Datasheet

LT1809/LT1810
19
180910fa
TYPICAL APPLICATIONS
Driving A/D Converters
The LT1809/LT1810 have a 27ns settling time to 0.1% of
a 2V step signal and 20 output impedance at 100MHz
making it ideal for driving high speed A/D converters. With
the rail-to-rail input and output and low supply voltage
operation, the LT1809 is also desirable for single supply
applications. As shown in Figure 2, the LT1809 drives a
10Msps, 12-bit ADC, the LTC1420. The lowpass fi lter, R3
and C1, reduces the noise and distortion products that
might come from the input signal. High quality capacitors
and resistors, an NPO chip capacitor and metal-fi lm surface
mount resistors, should be used since these components
can add to distortion. The voltage glitch of the converter,
due to its sampling nature, is buffered by the LT1809 and
the ability of the amplifi er to settle it quickly will affect the
spurious-free dynamic range of the system. Figure 2 to
Figure 7 depict the LT1809 driving the LTC1420 at different
confi gurations and voltage supplies. The FFT responses
show better than 90dB of SFDR for a ± 5V supply, and 80dB
on a 5V single supply for the 1.394MHz signal.
Figure 2. Noninverting A/D Driver
Figure 3. 4096 Point FFT Response
–
+
LT1809
LTC1420
PGA GAIN = 1
REF = 2.048V
5V
5V
12 BITS
10Msps
–5V
–5V
•
•
•
R2
1k
R1
1k
V
IN
1V
P-P
C1
470pF
–A
IN
1809 F02
+A
IN
R3
49.9Ω
FREQUENCY (MHz)
0
–120
AMPLITUDE (dB)
–100
–80
–60
–40
–20
0
1234
1809 F03
5
V
S
= ±5V
A
V
= +2
f
SAMPLE
= 10Msps
f
IN
= 1.394MHz
SFDR = 90dB
Figure 4. Inverting A/D Driver
Figure 5. 4096 Point FFT Response
–
+
LT1809
LTC1420
PGA GAIN = 1
REF = 2.048V
5V
5V
12 BITS
10Msps
–5V
–5V
•
•
•
1k
1k
V
IN
2V
P-P
470pF
–A
IN
1809 F04
+A
IN
49.9Ω
FREQUENCY (MHz)
0
–120
AMPLITUDE (dB)
–100
–80
–60
–40
–20
0
1234
1809 F05
5
V
S
= ±5V
A
V
= –1
f
SAMPLE
= 10Msps
f
IN
= 1.394MHz
SFDR = 90dB