LT1993-4 Linear Technology, LT1993-4 Datasheet - Page 12

• Manufacturer Part Number: LT1993-4
• Description: 900MHz Low Distortion - Low Noise Differential Amplifier/ADC Driver
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Single-Ended to Differential Operation
The LT1993-4’s performance with single-ended inputs is
comparable to its performance with differential inputs.
This excellent single-ended performance is largely due
to the internal topology of the LT1993-4. Referring to
the block diagram, if the +INA and +INB pins are driven
with a single-ended signal (while –INA and –INB are tied
to AC ground), then the +OUT and –OUT pins are driven
differentially without any voltage swing needed from
amplifi er C. Single-ended to differential conversion using
more conventional topologies suffers from performance
limitations due to the common mode amplifi er.
Driving ADCs
The LT1993-4 has been specifi cally designed to interface
directly with high speed Analog to Digital Converters
(ADCs). In general, these ADCs have differential inputs,
with an input impedance of 1k or higher. In addition, there
is generally some form of lowpass or bandpass fi ltering just
prior to the ADC to limit input noise at the ADC, thereby
improving system signal to noise ratio. Both the unfi ltered
and fi ltered outputs of the LT1993-4 can easily drive the
high impedance inputs of these differential ADCs. If the
fi ltered outputs are used, then cutoff frequency and the
type of fi lter can be tailored for the specifi c application if
needed.
Wideband Applications
(Using the +OUT and –OUT Pins)
In applications where the full bandwidth of the LT1993-4
is desired, the unfi ltered output pins (+OUT and –OUT)
should be used. They have a low output impedance;
therefore, gain is unaffected by output load. Capacitance
in excess of 5pF placed directly on the unfi ltered outputs
results in additional peaking and reduced performance.
When driving an ADC directly, a small series resistance
is recommended between the LT1993-4’s outputs and
the ADC inputs (Figure 4). This resistance helps eliminate
any resonances associated with bond wire inductances of
either the ADC inputs or the LT1993-4’s outputs. A value
between 10Ω and 25Ω gives excellent results.
LT1993-4
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The fi lter cutoff frequency is easily modifi ed with just a
few external components. To increase the cutoff frequency,
simply add 2 equal value resistors, one between +OUT and
+OUTFILTERED and the other between –OUT and –OUTFIL-
TERED (Figure 6). These resistors are in parallel with the
internal 25Ω resistor, lowering the overall resistance and
increasing fi lter bandwidth. To double the fi lter bandwidth,
for example, add two external 25Ω resistors to lower
the series resistance to 12.5Ω. The 36pF of capacitance
remains unchanged, so fi lter bandwidth doubles.
Filtered Applications
(Using the +OUTFILTERED and –OUTFILTERED Pins)
Filtering at the output of the LT1993-4 is often desired to
provide either anti-aliasing or improved signal to noise
ratio. To simplify this fi ltering, the LT1993-4 includes an
additional pair of differential outputs (+OUTFILTERED and
–OUTFILTERED) which incorporate an internal lowpass
fi lter network with a –3dB bandwidth of 175MHz (Figure
5). These pins each have an output impedance of 25Ω. In-
ternal capacitances are 12pF to V
plus an additional 12pF capacitor connected differentially
between the two fi ltered outputs. This resistor/capaci-
tor combination creates fi ltered outputs that look like a
series 25Ω resistor with a 36pF capacitor shunting each
fi ltered output to AC ground, giving a –3dB bandwidth of
175MHz.
Figure 5. LT1993-4 Internal Filter Topology –3dB BW ≈175MHz
Figure 4. Adding Small Series R at LT1993-4 Output
LT1993-4
LT1993-4
–OUT
+OUT
25
25
8
5
12pF
12pF
V
V
EE
EE
12pF
10Ω TO 25Ω
10Ω TO 25Ω
8
7
6
5
–OUT
–OUTFILTERED
+OUTFILTERED
+OUT
EE
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on each fi ltered output,
19934 F05
FILTERED OUTPUT
(175MHz)
ADC
19934 F04
19934fa