LTC2238 Linear, LTC2238 Datasheet - Page 17

• Manufacturer Part Number: LTC2238
• Description: (LTC2236 - LTC2238) LOW POWER 3V ADC
• Manufacturer: Linear

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.
ANALOG
ANALOG
INPUT
INPUT
Figure 4. Differential Drive with an Amplifier
0.1µF
DIFFERENTIAL
Figure 5. Single-Ended Drive
+
CM
–
HIGH SPEED
AMPLIFIER
U
+
–
1k
U
1k
25Ω
25Ω
0.1µF
25Ω
25Ω
12pF
2.2µF
12pF
W
2.2µF
V
A
A
V
CM
IN
IN
A
A
CM
IN
IN
+
–
+
–
LTC2238
LTC2237
LTC2236
LTC2238
LTC2237
LTC2236
223876 F05
223876 F04
U
LTC2238/LTC2237/LTC2236
ANALOG
ANALOG
ANALOG
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
INPUT
INPUT
INPUT
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 70MHz and 170MHz
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
0.1µF
0.1µF
0.1µF
0.1µF
0.1µF
0.1µF
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
T1
T1
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
25Ω
25Ω
25Ω
25Ω
25Ω
25Ω
6.8nH
12Ω
0.1µF
0.1µF
0.1µF
12Ω
6.8nH
2.2µF
8pF
2.2µF
2.2µF
A
V
V
A
V
A
A
A
A
IN
CM
CM
IN
CM
IN
IN
IN
IN
–
+
+
–
+
–
LTC2238
LTC2237
LTC2236
LTC2238
LTC2237
LTC2236
LTC2238
LTC2237
LTC2236
17
223876 F06
223876 F07
223876 F08
223876fa