LTC2247 Linear Technology, LTC2247 Datasheet - Page 16

• Manufacturer Part Number: LTC2247
• Description: (LTC2246 - LTC2248) 65/40/25Msps Low Power 3V ADCs
• Manufacturer: Linear Technology

www.datasheet4u.com
LTC2248/LTC2247/LTC2246
APPLICATIO S I FOR ATIO
16
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.
ANALOG
ANALOG
ANALOG
INPUT
INPUT
INPUT
Input Frequencies Between 170MHz and 300MHz
Input Frequencies Between 70MHz and 170MHz
Figure 6. Recommended Front End Circuit for
Figure 7. Recommended Front End Circuit for
Figure 8. Recommended Front End Circuit for
0.1µF
0.1µF
0.1µF
0.1µF
0.1µF
0.1µF
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS, INDUCTORS
ARE 0402 PACKAGE SIZE
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
Input Frequencies Above 300MHz
T1
T1
T1
T1 = MA/COM, ETC 1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
U
25Ω
25Ω
25Ω
25Ω
25Ω
25Ω
U
6.8nH
12Ω
0.1µF
0.1µF
0.1µF
12Ω
6.8nH
W
2.2µF
2.2µF
8pF
2.2µF
V
A
A
V
V
A
A
A
A
CM
IN
IN
CM
CM
IN
IN
IN
IN
+
–
+
–
+
–
LTC2248/47/46
LTC2248/47/46
LTC2248/47/46
U
224876 F08
224876 F06
224876 F07
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
the 2V range; tying the SENSE pin to V
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
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
TIE TO V
TIE TO V
0.5V < V
RANGE = 2 • V
CM
DD
SENSE
FOR 2V RANGE;
FOR 1V RANGE;
2.2µF
1µF
1µF
1.5V
< V
SENSE
Figure 9. Equivalent Reference Circuit
CM
•
FOR
1.1
1.5
2.2µF
SENSE
0.1µF
REFH
REFL
V
CM
LTC2248/47/46
CONTROL
DETECT
4Ω
RANGE
AND
CM
. This provides a high
1.5V BANDGAP
DIFF AMP
REFERENCE
INTERNAL ADC
HIGH REFERENCE
INTERNAL ADC
LOW REFERENCE
CM
1V
selects the 1V
BUFFER
0.5V
DD
selects
224876fa
224876 F09