LTC2229 LINER [Linear Technology], LTC2229 Datasheet - Page 14

• Manufacturer Part Number: LTC2229
• Description: 12-Bit, 80Msps Low Power 3V ADC
• Manufacturer: LINER [Linear Technology]

LTC2229
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
14
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
ANALOG
ANALOG
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 70MHz and 170MHz
INPUT
INPUT
ANALOG
INPUT
0.1µF
0.1µF
0.1µF
0.1µF
Figure 5. Single-Ended Drive
0.1µF
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
T1
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
U
1k
25Ω
25Ω
25Ω
25Ω
U
1k
25Ω
25Ω
0.1µF
12Ω
0.1µF
0.1µF
12Ω
12pF
2.2µF
W
V
A
A
CM
IN
IN
2.2µF
8pF
2.2µF
+
–
V
A
A
V
A
A
CM
IN
IN
CM
IN
IN
+
–
+
–
LTC2229
LTC2229
LTC2229
2229 F05
2229 F06
2229 F07
U
Figure 8, the series inductors are impedance matching
elements that maximize the ADC bandwidth.
Reference Operation
Figure 9 shows the LTC2229 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
TIE TO V
TIE TO V
RANGE = 2 • V
CM
0.5V < V
DD
CM
ANALOG
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
FOR 2V RANGE;
FOR 1V RANGE;
2.2µF
INPUT
1µF
1µF
1.5V
SENSE
selects the 1V range.
SENSE
Figure 9. Equivalent Reference Circuit
0.1µF
0.1µF
< 1V
FOR
DD
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
selects the 2V range; tying the SENSE
T1
2.2µF
SENSE
0.1µF
REFH
REFL
V
CM
25Ω
25Ω
LTC2229
6.8nH
CONTROL
4Ω
DETECT
0.1µF
RANGE
6.8nH
AND
1.5V BANDGAP
DIFF AMP
REFERENCE
2.2µF
INTERNAL ADC
HIGH REFERENCE
INTERNAL ADC
LOW REFERENCE
V
A
A
CM
IN
IN
+
–
1V
LTC2229
BUFFER
0.5V
2229 F08
2229fa
2229 F09