LT6402 Linear Technology Corporation, LT6402 Datasheet - Page 10

• 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
LT6402-20
Circuit Description
The LT6402-20 is a low noise, low distortion differential
amplifi er/ADC driver with:
• –3dB bandwidth
• Fixed gain independent of R
• Differential input impedance
• Low output impedance
• Built-in, user adjustable output fi ltering
• Requires minimal support circuitry
Referring to the block diagram, the LT6402-20 uses a
closed-loop topology which incorporates 3 internal am-
plifi ers. Two of the amplifi ers (A and B) are identical and
drive the differential outputs. The third amplifi er is used
to set the output common mode voltage. Gain and input
impedance are set by the 500Ω and 100Ω resistors in
the internal feedback network. Output impedance is low,
determined by the inherent output impedance of amplifi ers
A and B, and further reduced by internal feedback.
The LT6402-20 also includes built-in single-pole output
fi ltering. The user has the choice of using the unfi ltered
outputs, the fi ltered outputs (75MHz –3dB lowpass), or
modifying the fi ltered outputs to alter frequency response
by adding additional components. Many lowpass and
bandpass fi lters are easily implemented with just one or
two additional components.
The LT6402-20 has been designed to minimize the need
for external support components such as transformers or
AC-coupling capacitors. As an ADC driver, the LT6402-20
requires no external components except for power-supply
bypass capacitors. This allows DC-coupled operation for
applications that have frequency ranges including DC. At
10
DC to 300MHz
10V/V (20dB)
100Ω
U
U
LOAD
W
U
the outputs, the common mode voltage is set via the V
pin, allowing the LT6402-20 to drive ADCs directly. No
output AC-coupling capacitors or transformers are needed.
At the inputs, signals can be differential or single-ended
with virtually no difference in performance. Furthermore,
DC levels at the inputs can be set independently of the
output common mode voltage. These input characteristics
often eliminate the need for an input transformer and/or
AC-coupling capacitors.
Input Impedance and Matching Networks
Calculation of the input impedance of the LT6402-20 is
not straightforward from examination of the block diagram
because of the internal feedback network. In addition, the
input impedance when driven differentially is different than
when driven single-ended.
For single-ended 50Ω applications, a 121Ω shunt matching
resistor to ground will result in the proper input termina-
tion (Figure 1). For differential inputs there are several
termination options. If the input source is 50Ω differential,
then the input matching can be accomplished by either
a 100Ω shunt resistor across the inputs (Figure 3), or
equivalent 49.9Ω shunt resistors on each of the inputs
to ground (Figure 2). If additional AC gain is desired, an
impedance ratio transformer can also be used to better
match impedances.
LT6402-20
Figure 1. Input Termination for Single-Ended 50Ω
Input Impedance
IF IN
Z
SINGLE-ENDED
IN
=
50Ω
121Ω
0.1µF
Differential
100Ω
13
14
15
16
–INB
–INA
+INB
+INA
LT6402-20
–OUT
+OUT
8
5
Single-Ended
85.9Ω
6402 F01
640220fa
OCM