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參數資料
| 型號： | LTC1746 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | 14-Bit, 80Msps Low Power 3V ADC |
| 中文描述： | 14位，80Msps ADC的低功耗3V的 |
| 文件頁數： | 12/20頁 |
| 文件大?。?/td> | 464K |
| 代理商： | LTC1746 |
LTC2249
12
2249f
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.
APPLICATIOU
W
U
U
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
DD
selects the 2V range; tying the SENSE
pin to V
CM
selects the 1V range.
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
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 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 1
μ
F ceramic capacitor.
Figure 5. Single-Ended Drive
25
0.1
μ
F
ANALOG
INPUT
V
CM
A
IN
+
A
IN
–
10k
12pF
2249 F05
2.2
μ
F
10k
25
0.1
μ
F
LTC2249
The 25
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.
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 70MHz and 170MHz
25
25
12
12
0.1
μ
F
A
IN+
A
IN–
8pF
2.2
μ
F
V
CM
LTC2249
ANALOG
INPUT
0.1
μ
F
0.1
μ
F
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
2249 F06
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
25
25
0.1
μ
F
A
IN+
A
IN–
2.2
μ
F
V
CM
LTC2249
ANALOG
INPUT
0.1
μ
F
0.1
μ
F
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
2249 F07
25
25
0.1
μ
F
A
IN+
A
IN–
2.2
μ
F
V
CM
LTC2249
ANALOG
INPUT
0.1
μ
F
0.1
μ
F
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
2249 F08
6.8nH
6.8nH
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
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