LTC2209 Linear Technology Corporation, LTC2209 Datasheet - Page 20

• Manufacturer Part Number: LTC2209
• Description: 160Msps ADC
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
LTC2209
Figure 4a shows transformer coupling using a transmis-
sion line balun transformer. This type of transformer has
much better high frequency response and balance than
fl ux coupled center tap transformers. Coupling capacitors
are added at the ground and input primary terminals to
allow the secondary terminals to be biased at 1.25V. Figure
4b shows the same circuit with components suitable for
higher input frequencies.
ANALOG
Direct Coupled Circuits
Figure 5 demonstrates the use of a differential amplifi er to
convert a single ended input signal into a differential input
signal. The advantage of this method is that it provides
low frequency input response; however, the limited gain
bandwidth of any op amp or closed-loop amplifi er will de-
grade the ADC SFDR at high input frequencies. Additionally,
wideband op amps or differential amplifi ers tend to have
high noise. As a result, the SNR will be degraded unless
the noise bandwidth is limited prior to the ADC input.
20
ANALOG
INPUT
Figure 4b. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 250MHz to 500MHz
INPUT
Figure 4a. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 100MHz to 250MHz
0.1µF
0.1µF
0.1µF
0.1µF
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2µF
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2µF
1:1
T1
1:1
T1
25Ω
25Ω
25Ω
25Ω
0.1µF
0.1µF
4.7pF
2.2µF
2.2pF
2.2pF
5Ω
5Ω
2.2µF
4.7pF
4.7pF
5Ω
5Ω
V
A
A
V
A
A
CM
IN
IN
CM
IN
IN
+
–
+
–
LTC2209
LTC2209
2209 F04b
2209 F04a
Reference Operation
Figure 6 shows the LTC2209 reference circuitry consisting
of a 2.5V bandgap reference, a programmable gain ampli-
fi er and control circuit. The LTC2209 has three modes of
reference operation: Internal Reference, 1.25V external
reference or 2.5V external reference. To use the internal
reference, tie the SENSE pin to V
reference, simply apply either a 1.25V or 2.5V reference
voltage to the SENSE input pin. Both 1.25V and 2.5V applied
to SENSE will result in a full scale range of 2.25V
= 0). A 1.25V output, V
bias for input drive circuitry. An external bypass capacitor is
required for the V
low impedance path to ground for internal and external
circuitry. This is also the compensation capacitor for the
reference; it will not be stable without this capacitor. The
minimum value required for stability is 2.2µF.
TIE TO V
EXTERNAL 1.25V
EXTERNAL 2.5V
INTERNAL 2.5V
OR INPUT FOR
OR INPUT FOR
ANALOG
REFERENCE
REFERENCE
REFERENCE
Figure 5. DC Coupled Input with Differential Amplifi er
DD
INPUT
TO USE
AMPLIFIER = LTC6600-20,
LTC1993, ETC.
SENSE
2.2µF
V
DIFFERENTIAL
CM
–
+
CM
HIGH SPEED
CM
Figure 6. Reference Circuit
AMPLIFIER
output. This provides a high frequency
+
–
CM
BUFFER
is provided for a common mode
AND GAIN
CONTROL
SELECT
RANGE
DD
2.2µF
12pF
12pF
1.25V
. To use an external
V
A
A
CM
IN
IN
+
–
PGA
LTC2209
REFERENCE
BANDGAP
REFERENCE
P-P
INTERNAL
2.5V
2209 F05
ADC
(PGA
2209 F06
2209f