Datasheet

LT6202/LT6203/LT6204
21
620234fd
TYPICAL APPLICATIONS
Single-Supply 16-Bit ADC Driver
Figure 5 shows the LT6203 driving an LTC1864 unipolar
16-bit A/D converter. The bottom half of the LT6203 is
in a gain-of-one configuration and buffers the 0V nega-
tive full-scale signal V
LOW
into the negative input of the
LTC1864. The top half of the LT6203 is in a gain-of-ten
configuration referenced to the buffered voltage V
LOW
and
drives the positive input of the LTC1864. The input range
of the LTC1864 is 0V to 5V, but for best results the input
range of V
IN
should be from V
LOW
(about 0.4V) to about
0.82V. Figure 6 shows an FFT obtained with a 10.1318kHz
coherent input waveform, from 8192 samples with no
windowing or averaging. Spurious free dynamic range is
seen to be about 100dB.
Although the LTC1864 has a sample rate far below the
gain bandwidth of the LT6203, using this amplifier is not
necessarily a case of overkill. The designer is reminded that
A/D converters have sample apertures that are vanishingly
small (ideally, infinitesimally small) and make demands on
the upstream circuitry far in excess of what is implied by
the innocent-looking sample rate. In addition, when an A/D
converter takes a sample, it applies a small capacitor to
its inputs with a fair amount of glitch energy and expects
the voltage on the capacitor to settle to the true value very
quickly. Finally, the LTC1864 has a 20MHz analog input
bandwidth and can be used in undersampling applications,
again requiring a source bandwidth higher than Nyquist.
–
+
–
+
–
+
LTC1864
16-BIT
250ksps
5V
SERIAL
DATA
OUT
C1
470pF
R3
100Ω
R4
100Ω
R1
1k
R2
110Ω
V
IN
= 0.6V
DC
±200mV
AC
V
LOW
= 0.4V
DC
1/2 LT6203
1/2 LT6203
LT6202/03/04 F05
Figure 5. Single-Supply 16-Bit ADC Driver
FREQUENCY (kHz)
0
SFDR (dB)
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
–140
–150
37.5 62.5 100
LT6202/03/04 F06
12.5 25 50 75 82.5 112.5 125
f
S
= 250ksps
f
IN
= 10.131836kHz
Figure 6. FFT Showing 100dB SFDR