Datasheet

LT1806/LT1807
21
18067fc
TYPICAL APPLICATIONS
Driving A/D Converter
The LT1806/LT1807 have 60ns settling time to 0.01% on
a 2V step signal, and 20Ω output impedance at 100MHz,
that makes them ideal for driving high speed A/D convert-
ers. With the rail-to-rail input and output, and low supply
voltage operation, the LT1806/LT1807 are also desirable
for single supply applications. As shown in the application
on the front page of this data sheet, the LT1807 drives a
10Msps, 12-bit, LTC1420 ADC in a gain of 20. Driving the
LTC1420 differentially will optimize the signal-to-noise
ratio, SNR, and the total harmonic distortion, THD, of the
A/D converter. The lowpass fi lter, R5, R6 and C3 reduce
noise or distortion products that might come from the input
signal. High quality capacitors and resistors, 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 LT1807, and the ability of the
amplifi er to settle it quickly will affect the spurious free
dynamic range of the system. Figure 2 depicts the LT1806
driving LTC1420 at noninverting gain of 2 confi guration.
The FFT responses show a better than 92dB of spurious
free dynamic range, SFDR.
Figure 2. Noninverting A/D Driver
–
+
LT1806
LTC1420
PGA GAIN = 1
REF = 2.048V
5V
5V
12 BITS
10Msps
–5V
–5V
•
•
•
R2
1k
R1
1k
V
IN
1.5V
P-P
C1
470pF
–A
IN
18067 F02
+A
IN
R3
49.9Ω
Figure 3. 4096 Point FFT Response
FREQUENCY (MHz)
0
–120
AMPLITUDE (dB)
–100
–80
–60
–40
–20
0
1234
18067 F03
5
V
S
= p5V
A
V
= 2
f
SAMPLE
= 10Msps
f
IN
= 1.4086MHz
SFDR = 92.5dB