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LT6411CUD#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LT6411
10
6411f
APPLICATIONS INFORMATION
Input Considerations
The LT6411 input voltage range is from V
EE
+ 1V to
V
CC
– 1V. Therefore, on split supplies the LT6411 input
range is always as large as or larger than the output swing.
On a single positive supply with a gain of +2 and IN
–
con-
nected to ground, however, the input range limit of +1V
limits the linear output low swing to 2V (1V multiplied by
the internal gain of 2).
The inputs can be driven beyond the point at which the
output clips so long as input currents are limited to
±10mA. Continuing to drive the input beyond the output
limit can result in increased current drive and slightly
increased swing, but will also increase supply current
and may result in delays in transient response at larger
levels of overdrive.
DC Biasing Differential Amplifi er Applications
The inputs of the LT6411 must be DC biased within the
input common mode voltage range, typically V
EE
+ 1V to
V
CC
– 1V. If the inputs are AC coupled or DC biased be-
yond the input voltage range of a driven A-to-D converter,
DC biasing or level shifting will be required. In the basic
circuit confi gurations shown in Figure 1, the DC input
common mode voltage and the differential input signal
are both multiplied by the amplifi er gain. In the gain of
+2 confi guration, the DC common mode voltage gain can
be set to unity by adding a capacitor at the IN
–
pins as
shown in Figure 2.
If the inputs are AC coupled or the LT6411 is preceded
by a highpass fi lter, the input common mode voltage can
be set by resistor dividers as shown in Figure 3. Adding
the blocking capacitor to the gain setting resistors sets
the input and output DC common mode voltages equal.
When using the LT6411 to drive an A-to-D converter, the
DC common mode voltage level will affect the harmonic
distortion of the combined amplifi er/ADC system. Figure 4
shows the measured distortion of an LTC2249 ADC when
driven by the LT6411 at different common mode voltage
levels with the inputs confi gured as shown in Figure 3.
Adjusting the DC bias voltage can optimize the design for
the lowest possible distortion.
If the input signals are within the input voltage range
and output swing of the LT6411, but outside the input
range of an ADC or other circuit the LT6411 is driving,
+
–
+
–
+V
LT6411
IN+
IN–
C
LARGE
V
DC
OUT+
OUT–
V
DC
V
DC
V
DC
6411 F02
Figure 2. LT6411 Confi gured with a Differential Gain of 2
and Unity DC Common Mode Gain
+
–
+
–
+V
LT6411
IN+
IN–
OV
OV
OUT+
OUT–
V
DC
V
DC
V
DC
C
LARGE
C
LARGE
V
+
R1
R2
V
+
R1
R2
V
DC
6411 F03
Figure 3. Using Resistor Dividers to Set the
Input Common Mode Voltage When AC Coupling
V
CM
(V)
1.6
DISTORTION (dBc)
–70
–65
HD3
HD2
IM3
–60
2.4
6411 F04
–75
–80
–90
1.8
2.0
2.2
1.7 2.5
1.9
2.1
2.3
–85
–50
–55
V
CC
= 5V, V
EE
= 0V
A
V
= 2
T
A
= 25°C
Figure 4. Harmonic and Intermodulation Distortion of the
LT6411 Driving an LTC2249 Versus DC Common Mode
Voltage. Harmonic Distortion Measured with a –1dBFS Signal
at 30.2MHz. Intermodulation Distortion Measured with Two
–7dBFS Tones at 30.2MHz and 29.2MHz
LT6411
11
6411f
the output signals can be AC coupled and DC biased in a
manner similar to what is shown at the inputs in Figure
3. A simpler alternative when using an ADC such as the
LTC2249 is to use the ADC’s V
CM
pin to set the optimal
common mode voltage as shown in Figure 5.
If unity common mode gain and difference mode response
to DC is desired, there is another confi guration available.
Figure 6 shows the LT6411 connected to provide a differ-
ential signal gain of +3 with unity common mode gain. For
differential signal gain between unity and +3, three resistors
can be added to provide attenuation and set the differential
input impedance of the stage as illustrated in Figure 7. The
general expression for the differential gain is:
A
k
k
VDIFF()
•
=+
+
1
2
2
Scaling factor ‘k’ is the multiple between the two equal-
value series input resistors and the resistor connected
between the two positive inputs. The correct value of R for
the external resistors can be computed from the desired
differential input impedance, Z
IN
, as a function of k and
the 370Ω internal gain setting resistors, as described in
the equation:
R
Z
kZk
IN
IN
=
+
()
+
()
•
–
370
370 2 1
Ω
Ω
In Figure 7 k = 2 and R = 13.7Ω, setting the differential
gain to +2 and the differential input impedance to ap-
proximately 50Ω.
APPLICATIONS INFORMATION
+
–
+
–
+V
–V
C
LARGE
10k
LT6411
IN+
IN–
OV
OV
6411 F05
C
LARGE
10k
2.2µF
LTC2249
V
CM
+
–
+
–
+V
LT6411
IN+
IN–
V
CM
OUT+
OUT–
V
CM
V
CM
V
CM
6411 F06
Figure 6. LT6411 Confi gured for a Differential Gain of +3
and Unity Common Mode Gain with Response to DC
Figure 5. Level Shifting the Output Common Mode Voltage of the LT6411 Using the V
CM
Pin of an LTC2249
+
–
+
–
+V
LT6411
IN+
R = 13.7Ω
R = 13.7Ω
IN–
k • R = 27.4Ω
V
CM
OUT+
OUT–
V
CM
V
CM
V
CM
6411 F07
Figure 7. LT6411 Confi gured with a Differential Input Impedance
of 50Ω, a Differential Gain of +2 and Unity Common Mode Gain
LT6411
12
6411f
APPLICATIONS INFORMATION
Layout and Grounding
It is imperative that care is taken in PCB layout in order
to utilize the very high speed and very low crosstalk of
the LT6411. Separate power and ground planes are highly
recommended and trace lengths should be kept as short
as possible. If input or output traces must be run over a
distance of several centimeters, they should use a controlled
impedance with matching series and shunt resistances to
maintain signal fi delity.
Series termination resistors should be placed as close to
the output pins as possible to minimize output capacitance.
See the Typical Performance Characteristics section for
a plot of frequency response with various output capaci-
tors—only 12pF of parasitic output capacitance causes
6dB of peaking in the frequency response!
Low ESL/ESR bypass capacitors should be placed as close
to the positive and negative supply pins as possible. One
4700pF ceramic capacitor is recommended for both V
CC
and V
EE
. Additional 470pF ceramic capacitors with minimal
trace length on each supply pin will further improve AC
and transient response as well as channel isolation. For
high current drive and large-signal transient applications,
additional 1µF to 10µF tantalums should be added on each
supply. The smallest value capacitors should be placed
closest to the LT6411 package.
If the undriven input pins are not connected directly to a low
impedance ground plane, they must be carefully bypassed
to maintain minimal impedance over frequency. Although
crosstalk will be very dependent on the board layout, a
recommended starting point for bypass capacitors would
be 470pF as close as possible to each input pin with one
4700pF capacitor in parallel.
To maintain the LT6411’s channel isolation, it is benefi cial
to shield parallel input and output traces using a ground
plane or power supply traces. Vias between topside
and backside metal may be required to maintain a low
inductance ground near the part where numerous traces
converge.
ESD Protection
The LT6411 has reverse-biased ESD protection diodes
on all pins. If any pins are forced a diode drop above the
positive supply or a diode drop below the negative sup-
ply, large currents may fl ow through these diodes. If the
current is kept below 10mA, no damage to the devices
will occur.
Single-Ended to Differential Converter
Because the gains of each channel of the LT6411 can
be confi gured independently, the LT6411 can be used to
provide a gain of +2 when amplifying differential signals
and when converting single-ended signals to differential.
With both channels connected to a single-ended input,
one channel confi gured with a gain of +1 and the other
confi gured with a gain of –1, the output will be a differential
version of the input with twice the peak-to-peak (differential)
amplitude. Figure 8 shows the proper connections and
Figure 9 displays the resulting performance when driv-
ing an LTC2249. This confi guration can preserve signal
amplitude when converting single ended video signals to
differential signals when driving double terminated cables.
The 10k resistors in Figure 8 set the common mode volt-
age at the output.
Figure 8. Single-Ended to Differential Converter
with Gain of +2 and Common Mode Control
TYPICAL APPLICATIONS
5V
V
CC
V
EE
DGND
EN
LT6411
OUT1
370Ω370Ω
370Ω370Ω
OUT2
6411 F08
OUT
+
OUT
–
IN1
+
IN1
–
INPUT
1µF
IN2
–
IN2
+
10k0.1µF
+
+
–
–
10k
5V
V
CM
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LT6411CUD#TRPBF

Mfr. #:
Buy LT6411CUD#TRPBF
Manufacturer:
Analog Devices Inc.
Description:
High Speed Operational Amplifiers 650MHz Diff ADC Drvr/2x Sel Gain Amp
Lifecycle:
New from this manufacturer.
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