lt1993-2 Linear Technology Corporation, lt1993-2 Datasheet - Page 14

• Manufacturer Part Number: lt1993-2
• Description: 800mhz Low Distortion, Low Noise Differential Ampli? Er/ Adc Driver Av = 2v/v
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT1993-2
Circuit Description
The LT1993-2 is a low-noise, low-distortion differential
amplifi er/ADC driver with:
• DC to 800MHz –3dB bandwidth
• Fixed gain of 2V/V (6dB) independent of R
• 200Ω differential input impedance
• Low output impedance
• Built-in, user adjustable output fi ltering
• Requires minimal support circuitry
Referring to the block diagram, the LT1993-2 uses a closed-
loop topology which incorporates 3 internal amplifi ers.
Two of the amplifi ers (A and B) are identical and drive
the differential outputs. The third amplifi er (C) is used
to set the output common mode voltage. Gain and input
impedance are set by the 200Ω 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 LT1993-2 also includes built-in single-pole output
fi ltering. The user has the choice of using the unfi ltered
outputs, the fi ltered outputs (175MHz –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 LT1993-2 has been designed to minimize the need
for external support components such as transformers or
AC-coupling capacitors. As an ADC driver, the LT1993-2
requires no external components except for power-supply
bypass capacitors. This allows DC-coupled operation for
applications that have frequency ranges including DC.
At the outputs, the common mode voltage is set via the
V
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.
14
OCM
pin, allowing the LT1993-2 to drive ADCs directly. No
U
U
W
LOAD
U
Input Impedance and Matching Networks
Because of the internal feedback network, calculation of
the LT1993-2’s input impedance is not straightforward
from examination of the block diagram. Furthermore, the
input impedance when driven differentially is different than
when driven single-ended. When driven differentially, the
LT1993-2’s input impedance is 200Ω (differential); when
driven single-ended, the input impedance is 133Ω.
For single-ended 50Ω applications, an 80.6Ω shunt
matching resistor to ground will result in the proper input
termination (Figure 1). For differential inputs there are
several termination options. If the input source is 50Ω
differential, then input matching can be accomplished by
either a 67Ω shunt resistor across the inputs (Figure 3),
or a 33Ω shunt resistor on each of the inputs to ground
(Figure 2). If additional AC gain is desired, a 1:4 impedance
ratio transformer (like the Mini-Circuits TCM4-19) can also
be used to better match impedances and to provide an ad-
ditional 6dB of gain (Figure 4). With a 1:4 impedance ratio
transformer, ideal matching impedance at the transformer
output is 200Ω, so no termination resistors are required
to match the LT1993-2’s 200Ω input impedance.
Figure 2. Input Termination for Differential 50Ω Input Impedance
Figure 1. Input Termination for Single-Ended 50Ω
Input Impedance
IF IN
IF IN
IF IN
Z
SINGLE-ENDED
Z
DIFFERENTIAL
IN
IN
–
+
=
= 50Ω
50Ω
80.6Ω
0.1µF
33Ω
33Ω
13
14
15
16
13
14
15
16
–INB
–INA
+INB
+INA
–INB
–INA
+INB
+INA
LT1993-2
LT1993-2
–OUT
+OUT
–OUT
+OUT
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5
8
5
19932 F01
19932 F02
19932fa