The capacitance serves to provide the bulk

of the charge during the sampling process, while the two

resistors at the outputs of the LT6236/LT6237 are used to

dampen and attenuate any charge injected by the ADC. The

RC filter provides the benefit of band limiting broadband

output noise.

Thanks to the very low wideband noise of the LT6236/

LT6237, a wideband filter can be used between the

amplifier and the ADC without impacting SNR. This

is especially important with ADCs or applications that

require full settling in between each conversion.

The selection of an appropriate filter depends on the

specific ADC, however the following procedure is sug-

gested for choosing filter component values. Begin by

selecting an appropriate RC time constant for the input

APPLICATIONS INFORMATION

signal. Generally, longer time constants improve SNR at

the expense of settling time. Output transient settling to

18-bit accuracy will require over twelve RC time constants.

To select the resistor value, the resistors in the decoupling

network should be at least 10Ω. Keep in mind that these

resistors also serve to decouple the LT6236/LT6237

outputs from load capacitance. Too large of a resistor will

leave insufficient settling time. Too small of a resistor will

not properly dampen the load transient of the sampling

process, and prolong the time required for settling. For

lowest distortion, choose capacitors with low dielectric

absorption such as a C0G multilayer ceramic capacitor. In

general, large capacitor values attenuate the fixed nonlinear

charge kickback, however very large capacitor values will

detrimentally load the driver at the desired input frequency

and cause driver distortion. Smaller input swings allow

for larger filter capacitor values due to decreased loading

demands on the driver. This property may be limited by

the particular input amplitude dependence of differential

nonlinear kickback for the specific ADC used.

Series resistors should typically be placed at the inputs to

the ADC in order to further improve distortion performance.

These series resistors function with the ADC sampling

capacitor to filter potential ground bounce or other high

speed sampling disturbances. Additionally the resistors

limit the rise time of residual filter glitches that manage to

propagate to the driver outputs. Restricting possible glitch

propagation rise time to within the small signal bandwidth

of the driver enables less disturbed output settling.

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