LTC6421CUDC-20 Datasheet, PDF (8/16 Pages) Linear Integrated Systems – Dual Matched 1.3GHz Differential Amplifi ers/ADC Drivers

LTC6421-20
APPLICATIONS INFORMATION
Circuit Operation
Each of the two channels of the LTC6421-20 is composed
of a fully differential amplifier with on chip feedback and
output common mode voltage control circuitry. Differential
gain and input impedance are set by 100Ω/1000Ω
resistors in the feedback network. Small output resistors
of 12.5Ω improve the circuit stability over various load
conditions.
The LTC6421-20 is very flexible in terms of I/O coupling.
It can be AC- or DC-coupled at the inputs, the outputs or
both. If the inputs are AC-coupled, the input common mode
voltage is automatically biased close to V_OCM and thus
no external circuitry is needed for bias. The LTC6421-20
provides an output common mode voltage set by V_OCM,
which allows driving an ADC directly without external
components such as a transformer or AC coupling capacitors.
The input signal can be either single-ended or differential
with only minor differences in distortion performance.
Input Impedance and Matching
The differential input impedance of the LTC6421-20 is
200Ω. If a 200Ω source impedance is unavailable, then
the differential inputs may need to be terminated to a
lower value impedance, e.g. 50Ω, in order to provide
an impedance match for the source. Several choices
are available. One approach is to use a differential shunt
resistor (Figure 1). Another approach is to employ a wide
band transformer (Figure 2). Both methods provide a
wide band impedance match. The termination resistor or
the transformer must be placed close to the input pins in
order to minimize the reflection due to input mismatch.
Alternatively, one could apply a narrowband impedance
match at the inputs of the LTC6421-20 for frequency
selection and/or noise reduction.
1/2 LTC6421-20
25Ω
100Ω
1000Ω
+IN
–+ ^VIN
66.5Ω
IN ⁺
OUT ^–
IN ^–
OUT ⁺
25Ω
100Ω
–IN
1000Ω
642120 F01
Figure 1. Input Termination for Differential 50Ω Input Impedance
Using Shunt Resistor
1/2 LTC6421-20
25Ω
100Ω
1000Ω
+IN
1:4
–+ ^VIN
••
IN⁺
OUT ^–
IN^–
OUT ⁺
25Ω
100Ω
–IN
1000Ω
642120 F02
Figure 2. Input Termination for Differential 50Ω Input Impedance
Using a 1:4 Balun
642120fb
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