Datasheet

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