Datasheet

LT6236/LT6237/LT6238
20
623637fa
For more information www.linear.com/LT6236
Interfacing to ADCs
When driving an ADC, a single-pole, passive RC filter should
be used between the outputs of the LT6236/LT6237/LT6238
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/LT6238 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/LT6238, 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 suggested for
choosing filter component values. Begin by selecting an
appropriate RC time constant for the input signal. Gener-
APPLICATIONS INFORMATION
ally, 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 resis-
tors also serve to decouple the LT6236/LT6237/LT6238
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.