LTC6401-14
9
640114f
APPLICATIONS INFORMATION
Circuit Operation
The LTC6401-14 is a low noise and low distortion fully
differential op amp/ADC driver with:
• Operation from DC to 2GHz –3dB bandwidth
• Fixed gain of 5V/V (14dB)
• Differential input impedance 200Ω
• Differential output impedance 25Ω
• Differential impedance of output fi lter 100Ω
The LTC6401-14 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 200Ω/500Ω 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 LTC6401-14 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 approximately 250mV above V
OCM
and thus no external circuitry is needed for bias. The
LTC6401-14 provides an output common mode voltage
set by V
OCM
, which allows driving ADC directly without
external components such as transformer or AC coupling
capacitors. The input signal can be either single-ended
or differential with only minor difference in distortion
performance.
Input Impedance and Matching
The differential input impedance of the LTC6401-14 is
200Ω. Usually the differential inputs 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 resis-
tor (Figure 1). Another approach is to employ a wideband
transformer (Figure 2). Both methods provide a wideband
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 LTC6401-14 for frequency selection and/or noise
reduction.
Figure 1. Input Termination for Differential 50Ω Input Impedance
Using Shunt Resistor
Figure 2. Input Termination for Differential 50Ω Input Impedance
Using a Balun
640114 F01
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
100Ω
66.5Ω
12.5Ω
500Ω
LTC6401-14
100Ω
25Ω
25Ω
V
IN
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
+
–
640114 F02
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
MINI CIRCUITS
TCM4-19
–IN
100Ω 12.5Ω
500Ω
LTC6401-14
100Ω
25Ω
25Ω
V
IN
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
+
–
1:4
•
•
