LTC6400-20
11
640020f
APPLICATIONS INFORMATION
Circuit Operation
The LTC6400 is a low noise and low distortion fully dif-
ferential op amp/ADC driver with:
• Operation from DC to 1.8GHz (–3dB bandwidth)
• Fixed gain of 10V/V (20dB)
• Differential input impedance 200Ω
• Differential output impedance 25Ω
• On-Chip 590MHz output fi lter
The LTC6400 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. They also provide a possible
external fi ltering option, which is often desirable when the
load is an ADC.
Filter resistors of 50Ω are available for additional fi ltering.
Lowpass/bandpass fi lters are easily implemented with just
a couple of external components. Moreover, they offer
single-ended 50Ω matching in wideband applications and
no external resistor is needed.
The LTC6400-20 is very fl exible in terms of I/O coupling.
It can be AC- or DC-coupled at the inputs, the outputs or
both. Due to the internal connection between input and
output, users are advised to keep input common mode
voltage between 1V and 1.6V for proper operation. 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 LTC6400-20 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 LTC6400-20 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 imped-
ance 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 refl ection due to input mismatch. Alternatively, one
could apply a narrowband impedance match at the inputs
of the LTC6400-20 for frequency selection and/or noise
reduction.
Referring to Figure 3, LTC6400-20 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
matching network while the other input is connected to
the same matching network and a source resistor. Because
the return ratios of the two feedback paths are equal, the
two outputs have the same gain and thus symmetrical
Figure 1. Input Termination for Differential 50Ω Input Impedance
Using Shunt Resistor
Figure 2. Input Termination for Differential 50Ω Input Impedance
Using a 1:4 Balun
640020 F01
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
100Ω
66.5Ω
12.5Ω
1000Ω
LTC6400-20
100Ω
25Ω
25Ω
V
IN
1000Ω
12.5Ω
50Ω
50Ω
1.7pF
13
14
15
16
7
5
6
8
+
–
640020 F02
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
100Ω 12.5Ω
1000Ω
LTC6400-20
100Ω
25Ω
25Ω
V
IN
1000Ω
12.5Ω
50Ω
50Ω
1.7pF
13
14
15
16
7
5
6
8
+
–
1:4
••
