LTC6405 Linear Technology, LTC6405 Datasheet - Page 18

• Manufacturer Part Number: LTC6405
• Description: Rail-to-Rail Input Differential Amplifier/Driver
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC6405
Output Common Mode Voltage Range
The output common mode voltage is defi ned as the aver-
age of the two outputs:
The V
mode feedback loop which internally forces V
The output common mode range extends from 0.5V above
V
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
Thevenin equivalent input impedance presented by the
V
The V
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.
18
–
OCM
to typically 1V below V
V
OUTCM
OCM
pin).
OCM
LTC6405
OCM
+
–
Figure 7. LTC6405 Internal Filter Topology
pin sets this average by an internal common
pin should be bypassed with a high quality
= V
pin (as long as it is able to drive the 19kΩ
OCM
14
7
–OUT
+OUT
50Ω
50Ω
=
V
+OUT
13
8
1.25pF
1.25pF
+
–OUTF
+OUTF
. The V
1.25pF
+ V
2
V
V
–
–
6405 F07
–OUT
V
V
OCM
–
–
12
9
–OUTF
+OUTF
voltage is internally
FILTERED OUTPUT
OUTCM
= V
OCM
.
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 fi ltering, 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
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 8). 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.
Figure 8. LTC6405 Filter Topology Modifi ed for 2x Filter
Bandwidth (Two External Resistors)
LTC6405
+
–
–
on each fi ltered output, plus an additional
14
7
49.9Ω
49.9Ω
–OUT
+OUT
50Ω
50Ω
13
8
1.25pF
1.25pF
–OUTF
+OUTF
1.25pF
V
V
–
–
6405 F08
V
V
–
–
12
9
–OUTF
+OUTF
FILTERED OUTPUT
(1.7GHz)
6405fa