Datasheet

LTC6362
11
6362fa
APPLICATIONS INFORMATION
Input Common Mode Voltage Range
The LTC6362’s input common mode voltage (V
ICM
) is
defined as the average of the two input pins, V
+IN
and
V
–IN
. The inputs of the LTC6362 are capable of swinging
rail-to-rail and as such the valid range that can be used for
V
ICM
is V
–
to V
+
. However, due to external resistive divider
action of the gain and feedback resistors, the effective
range of signals 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 compo-
nent to the input common mode voltage. Applying only
V
INP
(setting V
INM
to zero), the input common 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
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.
current follows ∆I
B
/∆V
ICM
= 75nA/V, with I
B
at V
ICM
= 2.5V
typically below 75nA on a 5V supply. For common mode
voltages ranging from 1.1V below the positive supply to
0.2V below the positive supply, input bias current follows
∆I
B
/∆V
ICM
= 25nA/V, with I
B
at V
ICM
= 4.5V typically below
75nA on a 5V supply. Operating within these ranges allows
the amplifier to be used in applications with high source
resistances where errors due to voltage drops must be
minimized. For applications where V
ICM
is within 0.2V of
either rail, input bias current may reach values over 1µA.
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, a
termination resistor R1 should be chosen (see Figure2)
such that:
R1=
R
INM
•R
S
R
INM
–R
S
According to Figure 2, the input impedance looking into
the differential amp (R
INM
) reflects the single-ended source
case, given above. Also, R2 is chosen as:
R2=R1||R
S
=
R1•R
S
R1+R
S
Figure 1. Definitions and Terminology
–
+
R
F
V
–OUT
V
+OUT
V
OCM
V
INP
V
INM
V
OCM
6362 F01
R
F
R
I
R
I
V
–IN
V
+IN
+
–
+
–
V
CM
+
–
Input Bias Current
Input bias current varies according to V
ICM
. For common
mode voltages ranging from 0.2V above the negative
supply to 1.1V below the positive supply, input bias