LTC6400-26
10
640026fa
APPLICATIONS INFORMATION
Circuit Operation
The LTC6400-26 is a low noise and low distortion fully
differential op amp/ADC driver with:
• Operation from DC to 1.9GHz –3dB bandwidth
• Fixed gain of 20V/V (26dB)
• Differential input impedance 50Ω
• Differential output impedance 25Ω
• Differential impedance of output fi lter 100Ω
The LTC6400-26 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 25Ω/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 LTC6400-26 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 LTC6400-26 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 LTC6400-26 is 50Ω.
The interface between the input of LTC6400-26 and 50Ω
source is straightforward. One way is to directly connect
them if the source is differential (Figure 1). Another ap-
proach is to employ a wideband transformer if the source
is single ended (Figure 2). Both methods provide a wide-
band match. 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 LTC6400-26 for
frequency selection and/or noise reduction.
Referring to Figure 3, LTC6400-26 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 swing. In
general, the single-ended input impedance and termination
resistor R
T
are determined by the combination of R
S
, R
G
and R
F
. For example, when R
S
is 50Ω, it is found that the
single-ended input impedance is 75Ω and R
T
is 150Ω in
order to match to a 50Ω source impedance.
Figure 1. Input Termination for Differential 50Ω Input Impedance
Figure 2. Input Termination for Differential 50Ω
Input Impedance Using a Balun
640026 F01
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
25Ω 12.5Ω
500Ω
LTC6400-26
25Ω
25Ω
25Ω
V
IN
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
+
–
640026 F02
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
25Ω 12.5Ω
500Ω
LTC6400-26
25Ω
50Ω
V
IN
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
+
–
1:1
MACOM
MABA-007159-000000
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