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