LT1994 Linear Technology, LT1994 Datasheet - Page 13

• Manufacturer Part Number: LT1994
• Description: Low Noise - Low Distortion Fully Differential Input/Output Amplifier/Driver
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
With singled ended inputs, there is an input signal com-
ponent to the input common mode voltage. Applying only
V
approximately:
Output Common Mode Voltage Range
The output common mode voltage is defi ned as the aver-
age of the two outputs:
The V
feedback loop which internally forces V
output common mode range extends from approximately
1.1V above V
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
INP
V
V
V
ICM
CM
OUTCM
(setting V
V
OCM
CM
•
V
V
=
INM
⎛
⎜
⎝
INP
Figure 4. Circuit for Common Mode Range
sets this average by an internal common mode
R
V
=
IN
F
R
–
+
+
V
F
to approximately 0.8V below V
INM
V
OCM
R
R
R
OCM
+
2
I
I
I
V
⎞
⎟ +
⎠
to zero), the input common voltage is
V
V
IN
U
IN
IN
=
–
–
+
1
2
8
V
V
V
SHDNB
SHDN
7
≈
V
INP
+
R
–
R
OUT
2
OCM
F
F
V
U
OCM
3
6
•
+
V
S
LT1994
⎛
⎜
⎝
+
–
+
2
R
4
5
•
F
V
R
⎛
⎜
⎝
OUT
+
F
R
R
W
I
–
R
I
+
⎞
⎟
⎠
I
OUT
R
V
V
OUT
OUT
1994 F04
F
⎞
⎟ +
⎠
+
0.1µF
+
–
= –V
+
R
. The V
R
U
L
L
OUT
–
. The
OCM
the input common mode voltage, it can be directly tied
to the V
equivalent resistance that is tied to a mid-supply potential.
If an external reference drives the V
be bypassed with a high quality 0.1μF 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:
A plot of this equation and a plot of the noise generated
by the feedback components are shown in Figure 6.
The LT1994’s input referred voltage noise contributes the
equivalent noise of a 560Ω resistor. When the feedback
e
no
=
OCM
⎛
⎜
⎝
2
pin, but must be capable of driving a 40kΩ
e
•
e
e
ni
⎛
⎜
⎝
nRI2
nRI1
e
•
nRI
2
2
⎡
⎢
⎣
Figure 5. Noise Analysis
1
e
+
i
i
ncm
R
R
n–
n+
•
I2
I1
2
2
⎡
⎢
⎣
R
R
2
R
R
F
I
F
I
⎤
⎥
⎦
⎤
⎥
⎦
⎞
⎟ +
⎠
1
2
8
⎞
⎟ +
⎠
e
2
ni
7
2
V
–
+
2
OCM
2
R
R
–V
3
6
2
V
F2
F1
•
S
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•
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LT1994
(
/2
+
–
I R
e
n
OCM
nRF
4
5
•
e
e
F
nRF2
nRF1
2
pin, it should still
1994 F05
)
2
2
2
+
LT1994
e
no
2
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