Datasheet

14
LTC1744
1744f
APPLICATIO S I FOR ATIO
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Input Drive Circuits
Figure 3 shows the LTC1744 being driven by an RF
transformer with a center tapped secondary. The second-
ary center tap is DC biased with V
CM
, setting the ADC input
signal at its optimum DC level. Figure 3 shows a 1:1 turns
ratio transformer. Other turns ratios can be used if the
source impedence seen by the ADC does not exceed 100Ω
for each ADC input. A disadvantage of using a transformer
is the loss of low frequency response. Most small RF
transformers have poor performance at frequencies be-
low 1MHz.
frequencies higher than 40MHz, the capacitors may need
to be decreased to prevent excessive signal loss.
Reference Operation
Figure 5 shows the LTC1744 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
DD
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.
1:1
37Ω
0.1µF
ANALOG
INPUT
V
CM
A
IN
+
A
IN
–
100Ω 100Ω 18pF
18pF
18pF
1744 F03
4.7µF
37Ω
LTC1744
37Ω
5V
SINGLE-ENDED
INPUT
2.5V ±1/2 RANGE
V
CM
A
IN
+
A
IN
–
18pF
18pF
18pF
1744 F04a
4.7µF
37Ω
100Ω
500Ω 500Ω
LTC1744
–
+
1/2 LT1810
–
+
1/2 LT1810
Figure 3. Single-Ended to Differential
Conversion Using a Transformer
Figure 4a. Differential Drive with Op Amps
Figure 4b. Using the LT6600 as a Differential Driver
5V
1
4
3
GAIN = 402Ω/R
IN
MAXIMUM GAIN = 4
5
6
2
7
8
V
CM
A
IN
+
A
IN
–
18pF
1744 F04b
1µF
49.9Ω
49.9Ω
R
IN
402Ω
R
IN
402Ω
0.1µF
0.01µF
LTC1744
–
+
–
+
LT6600-20
V
OCM
V
MID
V
IN
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 37Ω resistors and 18pF capacitors on the analog
inputs serve two purposes: isolating the drive circuitry
from the sample-and-hold charging glitches and limiting
the wideband noise at the converter input. For input