LTC2239IUH View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2239IUH' PDF : 20 Pages View PDF
LTC2239
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
ANALOG
INPUT
0.1µF
1k 1k
25Ω
VCM
2.2µF
AIN+
LTC2239
12pF
25Ω
AIN–
0.1µF
2239 F05
Figure 5. Single-Ended Drive
0.1µF
ANALOG
INPUT
T1
0.1µF
12Ω
25Ω 0.1µF
25Ω 12Ω
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
VCM
2.2µF
AIN+
LTC2239
8pF
AIN–
2239 F06
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 70MHz and 170MHz
0.1µF
ANALOG
INPUT
T1
0.1µF
25Ω 0.1µF
25Ω
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
VCM
2.2µF
AIN+
LTC2239
AIN–
2239 F07
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
14
Figure 8, the series inductors are impedance matching
elements that maximize the ADC bandwidth.
Reference Operation
Figure 9 shows the LTC2239 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 VDD selects the 2V range; tying the SENSE
pin to VCM selects the 1V range.
VCM
ANALOG
INPUT
0.1µF
6.8nH
2.2µF
AIN+
25Ω 0.1µF
T1
0.1µF
25Ω 6.8nH
AIN–
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
LTC2239
2239 F08
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
LTC2239
1.5V
VCM
4Ω
1.5V BANDGAP
REFERENCE
2.2µF
1V 0.5V
TIE TO VDD FOR 2V RANGE;
TIE TO VCM FOR 1V RANGE;
RANGE = 2 • VSENSE FOR
0.5V < VSENSE < 1V
SENSE
1µF
REFH
RANGE
DETECT
AND
CONTROL
BUFFER
INTERNAL ADC
HIGH REFERENCE
2.2µF
1µF
0.1µF
REFL
DIFF AMP
INTERNAL ADC
LOW REFERENCE
Figure 9. Equivalent Reference Circuit
2239 F09
2239fa
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