LTC6406
17
6406fc
Output Common Mode Voltage Range
The output common mode voltage is defi ned as the aver-
age of the two outputs:
V
OUTCM
= V
OCM
=
V
+OUT
+ V
–OUT
2
The V
OCM
pin sets this average by an internal common
mode feedback loop which internally forces V
OUTCM
=
V
OCM
. The output common mode range extends from 0.5V
above V
–
to 1V below V
+
. The V
OCM
voltage is internally
set by a resistive divider between the supplies, develop-
ing a default voltage potential of 1.25V with a 3V supply.
In single supply applications, where the LTC6406 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 (as long as it is able to drive the 18kΩ
Thevenin equivalent input impedance presented by the
V
OCM
pin).
The V
OCM
pin should be bypassed with a high quality
ceramic bypass capacitor of at least 0.01μF to fi lter any
common mode noise rather than being converted to dif-
ferential noise and to prevent common mode signals on
this pin from being inadvertently converted to differential
signals by impedance mismatches both externally and
internally to the IC.
Output Filter Considerations and Use
Filtering at the output of the LTC6406 is often desired to
provide antialiasing or to improve signal to noise ratio.
To simplify this fi ltering, the LTC6406 in the QFN package
includes an additional pair of differential outputs (+OUTF
and –OUTF) which incorporate an internal lowpass RC
network with a –3dB bandwidth of 850MHz (Figure 6).
These pins each have an output resistance of 50Ω (toler-
ance ±12%). Internal capacitances are 1.25pF (tolerance
±15%) to V
–
on each fi ltered output, plus an additional
APPLICATIONS INFORMATION
1.25pF (tolerance ±15%) capacitor connected between the
two fi ltered outputs. This resistor/capacitor combination
creates fi ltered outputs that look like a series 50Ω resistor
with a 3.75pF capacitor shunting each fi ltered output to
AC ground, providing a –3dB bandwidth of 850MHz, and
a noise bandwidth of 1335MHz. The fi lter cutoff frequency
is easily modifi ed with just a few external components. To
increase the cutoff frequency, simply add two equal value
resistors, one between +OUT and +OUTF and the other
between –OUT and –OUTF (Figure 7). These resistors, in
parallel with the internal 50Ω resistors, lower the overall
resistance and therefore increase fi lter bandwidth. For
example, to double the fi lter bandwidth, add two external
50Ω resistors to lower the series fi lter resistance to 25Ω.
The 3.75pF of capacitance remains unchanged, so fi lter
bandwidth doubles. Keep in mind, the series resistance
also serves to decouple the outputs from load capacitance.
The outputs of the LTC6406 are designed to drive 5pF to
ground, so care should be taken to not lower the effec-
tive impedance between +OUT and +OUTF or –OUT and
–OUTF below 15Ω.
To decrease fi lter bandwidth, add two external capacitors,
one from +OUTF to ground, and the other from –OUTF to
ground. A single differential capacitor connected between
+OUTF and –OUTF can also be used, but since it is being
Figure 6. LTC6406 Internal Filter Topology
–
+
7
+OUT
8
+OUTF
14
–OUT
13
–OUTF
+OUTF
–OUTF
1.25pF
1.25pF
50Ω
50Ω
1.25pF
12
V
–
9
V
–
V
–
V
–
6406 F06
LTC6406
FILTERED OUTPUT
