Datasheet

LTC2389-16
20
238916f
applications inFormation
Input Filtering
The noise and distortion of the buffer amplifier and other
supporting circuitry must be considered since they add
to the ADC noise and distortion. A buffer amplifier with
low noise density must be selected to minimize SNR
degradation. A filter network should be placed between
the buffer output and ADC input to both minimize the
noise contribution of the buffer and reduce disturbances
reflected into the buffer from ADC sampling transients. A
simple one-pole lowpass RC filter is sufficient for many
applications. It is important that the RC time constants
of this filter be small enough to allow the analog inputs
to completely settle to 16-bit resolution within the ADC
acquisition time (t
ACQ
), as insufficient settling can limit
INL and THD performance. In many applications an RC
time constant of 10ns is fast enough to allow for sufficient
transient settling during acquisition while simultaneously
filtering driver wideband noise.
Often it is also beneficial to add small series resistors
between the primary lowpass RC filter and the ADC inputs.
These resistors, in conjunction with the ADC sampling
capacitance C
IN
and sampling switch resistance R
IN
,
form a second lowpass RC filter which further limits high-
frequency
driver noise as well as reduces the magnitude
of
the current spike drawn by the analog inputs when
entering acquisition. The time constant of this secondary
lowpass filter also directly affects settling of the analog
inputs during acquisition and must be kept fast. In many
applications 49.9Ω series resistors allow for sufficient
transient settling during acquisition while providing useful
additional filtering of wideband driver noise.
High quality capacitors and resistors should be used in the
RC filters since these components can add distortion. NPO
and silver mica type dielectric capacitors have excellent
linearity. Carbon surface mount resistors can 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.
Fully Differential Inputs
The LTC2389-16 accepts fully differential input signals
directly. For most fully differential applications, it is
recommended that the LTC2389-16 be driven using the
LT6201 ADC driver configured as two unity-gain buffers,
as shown in Figure 5a. The LT6201 combines fast settling
and good DC linearity with a 0.95nV/√Hz input-referred
noise density, enabling it to achieve the full ADC data
sheet SNR and THD specifications, as shown in the FFT
plot
in Figure 5b. This topology may also be used to buf-
fer single-ended signals and achieves full ADC data sheet
SNR and THD specifications in both pseudo-differential
input modes, as shown in the FFT plots in Figures 5c
and 5d.