Datasheet

LTC2248/LTC2247/LTC2246
16
224876fa
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.
APPLICATIO S I FOR ATIO
WUUU
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 70MHz and 170MHz
25Ω
25Ω
12Ω
12Ω
0.1µF
A
IN
+
A
IN
–
8pF
2.2µF
V
CM
LTC2248/47/46
ANALOG
INPUT
0.1µF
0.1µF
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
224876 F06
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
25Ω
25Ω
0.1µF
A
IN
+
A
IN
–
2.2µF
V
CM
LTC2248/47/46
ANALOG
INPUT
0.1µF
0.1µF
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
224876 F07
25Ω
25Ω
0.1µF
A
IN
+
A
IN
–
2.2µF
V
CM
LTC2248/47/46
ANALOG
INPUT
0.1µF
0.1µF
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
224876 F08
6.8nH
6.8nH
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
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
< V
CM
•
1.5V
REFL
2.2µF
2.2µF
INTERNAL ADC
HIGH REFERENCE
BUFFER
0.1µF
224876 F09
LTC2248/47/46
4Ω
DIFF AMP
1µF
1µF
INTERNAL ADC
LOW REFERENCE
1.5V BANDGAP
REFERENCE
1V
0.5V
RANGE
DETECT
AND
CONTROL
1.1
1.5
Figure 9. Equivalent Reference Circuit
Reference Operation
Figure 9 shows the LTC2248/LTC2247/LTC2246 refer-
ence circuitry consisting of a 1.5V bandgap reference, a
difference amplifier and switching and control circuit. The
internal voltage reference 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.
The 1.5V bandgap reference serves two functions: its
output provides a DC bias point for setting the common
mode voltage of any external input circuitry; additionally,
the reference is used with a difference amplifier to gener-
ate the differential reference levels needed by the internal
ADC circuitry. An external bypass capacitor is required for
the 1.5V reference output, V
CM
. This provides a high
frequency low impedance path to ground for internal and
external circuitry.
The difference amplifier generates the high and low refer-
ence for the ADC. High speed switching circuits are
connected to these outputs and they must be externally
bypassed. Each output has two pins. The multiple output