LTC6409
10
6409fa
applicaTions inForMaTion
that can be processed is even wider. The input common
mode range at the op amp inputs depends on the circuit
configuration (gain), V
OCM
and V
CM
(refer to Figure 1). For
fully differential input applications, where V
INP
= –V
INM
,
the common mode input is approximately:
V
ICM
=
V
+IN
+ V
–IN
2
≈ V
OCM
•
R
I
R
I
+R
F
+ V
CM
•
R
F
R
I
+R
F
With single-ended inputs, there is an input signal com-
ponent to the input common mode voltage. Applying
only V
INP
(setting V
INM
to zero), the input common mode
voltage is approximately:
V
ICM
=
V
+IN
+ V
–IN
2
≈ V
OCM
•
R
I
R
I
+R
F
+
V
CM
•
R
F
R
I
+R
F
+
V
INP
2
•
R
F
R
I
+R
F
(2)
This means that if, for example, the input signal (V
INP
)
is a sine, an attenuated version of that sine signal also
appears at the op amp inputs.
Input Impedance and Loading Effects
The low frequency input impedance looking into the V
INP
or V
INM
input of Figure 1 depends on how the inputs are
driven. For fully differential input sources (V
INP
= –V
INM
),
the input impedance seen at either input is simply:
R
INP
= R
INM
= R
I
For single-ended inputs, because of the signal imbalance
at the input, the input impedance actually increases over
the balanced differential case. The input impedance looking
into either input is:
R
INP
=R
INM
=
R
I
1–
1
2
•
R
F
R
I
+R
F
Input signal sources with non-zero output impedances can
also cause feedback imbalance between the pair of feedback
networks. For the best performance, it is recommended
that the input source output impedance be compensated.
If input impedance matching is required by the source,
Figure 1. Circuit for Common Mode Range
–
+
F
V
–OUT
V
+OUT
V
VOCM
V
OCM
6409 F01
R
F
I
R
I
+
–
V
INP
+
–
V
CM
–
+
V
INM
V
–IN
V
+IN
suited for pre-amplification, level shifting and conversion
of single-ended signals to differential output signals for
driving differential input ADCs.
Output Common Mode and V
OCM
Pin
The output common mode voltage is defined as the aver-
age of the two outputs:
V
OUTCM
= V
OCM
=
V
+OUT
+ V
–OUT
As the equation shows, the output common mode voltage
is independent of the input common mode voltage, and
is instead determined by the voltage on the V
OCM
pin, by
means of an internal common mode feedback loop.
If the V
OCM
pin is left open, an internal resistor divider
develops a default voltage of 1.25V with a 5V supply. The
V
OCM
pin can be overdriven to another voltage if desired.
For example, when driving an ADC, if the ADC makes a
reference available for setting the common mode voltage, it
can be directly tied to the V
OCM
pin, as long as the ADC is
capable of driving the 40k input resistance presented by the
V
OCM
pin. The Electrical Characteristics table specifies the
valid range that can be applied to the V
OCM
pin (V
OUTCMR
).
Input Common Mode Voltage Range
The LTC6409’s input common mode voltage (V
ICM
) is
defined as the average of the two input pins, V
+IN
and
V
–IN
. The valid range that can be used for V
ICM
has been
specified in the Electrical Characteristics table (V
ICMR
).
However, due to external resistive divider action of the
gain and feedback resistors, the effective range of signals
