AN-417
a
ONE TECHNOLOGY WAY • P.O. BOX 9106
•
NORWOOD, MASSACHUSETTS 02062-9106
APPLICATION NOTE
617/329-4700
•
Fast Rail-to-Rail Operational Amplifiers Ease Design Constraints in Low Voltage
High Speed Systems
by Eamon Nash
Movement towards lower power supply voltages is
driven by the demand that systems consume less and
less power coupled with the desire to reduce the number of power supply voltages in the system. Lowering
power supply voltages and reducing the number of supplies has obvious advantages. One such advantage is to
lower system power consumption. This has the additional benefit of saving space. Lowering overall power
consumption has a residual benefit in that there may no
longer be a need for cooling fans in the system.
However, as the traditional system power supply
voltages of ±15 V and ±12 V give way to lower bipolar
supplies of ±5 V and single supplies of +5 V and +3.3 V, it
is necessary for circuit designers to understand that designing in this new environment is not simply a matter
of finding components that are specified to operate at
lower voltages. Not all design principles used in the
past can be directly translated to a lower voltage
environment.
different power supply conditions (e.g., ±5 V, +5 V and
+3 V), along with corresponding loading conditions, are
useful and necessary here.
Rail-to-rail amplifiers are seen as a solution to the
dilemma of decreasing power supply voltages. The term
rail-to-rail, while not exactly defined, refers to devices
whose inputs and/or outputs can swing close to both
rails. This definition does not put an exact value on
“close to both rails”, nor does it specify the loading conditions under which rail-to-rail performance must be
maintained. Rail-to-rail op amps are a subset of singlesupply op amps which are devices that operate on a
single rail. The inputs and outputs of a single-supply op
amp may or may not be able to approach the rails. In
order to work successfully with rail-to-rail and singlesupply op amps, an basic understanding of some commonly used output stages is necessary.
+V
S
+V
S
Reducing the power supply voltage to a typical op amp
has a number of effects. Obviously, the signal swings
both at the input and output are reduced. The required
headroom between signal and rail (typically 1 V to 2 V in
conventional amplifiers), which is of lesser importance
with power supplies of ±15 V, now drastically reduces
the usable signal range. While this reduction does not
normally increase noise levels in the system, signal-tonoise ratios will be degraded. Because the designer can
no longer use techniques such as increasing power supply voltages and signal swings in order to “swamp”
noise levels, greater attention must be paid to noise
levels in the system.
Both bandwidth and slew rate decrease as power supplies drop. However, it should be noted that smaller
signal swings need lower slew rates to maintain the
same bandwidth. In choosing an operational amplifier,
close study of the data sheet is essential. Data sheet
specifications that list slew rate and bandwidth under
OUTPUT
–V
S
COMMON EMITTER
Figure 1. Common Op Amp Output Stages
Figure 1 shows two typical high speed op amp output
stages. The emitter-follower stage is widely used in low
distortion op amps. Its output voltage swing is limited to
slightly greater than one diode drop from the rails. In
reality, the headroom is closer to 1 V. In order to maintain low distortion at high frequencies, even more
headroom may be required, reducing the available
–V
S
EMITTER FOLLOWER
OUTPUT
R
L
50Ω TO
500Ω
peak-to-peak swing even further. Adding an external
load resistor (typically 50 Ω to 500 Ω) referenced to the
negative rail (this would be ground in a single-supply
application) provides a pull-down path to the output.
This, combined with the biasing on the bases of the NPN
and PNP transistors, allows the PNP transistor to shut
off. This allows the output to be pulled close to the negative rail so that the output stage behaves much like a
simple NPN follower. This only allows the voltage to approach the negative rail. The load resistor would have to
be referenced to the positive supply to bring the output
voltage close to the positive rail. Another potential
drawback of this configuration is the large load current
that would be drawn for signal swings greater than a
few hundred millivolts. Using a 50 Ω pull-down resistor,
for example, would draw a current from the op amp of
40 mA if a 2 V p-p swing was desired.
The common-emitter stage shown allows the output to
swing to within the transistor saturation voltage, V
CESAT
of both rails. For small amounts of load current (less
than 100 µA), the saturation voltage may be as low as 5
mV to 20 mV; but for higher load currents, the saturation
voltage may increase to several hundred millivolts (for
example, 500 mV at 50 mA). This type of output stage
has higher open-loop output impedance than an emitter
follower stage and is more likely to distort when driving
such nonlinear loads as flash converters. It is important
though not to look at open loop output impedance in
isolation. Closed loop output impedance, Z
, is given by
o
the formula
Z
Z
where
Z
is the open loop output impedance,
o
open loop gain and β is the feedback factor (
o
=
o
1+
a
β
o
a
is the
o
a
β is com-
o
monly referred to as Loop Gain). So a large open loop
gain, of 100 dB for example, would reduce the output
impedance of an op amp, connected as a unity gain
buffer, by a factor of 100,000. As frequency increases,
the decreasing open loop gain will cause the output
impedance to increase.
of the available voltage range. Step-up transformers can
increase voltages to an arbitrarily high level, but at the
cost of increased output current from the driving amplifier. The following collection of common high speed
applications seeks to illustrate the challenges involved
in designing low voltage analog circuits and looks specifically at the techniques involved in obtaining optimal
performance when using rail-to-rail op amps.
Driving High Speed ADCs
While most modern high speed ADCs operate from
single supplies, they are still most often used in signal
chains that have bipolar supplies. Because singlesupply ADCs typically have lower quiescent currents
than their dual supply equivalents, the main impetus
behind this trend is the power that is saved.
Bipolar signals usually need some form of level shifting
before being applied to a single-supply ADC. Because
,
the safe input voltage to an ADC should not generally
exceed the power supply voltages by more than a few
hundred millivolts, consideration must be given to the
protection of single-supply devices in a dual supply
environment.
Figure 2 shows an 8-bit 125 MSPS flash converter being
driven by a 240 MHz clamping amplifier. The ADC uses
ECL logic and is powered from a single –5.2 V supply.
The input voltage swing is 2 V (–1 V ± 1 V). The device’s
absolute maximum ratings specify a safe input voltage
range to be between –V
and +0.5 V. While choosing a
S
rail-to-rail amplifier to run from the same single supply
would inherently protect the ADC from overvoltage,
powering the op amp from a bipolar supply is more
appropriate in this example.
Even though a rail-to-rail amplifier running on a single
supply of –5.2 V would be capable of swinging most of
the way up to ground, signal distortion tends to degrade
significantly as voltages approach the rails. A more reasonable approach involves powering the op amp with
bipolar supplies so that there is a large amount of headroom (5 V on the positive side and 3 V on the negative
side) between the signal and the rails.
Even though rail-to-rail amplifiers can typically swing to
within a few tens of millivolts of the power supplies,
there is generally a tradeoff between distortion and
signal swing. Data sheets of op amps usually specify
optimum distortion with output signals that do not exercise the complete available voltage range. As signal
levels approach within a few hundred millivolts of the
rails, distortion performance degrades significantly. The
best distortion/signal level tradeoff in rail-to-rail op
amps, with common-emitter output stages, occurs when
there is a signal-to-rail headroom of about 500 mV to
each rail. This is a generalization and the optimal value
will also depend on loading.
In addition to using rail-to-rail amplifiers, there are a
number of techniques that can be used to increase signal swings without having to increase power supply
levels. Differential drive circuits make more efficient use
Using two resistor dividers, the input referred clamp
voltages of the op amp are set to ±0.55 V or 50 mV
greater than the normal maximum input voltages. In
order to map the ±0.5 V input voltage into the 0 V to –2 V
input range of the ADC, the op amp provides a gain of
two and uses a +2.5 V reference to give a level shift of
1
–1 V
. The output referred clamp voltages translate to
+0.1 V and –2.1 V. The 1N5712 Schottky diode provides
additional protection during power-up and actually
holds the maximum voltage at the ADC’s input to about
+0.3 V. A 50 Ω resistor in series with the op amp’s output
limits the current through the diode during overvoltage
as well as isolating the output stage from the signal
dependent capacitive load of the flash ADC
maximum value of 22 pF. The negative clamping level of
–2.1 V, while not necessary to protect the converter, prevents excessive negative overdrive of the analog input.
–2–
2
that has a
10µF
+5V
0.1µF
BIPOLAR
SIGNAL
+/–0.5 V
+5V
+
AD780
+2.5 V REF
0.1µF
R3
750Ω
R
75Ω
T
0.1µF
–5.2V
R1
499Ω
806Ω
AD8037
806Ω 100Ω
R2
301Ω
100Ω
V
= +0.55V
H
V
= –0.55V
L
1N5712
49.9Ω
AD8037 OUTPUT
CLAMPS AT +0.1 V, –2.1 V
AD9002
FLASH CONVERTER
(8-BITS, 125 MSPS)
V
= –1 +/–1V
IN
SUBSTRATE
DIODE
–5.2V
0.1µF
Figure 2. AD9002, 8-Bit, 125 MSPS Flash Converter
In addition to and perhaps more important than providing the necessary signal conditioning, a drive amplifier
must provide a low impedance source which does not
degrade the ADC’s dynamic capabilities. The signal to
noise plus distortion (S/(N+D) or SINAD) plot of the ADC
should generally be used as the first selection criterion
for the drive amplifier. This plot should be compared to
the op amp’s total harmonic distortion plus noise
(THD+N). Comparing like with like is important here and
both measurements should reference similar signal
levels, power supply voltages and bias conditions as will
be used in the actual circuit. The amplifier’s loading conditions should also be similar to those presented by the
ADC. As a general rule, in order to prevent the op amp
from degrading the dynamic performance of the ADC,
its THD+N should be 6 dB to 10 dB better than the ADC’s
S/(N+D) at the highest signal frequency
3
(usually but not
always the ADC’s Nyquist frequency). In some applications, such as spectral analysis, low distortion can be
more important than low noise. In such cases, comparing the op amp’s THD to the ADC’s distortion (usually
specified as spurious free dynamic range or SFDR) is
more meaningful. Once again, choosing an op amp
whose distortion is 6 dB to 10 dB better than the ADC’s is
appropriate.
This selection criterion can be used where the ADC’s input
impedance is fixed and does not change during the conversion process. This is usually the case with ADCs designed on bipolar processes. On the other hand, ADCs
designed on CMOS processes typically connect the
sample-and-hold switches directly to the analog input.
This generates transient currents during the conversion
that the external drive circuit must be able to deliver. In
addition to this, the (relatively low) on-impedance of
CMOS switches has some signal dependency. The
ADC’s analog input may, therefore, exhibit a signal-leveldependent input impedance, which leads to distortion.
Figure 3 shows a 12-bit 10 MSPS single-supply CMOS
ADC being driven by a differential amplifier, created using a single-supply dual op amp. The input stage of the
ADC is a differential sample-and-hold. The switches that
open and close at the sampling frequency are shown in
track mode. The capacitances denoted C
PARCPIN
are
about 16 pF and represent the combined stray capacitance of the switches and the input pins. C
and CH repre-
S
sent the sampling and hold capacitances respectively. In
the track mode, the differential input voltage is applied
to the C
capacitors. When it goes into hold mode, the
S
voltages on these capacitors are transferred to the hold
capacitors.
The input range of the ADC is set, by pin strapping, to 2 V
peak-to-peak. The differential drive amplifier sets up a
common-mode voltage of 2.5 V. From a signal distortion
point of view, this is the optimal configuration for a number of reasons.
In systems that truly operate on a single power supply, it
can often be difficult to maintain dc coupling from
source all the way to the ADC. In such systems, a virtual
ground is often created, usually centered halfway between the rails. This introduces the question of an optimum input voltage range for a single-supply ADC. At
first glance, it would seem that a zero-volt referenced input might be desirable. But in fact, this places some severe constraints on both the ADC and its driving
amplifier because both must maintain full linearity and
low distortion at or near 0 V.
A more optimum voltage range for both ADC and op
amp is one that includes neither ground nor the positive
supply. A range centered around V
/2 is usually opti-
S
mum. For example, an input range of 2 V p-p centered
around +2.5 V is bounded by +1.5 V and +3.5 V. If the
dynamic specifications of single-supply op amps are
stated for a midscale bias condition, a direct specification comparison can be made to help in making an
–3–
appropriate op amp ADC match. However, where a
single-supply ADC has a bias point substantially offset
from the ideal V
/2, the op amp’s distortion and other
S
dynamic specifications may degrade.
In the example shown, the differential amplifier, which
has a gain of two
4
, converts a ±0.5 V single-ended signal
to a 2 V peak-to-peak differential signal with a commonmode level of +2.5 V. Each of the op amps, however, is
only required to swing from 2 V to 3 V (i.e., 2.5 V ± 0.5 V).
This efficient use of signal range minimizes op amp
distortion because of the relatively large headroom of
2 V to each rail. This scheme also has benefits for the
converter. The on-resistance of the ADC’s CMOS sampling switches, that was mentioned earlier, is at a minimum when the input voltage is at midsupply.
Minimizing the voltage variation at each input decreases
the signal dependent impedance variation of the
switches and limits the resulting distortion.
This ADC can also be configured to accept an input voltage range, either single-ended or differential, of 5 V
peak-to-peak. Using the configuration shown for a differential input range of 5 V peak-to-peak, the drive amplifiers would be required to swing from 1.25 V to 3.75 V.
This still leaves 1.25 volts of headroom to both supplies.
Choosing this larger input range optimizes dc linearity
and signal-to-noise ratio. The increased signal range
will cause a slight degradation in distortion in the
converter.
From a safety point of view, the issue of clamping input
voltages in a single-supply signal chain is of lesser importance because both amplifier and ADC are usually
powered from the same source. However, the analog
inputs on some ADCs have absolute maximum ratings
that are less than the supply voltages. In these cases, the
issue of input protection through clamping must once
again be addressed.
Line Drivers
The Differential Gain and Differential Phase specifications are expressions of the variation of the gain and
phase of a small signal as the magnitude of a large signal, on which it is superimposed, changes. While these
specifications are primarily a function of amplifier architecture, the headroom between the signal and the
power supplies will affect the differential gain and phase
performance of an op amp. As a result, although composite video signals typically have maximum levels in
the 1 V to 2 V range, composite video line drivers have,
in the past, tended to run on power supplies of ±12 V
and ±15 V. Systems designed nowadays require differential gain and phase specifications that are at least as
good as those in the past. In order to save power, the
designer can no longer afford the luxury of a large
amount of headroom between signal and supplies.
BIPOLAR
SIGNAL
+/–0.5V
0.1µF
2.49kΩ
2.49kΩ
2.49kΩ
+5V
2.49kΩ
+5 V
R
1kΩ
+5V +5V +5V
0.1µF
+5V
0.1µF
R
F
1kΩ
1kΩ
0.1µF
1/2
AD8042
1kΩ
1kΩ
1/2
AD8042
1kΩ
0.1µF
VINA
VINB
V
REF
SENSE
CML
REFCOM DV
IN
0.1µF 0.1µF
DV
DD
C
PIN
C
PAR
16pF
S1
S2
16pF
C
PIN
C
PAR
SS
AV
DD
(additional pins omitted for clarity)
S3
AV
AV
DD
AD9220
12-BIT, 10 MSPS, ADC
C
S
4pF
C
H
4pF
C
H
4pF
C
S
4pF
AV
SS
SS
S6
S4
S7
S5
Figure 3. Driving a Single-Supply, Differential Input ADC with a Single-Ended to Differential Op Amp Configuration
–4–
+V
75Ω
10kΩ
4.99kΩ
4.99kΩ
10µF
0.1µF
10µF
1000µF
0.1µF
47µF
+
COMPOSITE
VIDEO IN
AD8041
220µF
R
G
1kΩ
R
F
1kΩ
R
T
75Ω
R
L
75Ω
V
OUT
+
+
75Ω
COAX
+5V
(+5 V to +15 V)
S
C3
V
IN
75Ω
R
G
649Ω
*AD8001 CAN BE USED ONLY WHERE +/–5 V POWER SUPPLIES ARE PRESENT
AD811/
AD8001*
–V
S
C1
0.1µF
C2
0.1µF
C4
(–5 V to –15 V)
R
FB
649Ω
C3, C4: 100µF/25 V
R
75Ω
R
75Ω
T1
T2
Figure 4. Traditional High Quality Video Line Driver
with Optional Video Distribution Function
Figure 4 shows a high performance video line driver,
that has optional distribution amplifier features. The op
amp stage operates at a gain of two, driving a pair of
75 Ω output lines through 75 Ω back terminations.
V
and V
OUT1
unity gain versions of V
are thus individually isolated/buffered
OUT2
. With the overall terminated
IN
gain of unity, this circuit serves well as a low distortion
buffer, or a video distribution amp.
Exactly as shown, using the AD811 op amp and operated from ±15 V supplies, the circuit has a –3 dB bandwidth of 120 MHz, and differential gain/phase of 0.01%/
0.01° with one line driven (R
= 150 Ω). Driving two lines,
L
the gain errors are essentially the same, while the phase
errors rise to about 0.04°. The gain flatness of this circuit
is within 0.1 dB to 35 MHz with ±15 V supplies. As expected, lower supplies do degrade performance some,
but differential phase is still less than 0.18° with ±5V
power. The –3 dB point falls to 80 MHz, and 0.1 dB gain
flatness is maintained to 25 MHz.
This example, which uses the AD811, illustrates the degree to which differential gain and phase degrade when
power supplies are reduced from ±15 V to ±5 V. A more
modern amplifier, like the AD8001 is only specified for
operation at ±5 V. This amplifier has much higher bandwidth and 0.1 dB gain flatness and can almost equal the
±15 V differential gain and phase specifications of the
AD811 and consumes less power.
For best accuracy and stability, the use of metal film
resistor types is recommended. Heavy decoupling is
also recommended. As a minimum, local low inductance/low ESR RF bypass caps should be used right at
the device supply pins, shown as C1/C2. These are 0.1 µF
surface mount chips (or other low inductance type).
When driving high peak current loads, these high frequency bypasses should be augmented by local, short
lead/large value, low ESR electrolytics, shown as C3/C4,
in the range of 47 µF to 100 µF. These capacitors will
carry the transient currents, and can be either tantalum,
or aluminum types rated for high frequency (i.e., switching supply types).
V
R
75Ω
V
R
75Ω
OUT
OUT
L2
1
L1
2
Figure 5. AC-Coupled Single-Supply Composite Video
Line Driver
Figure 5 shows a schematic of a single-supply gain-oftwo composite video line driver. Since the sync tips of a
composite video signal extend below ground, the input
must be ac-coupled and level-shifted positively. Setting
the optimal bias point requires some understanding of
the nature of composite video signals and the video performance of the op amp used.
After ac-coupling, signals of bounded peak-to-peak
amplitude that vary in duty cycle, require larger
dynamic swing capability than their peak-to-peak amplitude. As a worst case, the dynamic signal swing required will approach twice the peak-to-peak value. The
two bounding cases are for a duty cycle that is mostly
low, but occasionally goes high and vice versa. Composite video is not quite this demanding. One bounding extreme is a signal that is mostly black for an entire frame,
but that has a white (full intensity) minimum width spike
at least once per frame. The other extreme is for a video
signal that is full white everywhere. The blanking intervals and sync tips of such a signal will have negative
going excursions in compliance with composite video
specifications. The combination of horizontal and vertical blanking intervals limit such a signal to being at its
highest level (white) for only about 75% of the time.
As a result of the duty cycle variations between these
two extremes, an ac-coupled 2 V p-p composite video
signal requires about 3.2 V of dynamic voltage swing to
avoid clipping.
Some circuits use a sync tip clamp along with accoupling to hold the sync tips at a relatively constant
level in order to lower the amount of dynamic signal
swing required. However, these circuits can have artifacts like sync tip compression unless they are driven by
sources with very low output impedance.
Because the circuit shown uses an op amp with a rail-torail output stage, there is ample signal swing capability
to handle the dynamic range required without using a
sync tip clamp. As a test, the differential gain and phase
were measured while the supplies were varied. As the
lower supply is raised to approach the video signal, the
first effect to be observed is that the sync tips become
compressed before the differential gain and phase are
adversely affected. As the upper supply is lowered to
–5–
approach the video signal, the differential gain and
2V
50mV
1µs
0.5V
10
0%
100
90
STOP 5MHz
VERTICAL SCALE – 10dB/Div
START 0Hz
10
0%
100
90
1.5V
50mV
200ns
0.2V
phase were not significantly adversely affected until the
difference between the peak video output and the supply reached 0.6 V
Taking this test into account, it was found that the optimal point to bias the noninverting input was at 2.2 V dc.
Operating at this point, the worst case differential gain and
phase were measured at 0.06% and 0.06° respectively.
The ac-coupling capacitors used in the circuit appear
quite large at first glance. A composite video signal has
a lower frequency band edge of 30 Hz. The resistances at
the various ac-coupling points, especially at the output,
are quite small. In order to minimize phase shifts and
baseline tilt, the large value capacitors are required. For
video system performance that is not to be of the
highest quality, the value of these capacitors can be
reduced by a factor of up to five with only a slightly observable change in the picture quality.
A dc-coupled single-supply line driver presents a challenge if the voltage swing of output signal needs to go
close to ground. This is because the signal distortion increases as the output voltage approaches ground. The
AD8031 for example swings close to both rails. However
lowest distortion performance is achieved when the signal has a common-mode level half way between the
supplies and when there is about 500 mV of headroom
to each rail. If low distortion is required in single-supply
applications for signals which swing close to ground,
an emitter follower circuit can be used at the op amp
output.
Figure 7. Output Signal Swing of Low Distortion Line
Driver at 500 kHz
Figure 8. THD of Low Distortion Line Driver at 500 kHz
V
IN
49.9Ω
2.49kΩ 2.49kΩ 49.9Ω
Figure 6. Low Distortion Line Driver for Single-Supply
Ground Referenced Signals
Figure 6 shows the AD8031 configured as a dc-coupled
single-supply gain-of-2 line driver. With the output driving a back terminated 50 Ω line, the overall gain from V
to V
is unity. In addition to minimizing reflections, the
OUT
50 Ω back termination resistor protects the transistor
from damage if the cable is short circuited. The emitter
follower, which is inside the feedback loop, ensures that
the output voltage from the AD8031 stays about 700 mV
above ground. Using this circuit, very low distortion is
attainable even when the output signal swings to within
50 mV of ground. The circuit was tested at 500 kHz and
2 MHz. Figures 7 and 8 show the output signal swing
and frequency spectrum at 500 kHz. At this frequency,
the output signal (at V
swing of 1.95 V (50 mV to 2 V), has a THD of –68 dB.
3
2
7
AD8031
+5V
10µF
4
0.1µF
6
OUT
2N3904
200Ω
), which has a peak-to-peak
V
49.9Ω
OUT
Figure 9. Output Signal Swing of Low Distortion Line
Driver at 2 MHz
IN
VERTICAL SCALE – 10dB/Div
START 0Hz STOP 20MHz
Figure 10. THD of Low Distortion Line Driver at 2 MHz
–6–
Figures 9 and 10 show the output signal swing and frequency spectrum at 2 MHz. As expected, there is some
degradation in signal quality at the higher frequency.
When the output signal has a peak-to-peak swing of
1.45 V (swinging from 50 mV to 1.5 V), the THD is –55 dB.
This circuit could also be used to drive the analog input
of a single-supply high speed ADC whose input voltage
range is referenced to ground (e.g., 0 V to 2 V or 0 V to
4 V). In this case, a back termination resistor is not necessary (assuming a short physical distance from transistor to ADC). So the emitter of the external transistor
would be connected directly to the ADC input. The available output voltage swing of the circuit would therefore
be doubled.
Active Filters
Traditionally, when designing high speed active filters,
a designer could choose an amplifier whose gain
bandwidth product (GBP) was much higher than the
corner frequencies of the filter. Additionally, a supply
voltage of ±15 V or ±12 V meant that signal-to-rail headroom could be kept fairly large. This allowed the amplifier, from the point of view of bandwidth and signal
swing at least, to be viewed as an ideal component. The
advent of lower power supplies, which generally reduce
bandwidth and slew rate, coupled with the desire to
maximize signal range, means that in many cases, the
difference between the corner frequency of the filter and
the actual bandwidth of the amplifiers in the filter are no
longer as far apart as before. In choosing an op amp for
an active filter design, it is important to calculate beforehand, the bandwidth and phase shift that the amplifier
will exhibit in the circuit, given the power supply levels,
the desired signal swing and the required loading conditions. When considering signal swing, it is important
also to consider the signal levels on the internal nodes
of the circuit, not just the input and output levels. In
filters with Qs over 0.707 there will be peaking in the
response. The level of the peaking must be factored into
the dynamic range of the filter so that no clipping
occurs.
Many modern high speed op amps have a current feedback topology. Capacitance in the feedback loop of a
current feedback amplifier usually causes it to become
unstable. As a result, current feedback amplifiers are
generally not usable in filter topologies that configure
the op amp as an integrator
5
. An exception to this is the
Sallen-Key filter which does not incorporate integrators.
Figure 11 shows a circuit for a single-supply biquad
bandpass filter with a center frequency of 2 MHz. A 2.5 V
bias level is easily created by connecting the noninverting inputs of all three op amps to a resistor divider consisting of two 1 kΩ resistors connected between +5 V
and ground. This bias point is also decoupled to ground
with a 0.1 µF capacitor. The frequency response of the
filter is shown in Figure 12.
In order to maintain an accurate center frequency, it is
essential that the op amp has sufficient loop gain at
2 MHz. This requires the choice of an op amp with a sig-
nificantly higher unity gain crossover frequency. The
unity gain crossover frequency of the AD8031/AD8032 is
40 MHz. Multiplying the open-loop gain by the feedback
factors of the individual op amp circuits yields the loop
gain for each gain stage. From the feedback networks of
the individual op amp circuits, we can see that each op
amp has a loop gain of at least 21 dB. This level is high
enough to ensure that the center frequency of the filter
is not affected by the op amp’s bandwidth. If, for example, an op amp with a gain bandwidth product of
10 MHz was chosen in this application, the resulting center frequency would shift by 20% to 1.6 MHz.
R3
2kΩ
R6
1kΩ
1/2
AD8032
R4
2kΩ
+5V
0.1µF
R5
2kΩ
C2
50pF
1/2
AD8032
C1
50pF
R2
2kΩ
+5V
AD8031
1kΩ
0.1µF
V
OUT
R1
3kΩ
V
IN
1kΩ
0.1µF
Figure 11. A Single-Supply 2 MHz Biquad Bandpass
Filter Using AD8032 and AD8031
0
–10
–20
GAIN – dB
–30
–40
–50
10k 100M
100k 10M
1M
FREQUENCY – Hz
Figure 12. Frequency Response of Single-Supply
2 MHz Bandpass Filter
Transformer Drive Circuits
Even when using rail-to-rail amplifiers, an op amp’s signal swing is limited to the power supply voltages. Using
transformer coupling creates the possibility of increasing signal swings to voltages greater than the supply
rails. Additionally, a transformer coupled signal, being
differential, generally affords more immunity to external
interference. This can be critical where signals are being
transmitted over long distances.
The peak-to-peak amplitude of a signal can be increased
to an arbitrarily high level by choosing a step-up transformer with the appropriate turns ratio. However, the reflected impedance from secondary to primary of a
step-up transformer is equal to the secondary impedance divided by the square of the turns ratio. This leads
–7–
to a higher current demand on the op amp. In selecting a
Power
=10
log
10
V
peak –peak
2 ×
crest factor
2
/R
LOAD
1
mW
suitable op amp to drive a step-up transformer, the designer needs to look for good signal swing even when
the amplifier is delivering relatively high current.
HDSL Transceiver
HDSL or high bit-rate digital subscriber line is becoming
popular as a means of providing full duplex data communication at rates up to 2.048 Mbits/s over moderate
distances via conventional telephone twisted pair wires.
In order to achieve repeaterless transmission over distances of up to approximately 12,000 feet, a transmitted
power level of +13.5 dBm (assuming a load impedance
of 135 Ω) is required. Because the transceiver at the
customer’s end is sometimes powered via the twisted
pair from a power source at the central office, circuit
power consumption is critical.
Figure 13 shows a circuit powered from a single +5 V
supply that can deliver this power level. A dual op amp is
used to sum power into the two primary windings of the
transformer. These are effectively connected in parallel.
Both op amps are configured for a gain of 2. This allows
the output to swing rail-to-rail even though the
amplifier’s input range is not rail-to-rail (input range is
–0.2 V to +4 V). Although the output voltage is capable of
swinging quite close to both rails even under fairly
heavy loading conditions, a voltage swing from about
0.5 V to 4.2 V is more appropriate in order to maintain a
THD level of about –70 dB (measured at 500 kHz). A
100 µF capacitor, to which both primary transformers
are referenced, creates a virtual ground, equal to the
average dc value of the output signal (about 2.4 V). Each
primary has a reflected impedance from the secondary
of 29.78 Ω (134/1.5
2
/2). The primaries are each connected
in series with a resistance approximately equal to this
value. So the voltage across each primary is half the
voltage of the op amp driving it.
2.1
1/2
AD8042
2kΩ
1/2
AD8042
2kΩ
4.7Ω
0.47µF
29.4Ω
X
100µF
+5V
LUCENT
TECHNOLOGIES
2718AK
1:1.5
V
OUT
29.4Ω
Y
1µF
2kΩ
2kΩ
2kΩ
AD8041
13.4Ω
0.47µF
4.7Ω
+5V
V
OUT
0.3
V
IN
2kΩ
2kΩ
The divided down voltages from the two transmitter op
amps are also fed to the two inputs of the differential
receiver. These signals appear as a common-mode voltage to the receiver and are not amplified. In reality, the
voltages at Nodes X and Y are not exactly equal, so
some of the transmitted signal is amplified by the receiver. The transmitter to receiver rejection was measured at –20 dB.
The received signal couples on to both primaries. These
voltages however drive the differential receiver 180° out
of phase from each other. This results in a receiver gain
which is equal to the inverse of the turns ratio of the
transformer (1/1.5).
With each op amp delivering 3.5 V peak-to-peak at its
output, each primary has a peak-to-peak voltage of 1.75.
The secondary voltage of approximately 5.2 V peak-topeak is the sum of the primary voltages times the turns
ratio of 1.5. It corresponds to a power level of about
+14 dBm. This is calculated using the equation.
This power calculation is based upon a crest factor of
√
2. If a different crest factor is used in the calculation,
the resulting power will be more or less than this value.
If a higher signal swing is required, a transformer with a
higher turns ratio can be used. This will demand more
current from the op amps. In the configuration shown,
the op amps are delivering about 28 mA to their loads
which are referenced to +2.5 V. Because they are
capable of delivering up to 50 mA while maintaining a
signal swing of 0.5 V to 4.5 V, there is some scope for
increasing the signal swing on the secondary. Increasing the turns ratio will, however, decrease the amplitude of the received signal.
References
1.
Replacing Output Clamping Op Amps with Input
Clamping Amps
, Application Note AN-402, Analog
Devices, 1995, p. 3
2.
Amplifier Applications Guide
, Analog Devices, 1992,
pp. 7.49–52
3.
Practical Analog Design Techniques
, Analog Devices,
1995, pp. 4.12–15
4.
AD 8042, Dual 160 MHz Rail-to-Rail Amplifier
, Data
Sheet, Analog Devices, 1995, pp 12-13
5.
Amplifier Applications Guide
, Analog Devices, 1992,
pp. 6.27–29
E2184–10–10/96
PRINTED IN U.S.A.
2kΩ
1µF
Figure 13. Single-Supply HDSL Transceiver
–8–
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