Datasheet

13
1994fb
LT1994
Input Common Mode Voltage Range
The LT1994’s input common mode voltage (V
ICM
) is defi ned
as the average of the two input voltages, V
IN
+
, and V
IN
–
.
It extends from V
–
to approximately 1.25V below V
+
. The
input common mode range depends on the circuit con-
fi guration (gain), V
OCM
and V
CM
(refer to Figure 4). For
fully differential input applications, where V
INP
= –V
INM
,
the common mode input is approximately:
V
VV
V
R
RR
V
R
RR
ICM
IN IN
OCM
I
IF
CM
F
FI
=
+
≈
+
⎛
⎝
⎜
⎞
⎠
⎟
+
+
⎛
⎝
⎜
⎞
⎠
⎟
+ –
•
•
2
With singled-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 voltage is
approximately:
V
VV
V
R
RR
V
R
RR
VR
RR
ICM
IN IN
OCM
I
IF
CM
F
FI
INP F
FI
=
+
≈
+
⎛
⎝
⎜
⎞
⎠
⎟
+
+
⎛
⎝
⎜
⎞
⎠
⎟
+
+
⎛
⎝
⎜
⎞
⎠
⎟
+ –
•
••
2
2
Output Common Mode Voltage Range
The output common mode voltage is defi ned as the aver-
age of the two outputs:
VV
VV
OUTCM OCM
OUT OUT
==
+
+ –
2
The V
OCM
sets this average by an internal common mode
feedback loop which internally forces V
OUT
+
= –V
OUT
–
. The
output common mode range extends from approximately
1.1V above V
–
to approximately 0.8V below V
+
. The V
OCM
pin sits in the middle of an 80kΩ to 80kΩ voltage divider
that sets the default mid-supply open-circuit potential.
In single-supply applications, where the LT1994 is used
to interface to an ADC, the optimal common mode input
range to the ADC is often determined by the ADC’s refer-
ence. If the ADC makes a reference available for setting
the input common mode voltage, it can be directly tied
to the V
OCM
pin, but must be capable of driving a 40k
equivalent resistance that is tied to a mid-supply potential.
If an external reference drives the V
OCM
pin, it should still
be bypassed with a high quality 0.1F capacitor to a low
impedance ground plane to fi lter any thermal noise and
to prevent common mode signals on this pin from being
inadvertently converted to differential signals.
Noise Considerations
The LT1994’s input referred voltage noise is on the order
of 3nV/√Hz. Its input referred current noise is on the
order of 2.5pA/√Hz. In addition to the noise generated
by the amplifi er, the surrounding feedback resistors also
contribute noise. The output noise generated by both the
amplifi er and the feedback components is given by the
equation:
e
e
R
R
IR
e
R
R
e
no
ni
F
I
nF
nRI
F
I
nRF
=
+
⎡
⎣
⎢
⎤
⎦
⎥
⎛
⎝
⎜
⎞
⎠
⎟
+
()
+
⎡
⎣
⎢
⎤
⎦
⎥
⎛
⎝
⎜
⎞
⎠
⎟
+
•••
•• •
12
22
2
2
2
2
Figure 4. Circuit for Common Mode Range
V
CM
1994 F04
R
I
R
F
R
L
V
S
V
SHDNB
V
INM
V
INP
+
–
–
+
3
7
6
8
1
2
5
4
LT1994V
OCM
V
OCM
V
OUT
+
V
IN
–
V
OUT
–
V
IN
+
0.1µF
R
I
R
F
R
L
SHDN
APPLICATIONS INFORMATION