LTC6404
23
6404f
In single supply applications, where the LTC6404 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 the input
impedance presented by the V
OCM
as listed in the Electri-
cal 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 larger 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 LTC6404 is often desired to
provide either anti-aliasing or improved signal to noise
ratio. To simplify this fi ltering, the LTC6404 includes an
additional pair of differential outputs (OUTF
+
and OUTF
–
)
which incorporate an internal lowpass fi lter network with
a –3dB bandwidth of 88.5MHz (Figure 6).
These pins each have a DC output impedance of 50Ω. In-
ternal capacitances are 12pF to V
–
on each fi ltered output,
plus an additional 12pF capacitor connected differentially
between the two fi ltered outputs. This resistor/capacitor
combination creates fi ltered outputs that look like a series
50Ω resistor with a 36pF capacitor shunting each fi ltered
output to AC ground, providing a –3dB bandwidth of
APPLICATIONS INFORMATION
88.5MHz, and a noise bandwidth of 139MHz. The fi lter
cutoff frequency is easily modifi ed with just a few external
components. To increase the cutoff frequency, simply add 2
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 resistor, 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 36pF 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 unfi ltered outputs of the LTC6404 are designed to
drive 10pF to ground or 5pF differentially, so care should
be taken to not lower the effective impedance between
OUT
+
and OUTF
+
or OUT
–
and OUTF
–
below 25Ω.
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
driven differentially it will appear at each fi ltered output
as a single-ended capacitance of twice the value. To halve
the fi lter bandwidth, for example, two 36pF capacitors
could be added (one from each fi ltered output to ground).
Alternatively, one 18pF capacitor could be added between
the fi ltered outputs, again halving the fi lter bandwidth.
Combinations of capacitors could be used as well; a three
Figure 7. LTC6404 Filter Topology Modifi ed for 2x Filter
Bandwidth (2 External Resistors)
Figure 6. LTC6404 Internal Filter Topology
–
+
7
OUT
+
8
OUTF
+
14
OUT
–
13
OUTF
–
12
V
–
9
V
–
V
–
V
–
6404 F06
LTC6404
FILTERED OUTPUT
(88.5MHz)
50
12pF
12pF
12pF
50
–
+
7
OUT
+
8
OUTF
+
14
OUT
–
13
OUTF
–
12
V
–
9
V
–
V
–
V
–
6404 F07
LTC6404
FILTERED OUTPUT
(176MHz)
50
49.9
12pF
12pF
12pF
50
49.9
