LT6402-6 Datasheet, PDF (11/16 Pages) Linear Technology – 300MHz Low Distortion, Low Noise Differential Amplifi er/ ADC Driver (AV = 6dB)

LT6402-6
APPLICATIONS INFORMATION
The LT6402-6 has been designed to minimize the need
for external support components such as transformers or
AC-coupling capacitors. As an ADC driver, the LT6402-6
requires no external components except for power-supply
bypass capacitors. This allows DC-coupled operation for
applications that have frequency ranges including DC.
At the outputs, the common mode voltage is set via the
V_OCM pin, allowing the LT6402-6 to drive ADCs directly. No
output AC-coupling capacitors or transformers are needed.
At the inputs, signals can be differential or single-ended
with virtually no difference in performance. Furthermore,
DC levels at the inputs can be set independently of the
output common mode voltage. These input characteristics
often eliminate the need for an input transformer and/or
AC-coupling capacitors.
Input Impedance and Matching Networks
Calculation of the input impedance of the LT6402-6 is not
straightforward from examination of the block diagram
because of the internal feedback network. In addition, the
input impedance when driven differentially is different than
when driven single-ended.
LT6402-6
DIFFERENTIAL
200Ω
SINGLE-ENDED
133Ω
For single-ended 50Ω applications, an 80.6Ω shunt
matching resistor to ground will result in the proper input
termination (Figure 1). For differential inputs there are
several termination options. If the input source is 50Ω
differential, then the input matching can be accomplished
by either a 67Ω shunt resistor across the inputs (Figure 3),
or equivalent 33Ω shunt resistors on each of the inputs
to ground (Figure 2).
Single-Ended to Differential Operation
The LT6402-6’s performance with single-ended inputs is
comparable to its performance with differential inputs.
This excellent single-ended performance is largely due
to the internal topology of the LT6402-6. Referring to
the block diagram, if the +INA and +INB pins are driven
with a single-ended signal (while –INA and –INB are tied
to AC ground), then the +OUT and –OUT pins are driven
differentially without any voltage swing needed from
amplifier C. Single-ended to differential conversion using
more conventional topologies suffers from performance
limitations due to the common mode amplifier.
Driving ADCs
The LT6402-6 has been specifically designed to interface
directly with high speed Analog to Digital Converters
(ADCs). In general, these ADCs have differential inputs, with
an input impedance of 1kΩ or higher. In addition, there is
generally some form of lowpass or bandpass filtering just
prior to the ADC to limit input noise at the ADC, thereby
improving system signal to noise ratio. Both the unfiltered
and filtered outputs of the LT6402-6 can easily drive the
IF IN^–
Z_IN = 50Ω
33Ω
DIFFERENTIAL
IF IN⁺
33Ω
13 _–INB
14 _–INA
–OUT ⁸
LT6402-6
15 _+INB
16 _+INA
+OUT
5
64026 F02
Figure 2. Input Termination for Differential 50Ω Input Impedance
0.1μF
IF IN
80.6Ω
Z_IN = 50Ω
SINGLE-ENDED
13 _–INB
14 _–INA
–OUT ⁸
LT6402-6
15 _+INB
16 _+INA
+OUT
5
64026 F01
Figure 1. Input Termination for Single-Ended
50Ω Input Impedance
IF IN^–
Z_IN = 50Ω
DIFFERENTIAL
IF IN⁺
13 _–INB
14 _{–INA –OUT} ⁸
67Ω
LT6402-6
15 _{+INB +OUT}
16 _+INA
5
64026 F03
Figure 3. Alternate Input Termination for Differential
50Ω Input Impedance
64026fa
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