Datasheet

LTC2383-16
11
238316f
APPLICATIONS INFORMATION
time is important even for DC inputs, because the ADC
inputs draw a current spike when entering acquisition.
For best performance, a buffer amplifier should be used to
drive the analog inputs of the LTC2383-16. The amplifier
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 inputs draw.
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.
Another filter network consisting of LPF2 and the 100 series
input resistors should be used between the buffer and ADC
inputs to both minimize the noise contribution of the buffer
and to help minimize disturbances 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. With the 482kHz lowpass
filter shown in Figure 4, the LT6350 provides the full data
sheet performance of the LTC2383-16.
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.
50
3300pF
6600pF
50
500
100
100
LPF2
LPF1
BW = 482kHz
BW = 48kHz
SINGLE-ENDED-
TO-DIFFERENTIAL
DRIVER
SINGLE-ENDED-
INPUT SIGNAL
LTC2383-16
IN
+
IN
238316 F04
LT6350
Single-Ended-to-Differential Conversion
For single-ended input signals, a single-ended to differential
conversion circuit must be used to produce a differential
signal at the inputs of the LTC2383-16. The LT6350 ADC
driver is recommended for performing single-ended-to-
differential conversions.The LT6350 is flexible and may
be configured to convert single-ended signals of various
amplitudes to the ±2.5V differential input range of the
LTC2383-16. The LT6350 is also available in H-grade to
complement the extended temperature operation of the
LTC2383-16 up to 125°C.
Figure 5 shows the LT6350 being used to convert a 0V
to 2.5V single-ended input signal. In this case, the first
amplifier is configured as a unity gain buffer and the single-
ended input signal directly drives the high-impedance
input of the amplifier. As shown in the FFT of Figure 5a,
the LT6350 drives the LTC2383-16 to full datasheet
performance without degrading the SNR or THD .
Figure 4. Input Signal Chain
LT6350
V
CM
= V
REF
/2
2.5V to
0V
0V to
2.5V
0V to 2.5V
238316 F05
OUT1
R
INT
R
INT
OUT2
8
4
5
2
1
+
+
+
FREQUENCY (kHz)
0 100 200 300 400 500
–180
AMPLITUDE (dBFS)
–60
–40
–20
–80
–100
–120
–140
–160
0
238316 F05a
SNR = 92.2dB
THD = –106.2dB
SINAD = 92dB
SFDR = 110.4dB
Figure 5. LT6350 Converting a 0V-2.5V Single-Ended Signal
to a ±2.5V Differential Input Signal
Figure 5a. 32k Point FFT Plot for Circuit Shown in Figure 5