LTC6420IUDC-20#TRPBF Linear Technology, LTC6420IUDC-20#TRPBF Datasheet - Page 8

• Manufacturer Part Number: LTC6420IUDC-20#TRPBF
• Description: IC DRVR DIFF AMP/ADC DUAL 20QFN
• Manufacturer: Linear Technology
• Type: ADC Driverr

Specifications of LTC6420IUDC-20#TRPBF

• Applications: Data Acquisition
• Mounting Type: Surface Mount
• Package / Case: 20-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
LTC6420-20
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 100Ω/1000Ω
resistors in the feedback network. Small output resistors
of 12.5Ω 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
circuitry is needed for bias. The LTC6420-20 provides an
output common mode voltage set by V
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.
Figure 1. Input Termination for Differential 50Ω Input Impedance
Using Shunt Resistor
8
+
–
V
25Ω
25Ω
IN
66.5Ω
+ IN
– IN
1/2 LTC6420-20
100Ω
100Ω
IN
IN
Figure 3. Input Termination for Single-Ended 50Ω Input Impedance
+
–
OCM
1000Ω
1000Ω
and thus no external
OUT
OUT
OCM
640020 F01
+
–
–
+
R
66.5Ω
28.7Ω
V
50Ω
S
IN
R
//R
, which allows
S
R
T
T
0.1μF
0.1μF
+IN
– IN
1/2 LTC6420-20
100Ω
100Ω
IN
IN
+
–
Input Impedance and Matching
The differential input impedance of the LTC6420-20 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 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
Figure 2. Input Termination for Differential 50Ω Input Impedance
Using a 1:4 Balun
1000Ω
1000Ω
OUT
OUT
642020 F03
–
+
+
–
V
25Ω
25Ω
IN
•
1:4
•
+ IN
– IN
1/2 LTC6420-20
100Ω
100Ω
IN
IN
+
–
1000Ω
1000Ω
OUT
OUT
640020 F02
+
–
642020fb