LTC6405CUD Datasheet, PDF (18/26 Pages) Linear Technology – 2.7GHz, 5V, Low Noise,Rail-to-Rail Input Differential Amplifier/Driver

LTC6405
Applications Information
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
+
2
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.
LTC6405
+
–
14
13
–OUT –OUTF
–OUTF
1.25pF _V–
12
50Ω
V^–
1.25pF
50Ω
1.25pF ^V– _V^–
9
FILTERED OUTPUT
+OUT +OUTF
7
8
6405 F07
+OUTF
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-
LTC6405
+
–
49.9Ω
– OUTF
14
13
–OUT –OUTF
1.25pF _V–
12
50Ω
V^–
1.25pF
50Ω
1.25pF ^V– _V^–
9
FILTERED OUTPUT
(1.7GHz)
7 ^+OUT
8
+OUTF
6405 F08
49.9Ω
+OUTF
18
Figure 7. LTC6405 Internal Filter Topology
Figure 8. LTC6405 Filter Topology Modified for 2x Filter
Bandwidth (Two External Resistors)
6405fb
For more information www.linear.com/6405