LTC2250 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2250' PDF : 24 Pages View PDF
LTC2251/LTC2250
APPLICATIO S I FOR ATIO
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.
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, VCM. This provides a high
frequency low impedance path to ground for internal and
external circuitry.
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 VDD selects the 2V range; tying the
SENSE pin to VCM selects the 1V range.
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+
8pF
AIN–
LTC2251/
LTC2250
22510 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+
LTC2251/
LTC2250
AIN–
22510 F07
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 170MHz and 300MHz
VCM
ANALOG
INPUT
0.1µF
8.2nH
2.2µF
AIN+
25Ω 0.1µF
T1
0.1µF
25Ω 8.2nH
AIN–
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
LTC2251/
LTC2250
22510 F08
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 300MHz
LTC2251/LTC2250
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
1µF
SENSE
REFH
RANGE
DETECT
AND
CONTROL
BUFFER
INTERNAL ADC
HIGH REFERENCE
2.2µF
1µF
0.1µF
REFL
DIFF AMP
INTERNAL ADC
LOW REFERENCE
22510 F09
Figure 9. Equivalent Reference Circuit
22510fa
15
Share Link: 
