Datasheet

LTC2251/LTC2250
15
22510fa
APPLICATIO S I FOR ATIO
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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 transformer
gives better high frequency response than a flux coupled
center tapped transformer. The coupling capacitors 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.
Reference Operation
Figure 9 shows the LTC2251/LTC2250 reference circuitry
consisting of a 1.5V bandgap reference, a difference am-
plifier and switching and control circuit. The internal volt-
age 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.
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
ANALOG
INPUT
0.1µF
0.1µF
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
22510 F06
LTC2251/
LTC2250
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
25Ω
25Ω
0.1µF
A
IN
+
A
IN
–
2.2µF
V
CM
ANALOG
INPUT
0.1µF
0.1µF
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
22510 F08
8.2nH
8.2nH
LTC2251/
LTC2250
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
25Ω
25Ω
0.1µF
A
IN
+
A
IN
–
2.2µF
V
CM
ANALOG
INPUT
0.1µF
0.1µF
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
22510 F07
LTC2251/
LTC2250
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.
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
22510 F09
4Ω
DIFF AMP
1µF
1µF
INTERNAL ADC
LOW REFERENCE
1.5V BANDGAP
REFERENCE
1V
0.5V
RANGE
DETECT
AND
CONTROL
LTC2251/LTC2250
Figure 9. Equivalent Reference Circuit