LTC2242IUP-12 Linear Technology, LTC2242IUP-12 Datasheet - Page 17

• Manufacturer Part Number: LTC2242IUP-12
• Description: IC,A/D CONVERTER,SINGLE,12-BIT,CMOS,LLCC,64PIN
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
the sample-and-hold charging glitches and limiting the
wideband noise at the converter input. For input frequen-
cies higher than 100MHz, the capacitor may need to be
decreased to prevent excessive signal loss.
The A
package inductance. The two A
should be shorted together.
For input frequencies above 100MHz the input circuits of
Figure 6, 7 and 8 are recommended. The balun transformer
gives better high frequency response than a fl ux coupled
center-tapped transformer. The coupling capacitors allow
the analog inputs to be DC biased at 1.25V. In Figure 8 the
series inductors are impedance matching elements that
maximize the ADC bandwidth.
Reference Operation
Figure 9 shows the LTC2242-12 reference circuitry consist-
ing of a 1.25V bandgap reference, a difference amplifi er
and switching and control circuit. The internal voltage
reference can be confi gured for two pin selectable input
ranges of 2V (±1V differential) or 1V (±0.5V differential).
Tying the SENSE pin to V
the SENSE pin to V
The 1.25V 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 amplifi er to gener-
ate the differential reference levels needed by the internal
ADC circuitry. An external bypass capacitor is required
for the 1.25V reference output, V
frequency low impedance path to ground for internal and
external circuitry.
The difference amplifi er generates the high and low
reference for the ADC. High speed switching circuits are
connected to these outputs and they must be externally
bypassed. Each output has four pins: two each of REFHA
and REFHB for the high reference and two each of REFLA
and REFLB for the low reference. The multiple output pins
are needed to reduce package inductance. Bypass capaci-
tors must be connected as shown in Figure 9.
IN
+
and A
IN
–
inputs each have two pins to reduce
CM
selects the 1V range.
DD
selects the 2V range; typing
IN
CM
+
and the two A
. This provides a high
IN
–
pins
ANALOG
ANALOG
ANALOG
INPUT
INPUT
INPUT
Figure 6. Recommended Front End Circuit for
Input Frequencies Between 100MHz and 250MHz
Figure 7. Recommended Front End Circuit for
Input Frequencies Between 250MHz and 500MHz
Figure 8. Recommended Front End Circuit for
Input Frequencies Above 500MHz
0.1μF
0.1μF
0.1μF
0.1μF
0.1μF
0.1μF
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
T1
T1
T1
25Ω
25Ω
25Ω
25Ω
25Ω
25Ω
2.7nH
0.1μF
0.1μF
10Ω
12Ω
12Ω
0.1μF
10Ω
2.7nH
10Ω
LTC2242-12
2.2μF
8pF
2.2μF
2.2μF
V
A
A
A
A
V
V
A
A
A
A
A
A
A
A
CM
IN
IN
IN
IN
CM
IN
IN
IN
IN
CM
IN
IN
IN
IN
+
+
–
–
+
+
–
–
+
+
–
–
LTC2242-12
LTC2242-12
LTC2242-12
224212 F06
17
224212 F07
224212 F08
224212fb