Datasheet

LT1993-10
11
199310fb
APPLICATIO S I FOR ATIO
WUU
U
Circuit Description
The LT1993-10 is a low noise, low distortion differential
amplifi er/ADC driver with:
• DC to 700MHz –3dB bandwidth
• Fixed gain of 10V/V (20dB) independent of R
LOAD
• 100Ω differential input impedance
• Low output impedance
• Built-in, user adjustable output fi ltering
• Requires minimal support circuitry
Referring to the block diagram, the LT1993-10 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 (C) is
used to set the output common mode voltage. Gain and
input impedance are set by the 100Ω/500Ω 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 LT1993-10 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 LT1993-10 has been designed to minimize the need
for external support components such as transformers or
AC-coupling capacitors. As an ADC driver, the LT1993-10
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
OCM
pin, allowing the LT1993-10 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
Because of the internal feedback network, calculation of
the LT1993-10’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
LT1993-10’s input impedance is 100Ω (differential); when
driven single-ended, the input impedance is 85.9Ω.
For single-ended 50Ω applications, a 121Ω shunt match-
ing 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 100Ω shunt resistor across the inputs (Figure 3),
or a 49.9Ω shunt resistor on each of the inputs to ground
(Figure 2).
199310 F01
IF IN
0.1 F
LT1993-10
–INA
–INB
–OUT
+OUT
8
5
+INB
+INA
14
13
15
121
Z
IN
= 50
SINGLE-ENDED
16
Figure 1. Input Termination for Single-Ended 50Ω
Input Impedance
Figure 2. Input Termination for Differential 50Ω Input Impedance
199310 F02
IF IN
–
IF IN
+
LT1993-10
–INA
–INB
–OUT
+OUT
8
5
+INB
+INA
14
13
15
49.9
Z
IN
= 50
DIFFERENTIAL
16
49.9
Figure 3. Alternate Input Termination for Differential
50Ω Input Impedance
199310 F03
IF IN
–
IF IN
+
LT1993-10
–INA
–INB
–OUT
+OUT
8
5
+INB
+INA
14
13
15
Z
IN
= 50
DIFFERENTIAL
16
100