LT6204 Linear Technology, LT6204 Datasheet - Page 18

• Manufacturer Part Number: LT6204
• Description: (LT6202 - LT6204) Low Power Op Amps
• Manufacturer: Linear Technology

LT6202/LT6203/LT6204
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 negative
full-scale signal V
LTC1864. The top half of the LT6203 is in a gain-of-ten
configuration referenced to the buffered voltage V
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
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.
18
TYPICAL APPLICATIO S
IN
should be from V
V
LOW
LOW
= 0.4V
V
IN
200mV
= 0.6V
into the negative input of the
DC
DC
AC
LOW
+
–
U
1/2 LT6203
(about 0.4V) to about
1/2 LT6203
+
–
–100
–110
–120
–130
–140
–150
Figure 5. Single-Supply 16-Bit ADC Driver
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
Figure 6. FFT Showing 100dB SFDR
0
12.5 25
110
R1
R2
LOW
1k
37.5
and
FREQUENCY (kHz)
100
50
100
R4
R3
62.5
470pF
f
f
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 vanish-
ingly small (ideally, infinitesimally small) and make de-
mands 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.
S
IN
75 82.5
= 250ksps
C1
= 10.131836kHz
LT6202/03/04 F06
100
112.5 125
–
+
LTC1864
250ksps
16-BIT
5V
LT6202/03/04 F05
SERIAL
DATA
OUT
620234fa