LTC2238/LTC2237/LTC2236
17
223876fa
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 25
resistorsand12pFcapacitorontheanaloginputs
serve two purposes: isolating the drive circuitry from the
sample-and-hold charging glitches and limiting the
wideband noise at the converter input.
APPLICATIO S I FOR ATIO
WU
UU
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 70MHz and 170MHz
25
25
12
12
0.1
F
AIN
+
AIN
8pF
2.2
F
VCM
ANALOG
INPUT
0.1
F
0.1
F
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
223876 F06
LTC2238
LTC2237
LTC2236
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
25
25
0.1
F
AIN
+
AIN
2.2
F
VCM
ANALOG
INPUT
0.1
F
0.1
F
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
223876 F07
LTC2238
LTC2237
LTC2236
25
25
0.1
F
AIN
+
AIN
2.2
F
VCM
ANALOG
INPUT
0.1
F
0.1
F
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
223876 F08
6.8nH
LTC2238
LTC2237
LTC2236
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
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 5. Single-Ended Drive
Figure 4. Differential Drive with an Amplifier
25
25
12pF
2.2
F
VCM
223876 F04
+
CM
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
AIN
+
AIN
LTC2238
LTC2237
LTC2236
25
0.1
F
ANALOG
INPUT
VCM
AIN
+
AIN
1k
12pF
223876 F05
2.2
F
1k
25
0.1
F
LTC2238
LTC2237
LTC2236
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LTC2238IUH 制造商:Linear Technology 功能描述:ADC Single Pipelined 65Msps 10-bit Parallel 32-Pin QFN EP
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