LTC1743IFW View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1743IFW' PDF : 24 Pages View PDF
LTC1743
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.
SINGLE-ENDED INPUT
2.5V ±1/2 RANGE
VCM
5V
4.7µF
+
1/2 LT1819
–
37Ω
18pF
AIN+
LTC1743
18pF
100Ω
+
1/2 LT1819
–
37Ω
500Ω
500Ω
AIN–
18pF
1743 F04a
Figure 4a. Differential Drive with Op Amps
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 37Ω resistors and 18pF capacitors on the analog
inputs serve two purposes: isolating the drive circuitry
VCM
5V
1µF
VIN
RIN
402Ω
0.01µF
RIN
0.1µF
1
2
–
3
4
VOCM +
7
8
LT6600-20
VMID –
+5
49.9Ω
49.9Ω
AIN+
18pF
AIN–
6
GAIN = 402Ω/RIN
LTC1743
1743 F04b
402Ω
MAXIMUM GAIN = 4
Figure 4b. Using the LT6600 as a Differential Driver
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 VDD selects the 3.2V range.
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.
1743f
15
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