Datasheet

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