Datasheet

LTC6420-20
8
642020fb
APPLICATIONS INFORMATION
Figure 1. Input Termination for Differential 50 Input Impedance
Using Shunt Resistor
Figure 2. Input Termination for Differential 50 Input Impedance
Using a 1:4 Balun
Figure 3. Input Termination for Single-Ended 50 Input Impedance
642020 F03
+IN
IN
+
OUT
–
IN
–
OUT
+
–IN
100Ω
R
T
66.5Ω
0.1µF
1000Ω
1/2 LTC6420-20
100Ω
R
S
50Ω
R
S
//R
T
28.7Ω
V
IN
1000Ω
+
–
0.1µF
Circuit Operation
Each of the two channels of the LTC6420-20 is composed
of a fully differential amplifi er with on chip feedback and
output common mode voltage control circuitry. Differential
gain and input impedance are set by 100/1000
resistors in the feedback network. Small output resistors
of 12.5 improve the circuit stability over various load
conditions.
The LTC6420-20 is very fl exible 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 LTC6420-20 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 LTC6420-20 is
200. If a 200 source impedance is unavailable, then
the differential inputs may need to be terminated to a
lower value impedance, e.g. 50, in order to provide an
impedance match for the source. Several choices are avail-
able. 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 refl ection due to input mismatch.
Alternatively, one could apply a narrowband impedance
match at the inputs of the LTC6420-20 for frequency
selection and/or noise reduction.
Referring to Figure 3, LTC6420-20 can be easily confi gured
for single-ended input and differential output without a
balun. The signal is fed to one of the inputs through a
640020 F01
+IN
IN
+
OUT
–
IN
–
OUT
+
–IN
100Ω
66.5Ω
1000Ω
1/2 LTC6420-20
100Ω
25Ω
25Ω
V
IN
1000Ω
+
–
640020 F02
+IN
IN
+
OUT
–
IN
–
OUT
+
–IN
100Ω
1000Ω
1/2 LTC6420-20
100Ω
25Ω
25Ω
V
IN
1000Ω
+
–
1:4
••