LTC2249 Linear Technology, LTC2249 Datasheet - Page 12

• Manufacturer Part Number: LTC2249
• Description: 80Msps Low Power 3V ADC
• Manufacturer: Linear Technology

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LTC2249
APPLICATIO S I FOR ATIO
Figure 5 shows a single-ended input circuit. The imped-
ance seen by the analog inputs should be matched. This
circuit is not recommended if low distortion is required.
The 25Ω resistors and 12pF capacitor 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 frequencies above 70MHz, 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 1.5V. In
Figure 8, the series inductors are impedance matching
elements that maximize the ADC bandwidth.
12
ANALOG
INPUT
ANALOG
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
ANALOG
INPUT
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 70MHz and 170MHz
INPUT
0.1µF
0.1µF
0.1µF
0.1µF
0.1µF
Figure 5. Single-Ended Drive
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
10k
T1
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
U
10k
0.1µF
25Ω
25Ω
25Ω
25Ω
25Ω
25Ω
U
12Ω
12pF
0.1µF
2.2µF
12Ω
0.1µF
V
A
A
CM
IN
IN
+
–
LTC2249
W
2.2µF
8pF
2.2µF
V
A
A
CM
V
IN
IN
A
A
CM
IN
IN
+
–
2249 F05
+
–
LTC2249
LTC2249
2249 F06
2249 F07
U
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Reference Operation
Figure 9 shows the LTC2249 reference circuitry consisting
of a 1.5V 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 1V (±0.5V differential). Tying the
SENSE pin to V
pin to V
The 1.5V 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. An external bypass capacitor is required for
the 1.5V reference output, V
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 two pins. The multiple output
pins are needed to reduce package inductance. Bypass
capacitors must be connected as shown in Figure 9.
Other voltage ranges in-between the pin selectable ranges
can be programmed with two external resistors as shown
in Figure 10. An external reference can be used by applying
its output directly or through a resistor divider to SENSE.
It is not recommended to drive the SENSE pin with a logic
device. The SENSE pin should be tied to the appropriate
level as close to the converter as possible. If the SENSE pin
is driven externally, it should be bypassed to ground as
close to the device as possible with a 1µF ceramic capacitor.
ANALOG
CM
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
INPUT
selects the 1V range.
0.1µF
0.1µF
DD
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
T1
selects the 2V range; tying the SENSE
25Ω
25Ω
6.8nH
0.1µF
6.8nH
CM
. This provides a high
2.2µF
V
A
A
CM
IN
IN
+
–
LTC2249
2249 F08
2249f