LT6402 Linear Technology Corporation, LT6402 Datasheet - Page 13

• Manufacturer Part Number: LT6402
• Description: 300mhz Low Distortion, Low Noise Differential Ampli?er/ Adc Driver Av = 20db
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
solution of 14pF from each fi ltered output to ground plus
a 14pF capacitor between the fi ltered outputs would also
halve the fi lter bandwidth (Figure 7).
Bandpass fi ltering is also easily implemented with just a
few external components. An additional 560pF and 62nH,
each added differentially between +OUTFILTERED and
–OUTFILTERED creates a bandpass fi lter with a 26MHz
center frequency, –3dB points of 23MHz and 30MHz, and
1.6dB of insertion loss (Figure 8).
Output Common Mode Adjustment
The LT6402-20’s output common mode voltage is set by
the V
setting the output common mode voltage anywhere in
a range from 1.1V to 3.6V. Bandwidth of the V
typically 200MHz, so for applications where the V
is tied to a DC bias voltage, a 0.1µF capacitor at this pin is
recommended. For best distortion performance, the voltage
at the V
When interfacing with most ADCs, there is generally a
V
of the ADC. For 5V ADCs such as the LTC17XX family, this
V
addition of a 0.1µF capacitor) to the input V
LT6402-20. For 3V ADCs such as the LTC22XX families,
the LT6402-20 will function properly using the 1.65V from
the ADC’s V
Dynamic Range (SFDR) and distortion performance can
be achieved by level-shifting the LTC22XX’s V
voltage up to at least 1.8V. This can be accomplished as
shown in Figure 9 by using a resistor divider between the
LTC22XX’s V
the LT6402-20’s V
common mode voltage above 1.9V, AC coupling capacitors
are recommended between the LT6402-20 and LTC22XX
ADCs because of the input voltage range constraints of
the ADC.
Large Output Voltage Swings
The LT6402-20 has been designed to provide the
3.2V
of 14-bit low-noise ADCs. This additional output swing
improves system SNR by up to 4dB.
OCM
OCM
P-P
OCM
output pin that is at about half of the supply voltage
output pin should be connected directly (with the
OCM
output swing needed by the LTC1748 family
pin. It is a high-impedance input, capable of
CM
pin should be between 1.8V and 2.6V.
CM
reference pin, but improved Spurious Free
output pin and V
OCM
U
pin with a 0.1µF capacitor. For a
U
CC
W
and then bypassing
OCM
CM
OCM
U
pin of the
reference
OCM
pin is
pin
Input Bias Voltage and Bias Current
The input pins of the LT6402-20 are internally biased to
the voltage applied to the V
resistors are needed, even for AC-coupled operation. The
input bias current is determined by the voltage difference
between the input common mode voltage and the V
pin (which sets the output common mode voltage). For
example, if the inputs are tied to 2.5V with the V
at 2.2V, then a total input bias current of 1mA will fl ow
into the LT6402-20’s +INA and +INB pins. Furthermore,
an additional input bias current totaling 1mA will fl ow into
the –INA and –INB inputs.
Application (Demo) Boards
The DC954A Demo Board has been created for stand-alone
evaluation of the LT6402-20 with either single-ended or
differential input and output signals. As shown, it accepts
a single-ended input and produces a single-ended output
so that the LT6402-20 can be evaluated using standard
laboratory test equipment. For more information on this
Demo Board, please refer to the layout and schematic
diagrams found later in this data sheet.
There are also additional demo boards available that
combine the LT6402-20 with a variety of different Linear
Technology ADCs. Please contact the factory for more
information on these demo boards.
IF IN
0.1µF
121Ω
Figure 9. Level Shifting 3V ADC V
Improved SFDR
13
14
15
16
–INB
–INA
+INB
+INA
+OUTFILTERED
–OUTFILTERED
LT6402-20
V
2
OCM
OCM
6
7
0.1µF
pin. No external biasing
10Ω
10Ω
CM
LT6402-20
Voltage for
1
2
AIN
AIN
+
–
4.02k
11k
LTC22xx
31
V
3V
OCM
13
CM
1.5V
1.9V
640220fa
OCM
pin
6402 F9