Datasheet

LTC6403-1
14
64031fa
APPLICATIONS INFORMATION
Input Common Mode Voltage Range
The LTC6403-1’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 1.4V below V
+
.
For fully differential input applications, where V
INP
= –V
INM
,
the input common mode voltage is approximately (Refer
to Figure 5):
V
VV
V
R
RR
V
R
R
ICM
IN IN
VOCM
I
IF
CM
F
=
+
≈
+
⎛
⎝
⎜
⎞
⎠
⎟
+
+ –
•
•
2
FFI
R+
⎛
⎝
⎜
⎞
⎠
⎟
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
R
ICM
IN IN
VOCM
I
IF
CM
F
=
+
≈
+
⎛
⎝
⎜
⎞
⎠
⎟
+
+ –
•
•
2
FFI
INP F
FI
R
VR
RR+
⎛
⎝
⎜
⎞
⎠
⎟
+
+
⎛
⎝
⎜
⎞
⎠
⎟
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 VOCM
OUT OUT
==
+
+ –
2
The V
OCM
pin sets this average by an internal common
mode feedback loop. The output common mode range
extends from 1.1V above V
–
to 1V below V
+
. The V
OCM
pin sits in the middle of an internal voltage divider which
sets the default mid-supply open circuit potential.
In single supply applications, where the LTC6403-1 is
used to interface to an ADC, the optimal common mode
input range to the ADC is often determined by the ADC’s
reference. If the ADC makes a reference available for set-
ting the input common mode voltage, it can be directly
tied to the V
OCM
pin, but must be capable of driving the
input impedance presented by the V
OCM
as listed in the
Electrical Characteristics Table. This impedance can be
assumed to be connected to a mid-supply potential. If an
external reference drives the V
OCM
pin, it should still be
bypassed with a high quality 0.01μF or higher 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.
Output Filter Considerations and Use
Filtering at the output of the LTC6403-1 is often desired
to provide either anti-aliasing or improved signal to noise
ratio. To simplify this fi ltering, the LTC6403-1 includes an
additional pair of differential outputs (+OUTF and –OUTF)
which incorporate an internal lowpass fi lter network with
a –3dB bandwidth of 44.2MHz (Figure 6).
Figure 4. Optimal Compensation for Signal Source Impedance
Figure 5. Circuit for Common Mode Range
V
S
+
–
–
+
R
F
R
F
R
I
R
INM
V
OCM
R
S
R
I
R2 = R
S
|| R1
R1 CHOSEN SO THAT R1 || R
INM
= R
S
R2 CHOSEN TO BALANCE R
S
|| R1
R1
64031 F04
V
–
V
–
V
+
0.1μF
0.1μF
0.1μF
0.1μF
0.1μF
V
CM
–
+
1
SHDN
5 6
–IN
7
+OUT
8
+OUTF
16 15
+IN
NC
NC
14
–OUT
13
–OUTF
V
–OUTF
R
F
V
+OUTF
V
–OUT
V
+OUT
2
V
+
3
V
–
V
+
V
+
V
–
V
+
V
–
4
V
OCM
V
SHDN
V
VOCM
V
OCM
12
V
–
11
V
+
10
V
+
9
V
–
V
–
V
–
64031 F05
LTC6403-1
SHDN
0.1μF
0.01μF
R
F
R
I
R
I
+
–
V
INP
–
+
V
INM
V
–IN
V
+IN