LTC6405
18
6405fb
For more information www.linear.com/6405
Output Common Mode Voltage Range
The output common mode voltage is defined as the aver-
age of the two outputs:
V
OUTCM
= V
OCM
=
V
+OUT
V
–OUT
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 typically 1V below V
+
. The V
OCM
voltage is internally
set by a resistive divider between the supplies, developing
a default voltage potential of 2.5V with a 5V supply.
In single supply applications, where the LTC6405 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 19kΩ 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 filter 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.
applications inForMation
Output Filter Considerations and Use
Filtering at the output of the LTC6405 is often desired to
provide
anti-aliasing or to improve signal to noise ratio.
To simplify this filtering, the LTC6405 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 7).
These pins each have an output resistance of 50Ω (toler-
ance ±12%). Internal capacitances are 1.25pF (tolerance
±15%) to V
–
on each filtered output, plus an additional
1.25pF (tolerance ±15%) capacitor connected between the
two filtered outputs. This resistor/capacitor combination
creates filtered outputs that look like a series 50Ω resistor
with a 3.75pF capacitor shunting each filtered output to
AC ground, providing a –3dB bandwidth of 850MHz, and
a noise bandwidth of 1335MHz. The filter cutoff frequency
is easily modified 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 8). These resistors, in
parallel with the internal 50Ω resistors, lower the overall
resistance and therefore increase filter bandwidth. For
example, to double the filter bandwidth, add two external
50Ω resistors to lower the series filter resistance to 25Ω.
The 3.75pF
of capacitance remains unchanged, so filter
bandwidth doubles. Keep in mind, the series resistance
also serves to decouple the outputs from load capaci-
Figure 7. LTC6405 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
–
6405 F07
LTC6405
FILTERED OUTPUT
Figure 8. LTC6405 Filter Topology Modified for 2x Filter
Bandwidth (Tw o External Resistors)
–
+
7 8
14 13
12
V
–
9
V
–
V
–
V
–
6405 F08
LTC6405
FILTERED OUTPUT
(1.7GHz)
+OUTF
–OUTF
49.9Ω
49.9Ω
1.25pF
1.25pF
50Ω
50Ω
1.25pF
+OUT +OUTF
–OUT –OUTF
