Datasheet

LTC2369-18
11
236918fa
APPLICATIONS INFORMATION
INPUT DRIVE CIRCUITS
A low impedance source can directly drive the high im-
pedance input of the LTC2369-18 without gain error. A
high impedance source should be buffered to minimize
settling time during acquisition and to optimize the dis-
tortion performance of the ADC. Minimizing settling time
is important even for DC inputs, because the ADC input
draws a current spike when entering acquisition.
For best performance, a buffer amplifier should be used
to drive the analog input of the LTC2369-18. The ampli-
fier provides low output impedance, which produces fast
settling of the analog signal during the acquisition phase.
It also provides isolation between the signal source and
the current spike the ADC input draws.
Input Filtering
The noise and distortion of the buffer amplifier and signal
source must be considered since they add to the ADC noise
and distortion. Noisy input signals should be filtered prior
to the buffer amplifier input with an appropriate filter to
minimize noise. The simple 1-pole RC lowpass filter (LPF1)
shown in Figure 4 is sufficient for many applications.
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.
Pseudo-Differential Unipolar Inputs
For most applications, we recommend the low power
LT6202 ADC driver to drive the LTC2369-18. With a low
noise density of 1.9nV/√Hz and a low supply current of
3mA, the LT6202 is flexible and may be configured to
convert signals of various amplitudes to the 0V to 5V input
range of the LTC2369-18.
To achieve the full distortion performance of the
LTC2369-18, a low distortion single-ended signal source
driven through the LT6202 configured as a unity-gain buf-
fer as shown in Figure 4 can be used to get the full data
sheet THD specification of –120dB.
The LT6202 can also be used to buffer and convert large
true bipolar signals which swing below ground to the 0V
to 5V input range of the LTC2369-18. Figure 5a shows the
LT6202 being used to convert a ±10V true bipolar signal
for use by the LTC2369-18. In this case, the LT6202 is
configured as an inverting amplifier stage, which acts to
attenuate and level shift the input signal to the 0V to 5V input
range of the LTC2369-18. In the inverting configuration, the
single-ended input signal source no longer directly drives
a high impedance input. The input impedance is instead
set by resistor R
IN
. R
IN
must be chosen carefully based on
the source impedance of the signal source. Higher values
of R
IN
tend to degrade both the noise and distortion of
the LT6202 and LTC2369-18 as a system. Table 1 shows
the resulting SNR and THD for several values of R
IN
, R1,
R2, R3 and R4 in this configuration. Figure 5b shows the
resulting FFT when using the LT6202 as shown in Figure 5a.
Figure 4. Input Signal Chain
Another filter network consisting of LPF2 should be used
between the buffer and ADC input to both minimize the
noise contribution of the buffer and to help minimize distur-
bances reflected into the buffer from sampling transients.
Long RC time constants at the analog inputs will slow
down the settling of the analog inputs. Therefore, LPF2
requires a wider bandwidth than LPF1. A buffer amplifier
with a low noise density must be selected to minimize
degradation of the SNR.
5.1Ω
10nF
66nF
50Ω
LPF2
LPF1
BW = 3.2MHz
BW = 48kHz
LTC2369-18
IN
+
IN
–
236918 F04
–
+
LT6202
V
REF
0V