
14
LTC2239
2239fa
APPLICATIO S I FOR ATIO
WU
UU
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 (
±1Vdifferential)or1V(±0.5Vdifferential).Tyingthe
SENSE pin to VDD selects the 2V range; tying the SENSE
pin to VCM selects the 1V range.
Figure 5. Single-Ended Drive
25
0.1
F
ANALOG
INPUT
VCM
AIN
+
AIN
–
1k
12pF
2239 F05
2.2
F
1k
25
0.1
F
LTC2239
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.
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
25
25
12
12
0.1
F
AIN
+
AIN
–
8pF
2.2
F
VCM
LTC2239
ANALOG
INPUT
0.1
F
0.1
F
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
2239 F06
25
25
0.1
F
AIN
+
AIN
–
2.2
F
VCM
LTC2239
ANALOG
INPUT
0.1
F
0.1
F
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
2239 F07
25
25
0.1
F
AIN
+
AIN
–
2.2
F
VCM
LTC2239
ANALOG
INPUT
0.1
F
0.1
F
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
2239 F08
6.8nH
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
VCM
REFH
SENSE
TIE TO VDD FOR 2V RANGE;
TIE TO VCM FOR 1V RANGE;
RANGE = 2 VSENSE FOR
0.5V < VSENSE < 1V
1.5V
REFL
2.2
F
2.2
F
INTERNAL ADC
HIGH REFERENCE
BUFFER
0.1
F
2239 F09
LTC2239
4
DIFF AMP
1
F
1
F
INTERNAL ADC
LOW REFERENCE
1.5V BANDGAP
REFERENCE
1V
0.5V
RANGE
DETECT
AND
CONTROL
Figure 9. Equivalent Reference Circuit
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz