LTC2217 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2217' PDF : 32 Pages View PDF
LTC2217
APPLICATIONS INFORMATION
Figure 4 shows transformer coupling using a transmis-
sion line balun transformer. This type of transformer has
much better high-frequency response and balance than
flux coupled center-tap transformers. Coupling capacitors
are added at the ground and input primary terminals to
allow the secondary terminals to be biased at 1.575V.
VCM
0.1μF
ANALOG
INPUT
T1
1:1
0.1μF
5Ω
10Ω
25Ω 0.1μF
25Ω 10Ω
2.2μF
5Ω AIN+
4.7pF
LTC2217
4.7pF
5Ω AIN–
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
4.7pF
2217 F04
Figure 4. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 100MHz to 250MHz
Direct Coupled Circuits
Figure 5 demonstrates the use of a differential amplifier 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 amplifier will de-
grade the ADC SFDR at high input frequencies. Additionally,
wideband op amps or differential amplifiers tend to have
high noise. As a result, the SNR will be degraded unless
the noise bandwidth is limited prior to the ADC input.
Reference Operation
Figure 6 shows the LTC2217 reference circuitry consisting
of a 2.5V bandgap reference, a programmable gain ampli-
fier and control circuit. The LTC2217 has three modes of
reference operation: Internal Reference, 1.25V external
reference or 2.5V external reference. To use the internal
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER 25Ω
++
CM
––
25Ω
AMPLIFIER = LTC6600-20,
LTC1993, ETC.
VCM
2.2μF
AIN+
12pF
LTC2217
AIN–
12pF
2217 F05
Figure 5. DC Coupled Input with Differential Amplifier
reference, tie the SENSE pin to VDD. To use an external
reference, simply apply either a 1.25V or 2.5V reference
voltage to the SENSE input pin. Both 1.25V and 2.5V ap-
plied to SENSE will result in a full scale range of 2.75VP-P. A
1.575V output, VCM, is provided for a common mode bias
for input drive circuitry. An external bypass capacitor is
required for the VCM output. This provides a high frequency
low impedance path to ground for internal and external
circuitry. This is also the compensation capacitor for the
reference; which will not be stable without this capacitor.
The minimum value required for stability is 2.2μF.
TIE TO VDD TO USE
INTERNAL 2.5V
REFERENCE
OR INPUT AN
EXTERNAL 2.5V
REFERENCE
OR INPUT AN
EXTERNAL 1.25V
REFERENCE
SENSE
VCM
2.2μF
RANGE
SELECT
AND GAIN
CONTROL
INTERNAL
ADC
REFERENCE
PGA
BUFFER
1.575V
2.5V
BANDGAP
REFERENCE
2217 F06
Figure 6. Reference Circuit
2217f
20
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