LTC1743 Linear Technology, LTC1743 Datasheet - Page 15

• Manufacturer Part Number: LTC1743
• Description: 12-Bit 50Msps ADC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Figure 4a demonstrates the use of operational amplifiers
to convert a single ended input signal into a differential
input signal. The advantage of this method is that it
provides low frequency input response; however, the
limited gain bandwidth of most op amps will limit the
SFDR at high input frequencies.
Figure 4b shows the LT6600, a low noise differential
amplifier and lowpass filter, used as an input driver. The
LT6600 provides two functions: it serves as a 4th order
lowpass filter and as a single-ended to differential con-
verter. Additionally it can be programmed with one exter-
nal resistor to provide a gain from 1 to 4. Three versions
of this device are available having lowpass filter band-
widths of 2.5MHz, 10MHz or 20MHz.
The 37
inputs serve two purposes: isolating the drive circuitry
SINGLE-ENDED INPUT
2.5V 1/2 RANGE
Figure 4a. Differential Drive with Op Amps
resistors and 18pF capacitors on the analog
100
500
U
+
–
+
–
1/2 LT1819
1/2 LT1819
U
5V
500
W
37
37
18pF
4.7 F
18pF
18pF
V
A
A
CM
IN
IN
U
+
–
LTC1743
1743 F04a
from the sample-and-hold charging glitches and limiting
the wideband noise at the converter input. For input
frequencies higher than 40MHz, the capacitors may need
to be decreased to prevent excessive signal loss.
Reference Operation
Figure 5 shows the LTC1743 reference circuitry consisting
of a 2.5V bandgap reference, a difference amplifier and
switching and control circuit. The internal voltage refer-
ence can be configured for two pin selectable input ranges
of 2V( 1V differential) or 3.2V( 1.6V differential). Tying
the SENSE pin to ground selects the 2V range; tying the
SENSE pin to V
The 2.5V bandgap reference serves two functions: its
output provides a DC bias point for setting the common
mode voltage of any external input circuitry; additionally,
the reference is used with a difference amplifier to gener-
ate the differential reference levels needed by the internal
ADC circuitry.
V
Figure 4b. Using the LT6600 as a Differential Driver
IN
0.01 F
402
402
R
R
IN
IN
DD
1
2
7
8
selects the 3.2V range.
–
V
V
+
LT6600-20
OCM
MID
5V
3
6
+
–
4
5
GAIN = 402 /R
MAXIMUM GAIN = 4
0.1 F
49.9
49.9
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18pF
IN
1 F
V
A
A
CM
IN
IN
+
–
LTC1743
LTC1743
1743 F04b
15
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