LTC2240UP-10 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC2240UP-10 Datasheet PDF : 28 Pages
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APPLICATIONS INFORMATION
the sample-and-hold charging glitches and limiting the
wideband noise at the converter input. For input frequen-
cies higher than 100MHz, the capacitor may need to be
decreased to prevent excessive signal loss.
The AIN+ and AIN– inputs each have two pins to reduce
package inductance. The two AIN+ and the two AIN– pins
should be shorted together.
For input frequencies above 100MHz the input circuits of
Figure 6, 7 and 8 are recommended. The balun transformer
gives better high frequency response than a flux coupled
center tapped transformer. The coupling capacitors allow
the analog inputs to be DC biased at 1.25V. In Figure 8 the
series inductors are impedance matching elements that
maximize the ADC bandwidth.
Reference Operation
Figure 9 shows the LTC2240-10 reference circuitry consist-
ing of a 1.25V bandgap reference, a difference amplifier
and switching and control circuit. The internal voltage
reference can be configured for two pin selectable input
ranges of 2V (±1V differential) or 1V (±0.5V differential).
Tying the SENSE pin to VDD selects the 2V range; typing
the SENSE pin to VCM selects the 1V range.
The 1.25V 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. An external bypass capacitor is required
for the 1.25V reference output, VCM. This provides a high
frequency low impedance path to ground for internal and
external circuitry.
The difference amplifier generates the high and low
reference for the ADC. High speed switching circuits are
connected to these outputs and they must be externally
bypassed. Each output has four pins: two each of REFHA
and REFHB for the high reference and two each of REFLA
and REFLB for the low reference. The multiple output pins
are needed to reduce package inductance. Bypass capaci-
tors must be connected as shown in Figure 9.
LTC2240-10
10Ω
0.1μF
ANALOG
INPUT
T1
0.1μF
12Ω
25Ω 0.1μF
25Ω 12Ω
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
VCM
2.2μF
AIN+
AIN+
8pF
AIN–
AIN–
LTC2240-10
224010 F06
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 100MHz and 250MHz
10Ω
0.1μF
ANALOG
INPUT
T1
0.1μF
25Ω 0.1μF
25Ω
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
VCM
2.2μF
AIN+
AIN+ LTC2240-10
AIN–
AIN–
224010 F07
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 250MHz and 500MHz
10Ω
VCM
0.1μF
ANALOG
INPUT
T1
0.1μF
2.7nH
25Ω 0.1μF
25Ω 2.7nH
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
2.2μF
AIN+
AIN+ LTC2240-10
AIN–
AIN–
224010 F08
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 500MHz
224010fb
17
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