December 2009 - Using a Differential I/O Amplifier in Single-Ended Applications

L DESIGN IDEAS
Using a Differential I/O Amplifier
in Single-Ended Applications
by Glen Brisebois
Introduction
Recent advances in low voltage silicon
germanium and BiCMOS processes
have allowed the design and production of very high speed amplifiers.
Because the processes are low voltage,
most of the amplifier designs have
incorporated differential inputs and
outputs to regain and maximize total
output signal swing. Since many low
voltage applications are single-ended,
the questions arise, “How can I use a
differential I/O amplifier in a singleended application?” and “What are
the implications of such use?” This
article addresses some of the practical
implications and demonstrates specific single-ended applications using
the 3GHz gain-bandwidth LTC6406
differential I/O amplifier.
Background
A conventional op amp has two differential inputs and an output. The
gain is nominally infinite, but control is
maintained by virtue of feedback from
the output to the negative “inverting”
input. The output does not go to infinity, but rather the differential input is
kept to zero (divided by infinity, as it
were). The utility, variety and beauty
of conventional op amp applications
are well documented, yet still appear
inexhaustible. Fully differential op
amps have been less well explored.
Figure 1 shows a differential op
amp with four feedback resistors. In
this case the differential gain is still
nominally infinite, and the inputs
kept together by feedback, but this
is not adequate to dictate the output voltages. The reason is that the
common mode output voltage can be
anywhere and still result in a “zero”
differential input voltage because the
feedback is symmetric. Therefore, for
any fully differential I/O amplifier,
there is always another control voltage
to dictate the output common mode
voltage. This is the purpose of the
VOCM pin, and explains why fully dif32
RI2
RF2
VOUT–
+
LTC6406
VIN
–
RI1
VOCM
VOUT+
RF1
0.1µF
Figure 1. Fully differential I/O amplifier
showing two outputs and an additional
VOCM pin
ferential amplifiers are at least 5-pin
devices (not including supply pins)
rather than 4-pin devices. The differential gain equation is VOUT(DM) =
VIN(DM) • R2/R1. The common mode
output voltage is forced internally to
the voltage applied at VOCM. One final
observation is that there is no longer a
single inverting input: both inputs are
inverting and noninverting depending
on which output is considered. For
the purposes of circuit analysis, the
inputs are labeled with “+” and “–”
in the conventional manner and one
output receives a dot, denoting it as
the inverted output for the “+” input.
Anybody familiar with conventional
op amps knows that noninverting applications have inherently high input
impedance at the noninverting input,
approaching GΩ or even TΩ. But in the
case of the fully differential op amp
in Figure 1, there is feedback to both
inputs, so there is no high impedance
node. Fortunately this difficulty can
be overcome.
Simple Single-Ended
Connection of a Fully
Differential Op amp
Figure 2 shows the LTC6406 connected as a single-ended op amp. Only
one of the outputs has been fed back
and only one of the inputs receives
feedback. The other input is now high
impedance.
The LTC6406 works fine in this
circuit and still provides a differential
output. However, a simple thought
experiment reveals one of the downsides of this configuration. Imagine
that all of the inputs and outputs are
sitting at 1.2V, including VOCM. Now
imagine that the VOCM pin is driven
an additional 0.1V higher. The only
output that can move is VOUT – because VOUT + must remain equal to
VIN, so in order to move the common
mode output higher by 100mV the
amplifier has to move the VOUT – output a total of 200mV higher. That’s a
200mV differential output shift due to
continued on page 37
0.2pF
VIN
VOUT–
+
LTC6406
–
VOCM
0.1µF
Figure 2. Feedback is single-ended only. This
circuit is stable, with a Hi-Z input like the
conventional op amp. the closed loop output
(VOUT+ in this case) is low noise. Output is
best taken single-ended from the closed loop
output, Providing a 3dB bandwidth Of 1.2GHz.
The Open Loop Output (VOUT–) has a noise
gain of two from VOCM, but is well behaved to
about 300MHz, above which it has significant
passband ripple.
20k 1%
3V
VOUT+
NXP
BF862
OSRAM
SFH213
715
VOCM
3V
VOUT–
+
LTC6406
VOUT+
–
3V
0.1µF
10k
0.1µF
Figure 3. Transimpedance amplifier. Ultralow
noise JFET buffers the current noise of the
bipolar LTC6406 input trim the pot for 0V
differential output under no-light conditions.
Linear Technology Magazine • December 2009
DESIGN IDEAS L
Signal Chain Topology
Figure 1 details a signal chain optimized for a 70MHz center frequency
and a 20MHz bandwidth driving the
LTC2274. The final filter and circuitry
around the ADC are shown in detail.
The earlier stages of the chain can be
changed to suit a target application.
The first stage of amplification
in the chain uses an AH31 from
TriQuint Semiconductor. This GaAs
FET amplifier offers a low noise figure
and high IP3 point, which minimizes
distortion caused by the amplifier
stage. It provides 14dB of gain over
a wide frequency region. The high
IP3 prevents intermodulation distortion between frequencies outside the
passband of the surface acoustic wave
(SAW) filter.
A SAW filter follows the amplification stage for band selection. The SAW
filter offers excellent selectivity and a
flat passband if matched correctly.
Gain before the SAW must not be
–50
–60
AMPLITUDE (dB)
required to transmit output data. The
LTC2274 has 77dB of SNR, and 100dB
of spurious free dynamic range.
–70
–80
–90
–100
–110
–120
0
4
8 12 16 20 24 28 32 36 40 44 48 52
FREQUENCY (MHz)
Figure 2. Typical spread spectrum performance
higher than the maximum input power
rating of the SAW; otherwise it leads
to distortion. A digitally controlled
step attenuator may be required in
the signal path to control the power
going into the SAW filter.
The second stage of amplification is
used to recover the loss in the SAW filter. The insertion loss of the SAW filter
is about –15dB, so the final amplifier
should have at least this much gain,
plus enough gain to accommodate
the final filter. By splitting the gain
between two amplifiers, the noise and
distortion can be optimized without
overdriving the SAW filter. It also allows
for a final filter with better suppres-
sion of noise from the final amplifier,
improving SNR and selectivity.
The output stage of the final filter
needs to be absorptive to accommodate
the ADC front end. This suppresses
glitches reflected back from the direct
sampling process.
This signal chain will not degrade
the performance of the LTC2274.
When receiving a 4-channel WCDMA
signal with a 20MHz bandwidth, centered at 70MHz, the ACPR is 71.5dB
(see Figure 2).
Conclusion
The LTC2274 can be used to receive
high IF frequencies, but getting the
most out of this high performance
ADC requires a carefully designed
analog front end. The performance of
the LTC2274 is such that it is possible
to dispense with the automatic gain
control and build a receiver with a
low fixed gain. The LTC2274 is a part
of a family of 16-bit converters that
range in sample rate from 65Msps to
105Msps. For complete schematics
of this receiver network, visit www.
linear.com. L
LTC6406, continued from page 32
a 100mV VOCM shift. This illustrates
the fact that single-ended feedback
around a fully differential amplifier
introduces a noise gain of two from
the VOCM pin to the “open” output.
In order to avoid this noise, simply
do not use that output, resulting in a
fully single-ended application. Or, you
can take the slight noise penalty and
use both outputs.
A Single-Ended
Transimpedance Amplifier
Figure 3 shows the LTC6406 connected as a single-ended transimpedance
amplifier with 20kΩ of transimpedance
gain. The BF862 JFET buffers the
LTC6406 input, drastically reducing
the effects of its bipolar input transistor current noise. The VGS of the JFET
is now included as an offset, but this is
typically 0.6V so the circuit still functions well on a 3V single supply and
Linear Technology Magazine • December 2009
Figure 4. Time domain response of circuit of Figure 3, showing both outputs each with 20kΩ of
TIA gain. Rise time is 16ns, indicating a 20MHz bandwidth.
the offset can be dialed out with the
10k potentiometer. The time domain
response is shown in Figure 4. Total
output noise on 20MHz bandwidth
measurements shows 0.8mVRMS on
VOUT + and 1.1mVRMS on VOUT –. Taken
differentially, the transimpedance gain
is 40kΩ.
Conclusion
New families of fully differential op
amps like the LTC6406 offer unprecedented bandwidths. Fortunately, these
op amps can also function well in
single-ended and 100% feedback applications. L
37
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