LTC2220-1 Datasheet, PDF (17/28 Pages) Linear Technology – 12-Bit,185Msps ADC

LTC2220-1
APPLICATIO S I FOR ATIO
The A_IN+ and A_IN– inputs each have two pins to reduce
package inductance. The two A_IN+ and the two A_IN– pins
should be shorted together.
For input frequencies above 100MHz the input circuits of
Figure 6, 7 and 8 are recommended. The balun trans-
former gives better high frequency response than a flux
coupled center tapped transformer. The coupling capaci-
tors allow the analog inputs to be DC biased at 1.6V. In
Figure 8 the series inductors are impedance matching
elements that maximize the ADC bandwidth.
V_CM
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
2.2µF
A_IN+
A_IN+
8pF
A_IN–
A_IN–
LTC2220-1
22201 F06
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 100MHz and 250MHz
0.1µF
ANALOG
INPUT
T1
0.1µF
25Ω 0.1µF
25Ω
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
V_CM
2.2µF
A_IN+
A_IN+
LTC2220-1
A_IN–
A_IN–
22201 F07
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 250MHz and 500MHz
0.1µF
ANALOG
INPUT
T1
0.1µF
4.7nH
25Ω 0.1µF
25Ω _4.7nH
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
V_CM
2.2µF
A_IN+
A_IN+
2pF
A_IN–
A_IN–
LTC2220-1
22201 F08
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 500MHz
Reference Operation
Figure 9 shows the LTC2220-1 reference circuitry consist-
ing of a 1.6V 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 1V (±0.5V differential). Tying the
SENSE pin to V_DD selects the 2V range; typing the SENSE
pin to V_CM selects the 1V range.
The 1.6V 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 1.6V reference output, V_CM. This provides a high
frequency low impedance path to ground for internal and
external circuitry.
The difference amplifier generates the high and low refer-
ence 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.
LTC2220-1
1.6V
V_CM
4Ω
1.6V BANDGAP
REFERENCE
2.2µF
1V 0.5V
TIE TO V_DD FOR 2V RANGE;
TIE TO V_CM FOR 1V RANGE;
RANGE = 2 • V_SENSE FOR
0.5V < V_SENSE < 1V
1µF
SENSE
REFLB
0.1µF
REFHA
RANGE
DETECT
AND
CONTROL
BUFFER
INTERNAL ADC
HIGH REFERENCE
2.2µF
DIFF AMP
1µF
REFLA
0.1µF
REFHB
INTERNAL ADC
LOW REFERENCE
22201 F09
Figure 9. Equivalent Reference Circuit
2220_1fa
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