Analog Devices AD813x differential amplifier Application note

a
AN-584
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com
Using the AD813x
THEORY OF OPERATION
The AD813x differs from conventional op amps by the
external presence of an additional input and output. The
additional input, VOGM, controls the output commonmode voltage. The additional output is the analog
complement of the single output of a conventional op
amp. For its operation, the AD813x makes use of two
feedback loops as compared to the single loop of conventional op amps. While this provides significant
freedom to create various novel circuits, basic op amp
theory can still be used to analyze the operation.
One of the feedback loops controls the output commonmode voltage, VOUT,cm. Its input is VOCM (Pin 2) and the
output is the common-mode, or average voltage, of the
two differential outputs (+OUT and –OUT). The gain of
this circuit is internally set to unity. When the AD813x is
operating in its linear region, this establishes one of the
operational constraints: VOUT,cm = VOCM.
The second feedback loop controls the differential operation. Similar to an op amp, the gain and gain-shaping of
the transfer function is controllable by adding passive
feedback networks. However, only one feedback network is required to “close the loop” and fully
constrain the operation. But depending on the function
desired, two feedback networks can be used. This is
possible as a result of having two outputs that are each
inverted with respect to the differential inputs.
DEFINITION OF TERMS
Differential voltage refers to the difference between two
node voltages. For example, the output differential voltage (or equivalently output differential-mode voltage) is
defined as:
VOUT,dm = (V+OUT – V–OUT )
(1)
V+OUT and V–OUT refer to the voltages at the +OUT and –OUT
terminals with respect to a common reference.
Common-mode voltage refers to the average of two
node voltages. The output common-mode voltage is
defined as:
VOUT,cm = (V+OUT + V–OUT ) / 2
(2)
PIN FUNCTION DESCRIPTIONS
Pin No. Mnemonic
Function
1
2
–IN
VOCM
3
4
V+
+OUT
5
–OUT
6
7
8
V–
NC
+IN
Negative Input
Voltage applied to this pin sets the
common-mode output voltage with
a ratio of 1:1. For example, 1 V dc
on VOCM will set the dc bias level
on +OUT and –OUT to 1 V.
Positive Supply Voltage
Positive Output. Note: The voltage
at –DIN is inverted at +OUT.
Negative Output. Note: The voltage
at +DIN is inverted at –OUT.
Negative Supply Voltage
No Connect
Positive Input
CF
RF
+DIN
RG
+IN
AD813x
VOCM
–DIN
–OUT
RG
–IN
RL,dm
+OUT
RF
CF
Figure 1. Circuit Definitions
REV. 0
VOUT,dm
GENERAL USAGE OF THE AD813x
Several assumptions are made here for a first-order
analysis, which are the typical assumptions used for the
analysis of op amps:
• The input impedances are arbitrarily large and their
loading effect can be ignored.
• The input bias currents are sufficiently small so they
can be neglected.
• The output impedances are arbitrarily low.
• The open-loop gain is arbitrarily large, which drives
the amplifier to a state where the input differential
voltage is effectively zero.
• Offset voltages are assumed to be zero.
© Analog Devices, Inc., 2002
AN-584
While it is possible to operate the AD813x with a purely
differential input, many of its applications call for a circuit
that has a single-ended input with a differential output.
Like a conventional op amp, the AD813x has two
differential inputs that can be driven with both a differential-mode input voltage, VIN,dm, and a common-mode input
voltage, VIN,cm. Another input, VOCM, is not present on
conventional op amps, but provides another input to
consider on the AD813x. It is totally separate from the
above inputs. There are also two complementary outputs
whose response can be defined by a differential-mode
output, VOUT,dm and a common-mode output, VOUT,cm.
For a single-ended-to-differential circuit, the RG of the
undriven input will be tied to a reference voltage. For
now this is ground. Other conditions will be discussed
later. Also, the voltage at VOCM, and hence VOUT,cm will be
assumed to be ground for now. Figure 2 shows a generalized schematic of such a circuit using an AD813x with
two feedback paths.
Table I indicates the gain from any type of input to either
type of output.
RF1
RG1
Table I. Differential and Common-Mode Gains
+
RG2
RF2
Figure 2. Typical Four-Resistor Feedback Circuit
For each feedback network, a feedback factor can be
defined, which is the fraction of the output signal that is
fed back to the opposite-sign input. These terms are:
β1 = RG1 / (RG1 + R F 1)
(3)
β2 = RG2 / (RG2 + R F 2 )
(4)
VOUT,dm
VOUT,cm
VIN,dm
VIN,cm
VOCM
RF/RG
0
0
0 (By Design)
0 (By Design)
1 (By Design)
The differential output (VOUT,dm) is equal to the differential input voltage (VIN,dm) times RF/RG. In this case, it does
not matter if both differential inputs are driven, or only
one output is driven and the other is tied to a reference
voltage, like ground. As can be seen from the two zero
entries in the first column, neither of the common-mode
inputs has any effect on this gain.
The feedback factor ␤1 is for the side that is driven, while
the feedback factor ␤2 is for the side that is tied to a reference voltage (ground for now). Note also that each
feedback factor can vary anywhere between 0 and 1.
The gain from VIN,dm to VOUT,cm is 0 and to first-order does
not depend on the ratio matching of the feedback networks. The common-mode feedback loop within the
AD813x provides a corrective action to keep this gain
term minimized. The term “balance error” describes the
degree to which this gain term differs from zero.
A single-ended-to-differential gain equation can be
derived that is true for all values of ␤1 and ␤2:
G = 2 × (1 – β1) / ( β1 + β 2)
Input
(5)
The gain from VIN,cm to VOUT,dm does directly depend on
the matching of the feedback networks. The analogous
term for this transfer function, which is used in conventional op amps, is “common-mode rejection ratio” or
CMRR. Thus, if it is desirable to have a high CMRR, the
feedback ratios must be well matched.
This expression is not very intuitive. One observation
that can be made right away is that a tolerance error in
␤1 does not have the same effect on gain as the same
tolerance error in ␤2.
For RF1/RG1 = RF2/RG2 the gain equation simplifies to G = RF/RG.
The gain from VIN,cm to VOUT,cm is also ideally 0, and is
first-order independent of the feedback ratio matching.
As in the case of VIN,dm to VOUT,cm, the common-mode
feedback loop keeps this term minimized.
BASIC CIRCUIT OPERATION
One of the more useful and easy to understand ways to
use the AD813x is to provide two equal-ratio feedback
networks. To match the effect of parasitics, these networks should actually be comprised of two equal-value
feedback resistors, RF and two equal-value gain resistors, RG. This circuit is diagrammed in Figure 1.
The gain from VOCM to VOUT,dm is ideally 0 only when the
feedback ratios are matched. The amount of differential
output signal that will be created by varying V OCM is
related to the degree of mismatch in the feedback networks.
–2–
REV. 0
AN-584
VOCM controls the output common-mode voltage VOUT,cm
with a unity-gain transfer function. With equal-ratio
feedback networks (as assumed above), its effect on
each output will be the same, which is another way to
say that the gain from VOCM to VOUT,dm is zero. If not
driven, the output common-mode will be at mid-supplies.
It is recommended that a 0.1 µF bypass resistor be connected to VOCM.
In the case of a single-ended input signal (for example if
–DIN is grounded and the input signal is applied to +DIN),
the input impedance becomes:
R IN,dm
(
When unequal feedback ratios are used, the two gains
associated with VOUT,dm become nonzero. This significantly complicates the mathematical analysis along
with any intuitive understanding of how the part operates. Some of these configurations will be in another
section.
(6)
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The AD813x’s VOCM pin is internally biased at a voltage
approximately equal to the mid-supply point (average
value of the voltages on V+ and V–). Relying on this
internal bias will result in an output common-mode voltage
that is within about 100 mV of the expected value. In
cases where more accurate control of the output commonmode level is required, it is recommended that an external
source, or resistor divider (with R SOURCE < 10 kΩ), be used.
Table II. Recommended Resistor Values and Noise Performance
for Specific Gains
RG RF
Gain (⍀) (⍀)
Output
Bandwidth Noise
–3 dB
AD813x
Output
Noise
AD813x + RG, RF
1
2
5
10
360 MHz
160 MHz
65 MHz
20 MHz
17 nV/Hz
26.1 nV/Hz
53.3 nV/Hz
98.6 nV/Hz
499
1.0 k
2.49 k
4.99 k
16 nV/Hz
24.1 nV/Hz
48.4 nV/Hz
88.9 nV/Hz
APPLICATION NOTES FOR THE AD813x DIFFERENTIAL AMPS
ADC DRIVING
High-Performance ADC Driving
The circuit in Figure 3 shows a simplified front-end
connection for an AD813x driving an AD9224, a 12-bit,
40 MSPS A/D converter. The A/D works best when
driven differentially, which minimizes its distortion as
described in its data sheet. The AD813x eliminates the
need for a transformer to drive the ADC and performs
single-ended-to-differential conversion, common-mode
level-shifting, and buffering of the driving signal.
CALCULATING AN APPLICATION CIRCUIT’S INPUT
IMPEDANCE
The effective input impedance of a circuit such as that in
Figure 1, at +DIN and –DIN, will depend on whether the
amplifier is being driven by a single-ended or differential signal source. For balanced differential input signals,
the input impedance (RIN,dm) between the inputs (+DIN
and –DIN) is simply:
R IN,dm = 2 × R G
REV. 0
(8)
INPUT COMMON-MODE VOLTAGE RANGE IN SINGLESUPPLY APPLICATIONS
The AD813x is optimized for level-shifting “ground” referenced input signals. For a single-ended input this
would imply, for example, that the voltage at –D IN in Figure 1
would be zero volts when the amplifier’s negative power
supply voltage (at V–) was also set to zero volts.
To compute the total output referred noise for the circuit
of Figure 1, consideration must also be given to the
contribution of the resistors RF and RG. Refer to Table II
for estimated output noise voltage densities at various
closed-loop gains.
499
499
499
499
)






The circuit’s input impedance is effectively higher than it
would be for a conventional op amp connected as an
inverter because a fraction of the differential output voltage
appears at the inputs as a common-mode signal, partially
bootstrapping the voltage across the input resistor RG.
ESTIMATING THE OUTPUT NOISE VOLTAGE
Similar to the case of a conventional op amp, the differential output errors (noise and offset voltages) can be
estimated by multiplying the input referred terms, at +IN
and –IN, by the circuit noise gain. The noise gain is
defined as:
R 
GN = 1+  F 
 RG 


RG
=
RF

 1– 2 × R + R

G
F
The positive and negative outputs of the AD813x are
connected to the respective differential inputs of the
AD9224 via a pair of 49.9 Ω resistors to minimize the effects
of the switched-capacitor front-end of the AD9224. For
best distortion performance it is run from supplies of ±5 V.
(7)
–3–
AN-584
The AD813x can also be configured with unity gain for a
single-ended input-to-differential output. The additional
23 Ω, 522 Ω total, at the input to –IN is to balance the
parallel impedance of the 50 Ω source and its 50 Ω termination that drives the noninverting input.
SINGLE 3 V SUPPLY DIFFERENTIAL A-TO-D DRIVER
Many newer A-to-D converters can run from a single 3 V
supply, which can save significant system power. In order
to increase the dynamic range at the analog input, they
have differential inputs, which doubles the dynamic
range with respect to a single-ended input. An added
benefit of using a differential input is that the distortion
can be improved.
The signal generator has a ground-referenced, bipolar
output, i.e., it drives symmetrically above and below
ground. Connecting VOCM to the CML pin of the AD9224
sets the output common-mode of the AD813x at 2.5 V,
which is the mid-supply level for the AD9224. This voltage is bypassed by a 0.1 µF capacitor.
The low distortion and ability to run from a single 3 V
supply make the AD813x suitable as an A-to-D driver for
some 10-bit, single-supply applications. Figure 4 shows
a schematic of a circuit for an AD813x driving an
AD9203, 10-bit, 40 MSPS A-to-D converter.
The full-scale analog input range of the AD9224 is set to
4 V p-p, by shorting the SENSE terminal to AVSS. This
has been determined to be the scaling to provide minimum harmonic distortion.
3V
3V
10k⍀
0.1␮F
For the AD813x to swing a 4 V p-p, each output swings
2 V p-p, while providing signals that are 180 degrees out
of phase. With a common-mode voltage at the output of
2.5 V, this means that each AD813x output will swing
between 1.5 V and 3.5 V.
1V p-p
+
10␮F
348⍀
10k⍀
60.4⍀
348⍀
20pF
49.9⍀
AD813x
0.1␮F
A ground-referenced 4 V p-p, 5 MHz signal at DIN+ was
used to test the circuit in Figure 3. When the combineddevice circuit was run with a sampling rate of 20 MHz
MSPS, the SFDR (spurious free dynamic range) was
measured at –85 dBc.
20pF
348⍀
60.4⍀
24.9⍀
348⍀
+5V
3V
499⍀
0.1␮F
0.1␮F
499⍀
50⍀
SOURCE
49.9⍀
+
AVDD
VOCM
49.9⍀
AD813x
523⍀
DRVDD
AINN
49.9⍀
AD9203
0.1pF
DIGITAL
OUTPUTS
AINP
499⍀
AVSS
DRVSS
–5V
Figure 4. AD813x Driving AD9203, a 10-Bit 40 MSPS A/D
Converter
+5V
0.1pF
0.1pF
The common-mode of the AD813x output is set at midsupply by the voltage divider connected to VOCM, and ac
bypassed with a 0.1 µF capacitor. This provides for
maximum dynamic range between the supplies at the
output of the AD813x. The 110 Ω resistors at the AD813x
output, along with the shunt capacitors form a one-pole,
low-pass filter for lowering noise and antialiasing.
VINB AVDD DRVDD
AD9224
VINA AVSS
SENSE CML
DRVSS
Figure 5 shows an FFT plot that was taken from the combined devices at an analog input frequency of 2.5 MHz
Figure 3. AD813x Driving an AD9224, a 12-Bit, 40 MSPS
A/D Converter
–4–
REV. 0
AN-584
and a 40 MSPS sampling rate. The performance of the
AD813x compares very favorably with a center-tapped
transformer drive, which has typically been the best way
to drive this A-to-D converter. The AD813x has the
advantage of maintaining dc performance, which a
transformer solution cannot provide.
Figure 6 shows a circuit of an AD813x driving a twistedpair line, like a Category 3 or Category 5 (Cat3 or Cat5),
already installed in many buildings for telephony and
data communications. The characteristic impedance of
such transmission lines is usually about 100 Ω. The outstanding balance of the AD813x output will minimize the
common-mode signal and therefore the amount of EMI
generated by driving the twisted pair.
10
FUND
fS = 40MHz
fIN = 2.5MHz
0
+5V
–10
–20
+
0.1␮F
OUTPUT – dBc
–30
–40
49.9⍀
–50
8
–60
–70
49.9⍀
2ND
–80
5TH
3RD
6TH
4TH
9TH 8TH
7TH
24.9⍀
–90
2
1
3
5
6
AD8130
4
RECEIVER
49.9⍀
+
0.1␮F
–110
10␮F
–5V
0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
20.0
Figure 6. Single-Ended-to-Differential 100 Ω Line Driver
INPUT FREQUENCY – MHz
The two resistors in series with each output terminate
the line at the transmit end. Since the impedances of the
outputs of the AD813x are very low, they can be thought
of as a short circuit, and the two terminating resistors
form a 100 Ω termination at the transmit end of the
transmission line. The receive end is directly terminated
by a 100 Ω resistor across the line.
Figure 5. FFT Response for AD813x Driving AD9203
BALANCED LINE DRIVING
TWISTED-PAIR LINE DRIVER
When driving a twisted-pair cable, it is desirable to drive
only a pure differential signal onto the line. If the signal
is purely differential (i.e., fully balanced), and the transmission line is twisted and balanced, there will be a
minimum radiation of any signal.
This back-termination of the transmission line divides
the output signal by two. The fixed gain-of-two of the
AD813x will create a net unity gain for the system from
end to end.
The complementary electrical fields will mostly be confined to the space between the two twisted conductors
and will not significantly radiate out from the cable. The
current in the cable will create magnetic fields that will
radiate to some degree. However, with each twist, the
two adjacent twists will have an opposite polarity magnetic field. If the twist pitch is tight enough, these small
magnetic field loops will contain most of the magnetic
flux, and the magnetic far-field strength will be negligible.
In this case, the input signal is provided by a signal
generator with an output impedance of 50 Ω. This is terminated with a 49.9 Ω resistor near +DIN of the AD813x.
The effective parallel resistance of the source and termination is 25 Ω. The 24.9 Ω resistor from –DIN to ground
matches the +DIN source impedance and minimizes any
dc and gain errors.
Any imbalance in the differential drive signal will appear
as a common-mode signal on the cable. This is the
equivalent of a single wire that is driven with the common-mode signal. In this case, the wire will act as an
antenna and radiate. Thus, in order to minimize radiation when driving differential twisted-pair cables, the
differential drive signal should be very well balanced.
If +DIN is driven by a low-impedance source over a short
distance, such as the output of an op amp, no termination resistor is required at +DIN. In this case, the –DIN can
be directly tied to ground.
TRANSMIT EQUALIZER
Any length of transmission line will attenuate the signals
it carries. This effect is worse at higher frequencies than
at low frequencies. One way to compensate for this is to
provide an equalizer circuit that boosts the higher frequencies in the transmitter circuit, so that at the receive
end of the cable the attenuation effects are diminished.
The common-mode feedback loop in the AD813x helps
to minimize the amount of common-mode voltage at the
output, and therefore can be used to create a well-balanced
differential line driver.
REV. 0
100⍀ AD8129/
AD813x
–100
–120
10␮F
–5–
AN-584
If the interwinding capacitance (CSTRAY) is assumed to be
uniformly distributed, a signal from the driving source
will couple to the secondary output terminal that is closest
to the primary’s driven side. On the other hand, no signal will be coupled to the opposite terminal of the
secondary, because its nearest primary terminal is not
driven (see Figure 9). The exact amount of this imbalance
will depend on the particular parasitics of the transformer, but will mostly be a problem at higher
frequencies.
By lowering the impedance of the RG component of the
feedback network at higher frequency, the gain can be
increased at high frequency. Figure 7 shows a gain of a
two line driver that has its RGs shunted by 10 pF resistors. The effect of this is shown in the frequency
response plot of Figure 8.
10pF
VIN
49.9⍀
499⍀
49.9⍀
249⍀
249⍀
100⍀
VOUT
49.9⍀
SIGNAL WILL BE COUPLED
ON THIS SIDE VIA CSTRAY
24.9⍀
10pF
499⍀
CSTRAY
Figure 7. Frequency Boost Circuit
VUNBAL
52.3⍀ PRIMARY
20
500⍀
0.005%
500⍀ SECONDARY VDIFF
0.005%
10
CSTRAY
0
NO SIGNAL IS COUPLED
ON THIS SIDE
VOUT/VIN – dB
–10
Figure 9. Transformer Single-Ended-to-Differential
Converter Is Inherently Imbalanced
–20
–30
The balance of a differential circuit can be measured by
connecting an equal-valued resistive voltage divider
across the differential outputs and then measuring the
center point of the circuit with respect ground. Since the
two differential outputs are supposed to be of equal
amplitude, but 180 degrees opposite phase, there
should be no signal present for perfectly balanced outputs.
–40
–50
–60
–70
–80
1
10
100
FREQUENCY – MHz
1000
Figure 8. Frequency Response for Transmit Boost Circuit
The circuit in Figure 9 shows a Minicircuits T1-6T transformer connected with its primary driven single-endedly
and the secondary connected with a precision voltage
divider across its terminals. The voltage divider is made
up of two 500 Ω, 0.005% precision resistors. The voltage
VUNBAL, which is also equal to the ac common-mode voltage, is a measure of how closely the outputs are
balanced.
MISCELLANEOUS APPLICATIONS
Balanced Transformer Driver
Transformers are among the oldest devices that have
been used to perform a single-ended-to-differential conversion (and vice versa). Transformers also can perform
the additional functions of galvanic isolation, step-up or
step-down of voltages, and impedance transformation.
For these reasons, transformers will always find uses in
certain applications.
The plots in Figure 10 show a comparison between the case
where the transformer is driven single-endedly by a signal
generator and driven differentially using an AD813x.
The top signal trace of Figure 10 shows the balance of
the single-ended configuration, while the bottom shows
the differentially driven balance response. The 100 MHz
balance is 35 dB better when using the AD813x.
However, when driving a transformer single-endedly
and then looking at its output, there is a fundamental
imbalance due to the parasitics inherent in the transformer. The primary (or driven) side of the transformer
has one side at dc potential (usually ground), while the
other side is driven. This can cause problems in systems
that require good balance of the transformer’s differential output signals.
–6–
REV. 0
AN-584
a 1N4148 can be used. The cathodes of the two diodes
are connected together and this output node is connected to ground by a 100 Ω resistor.
OUTPUT BALANCE ERROR – dB
0
–20
+5V
VUNBAL, FOR TRANSFORMER
WITH SINGLE-ENDED DRIVE
–40
VIN
–60
RG1
348⍀
RT1
49.9⍀
RT2
24.9⍀
–80
VUNBAL, DIFFERENTIAL DRIVE
RF1
348⍀
RG2
348⍀
5V
HP2835
RF2
348⍀
RL
100⍀
VOUT
–5V
–100
0.3
10k⍀
1
10
FREQUENCY – MHz
100
500
Figure 12. Full-Wave Rectifier
Figure 10. Output Balance Error for Circuits of Figure 9
and Figure 11
The diodes should be operated such that they are
slightly forward-biased when the differential output voltage
is zero. For the Schottky diodes, this is about 400 mV. The
forward biasing can be conveniently adjusted by CR1,
which, in this circuit, raises and lowers VOUT,cm without
creating a differential output voltage.
The well-balanced outputs of the AD813x will provide a
drive signal to each of the transformer’s primary inputs
that are of equal amplitude and 180 degrees out of
phase. Thus, depending on how the polarity of the secondary is connected, the signals that conduct across the
interwinding capacitance will either both assist the
transformer’s secondary signal equally, or both buck the
secondary signals. In either case, the parasitic effect will
be symmetrical and provide a well-balanced transformer output. (See Figure 11.)
One advantage of this circuit is that the feedback loop is
never momentarily opened while the diodes reverse
their polarity within the loop. This is the scheme that is
sometimes used for full-wave rectifiers that use conventional op amps. These conventional circuits do not work
well at frequencies above about 1 MHz.
499⍀
If there is not enough forward bias (VOUT,cm too low), the
lower sharp cusps of the full-wave rectified output
waveform will be rounded off. Also, as the frequency
increases, there tends to be some rounding of the lower
cusps. The forward bias can be increased to yield
sharper cusps at higher frequencies.
CSTRAY
49.9⍀
499⍀
+IN
OUT–
VUNBAL
AD813x
499⍀
–IN
49.9⍀
500⍀
0.005%
VDIFF
500⍀
0.005%
OUT+
CSTRAY
There is not a reliable, entirely quantifiable, means to
measure the performance of a full-wave rectifier. Since
the ideal waveform has periodic sharp discontinuities, it
should have (primarily even) harmonics that have no
upper bound on the frequency. However, for a practical
circuit, as the frequency increases, the higher harmonics
become attenuated and the sharp cusps that are present
at low frequencies become significantly rounded.
499⍀
Figure 11. AD813x Forms a Balanced Transformer Driver
Full-Wave Rectifier
The balanced outputs of the AD813x, along with a
couple of Schottky diodes, can create a very high-speed
full-wave rectifier. Such circuits are useful for measuring
ac voltages and other computational tasks.
The circuit was run at a frequency up to 300 MHz and,
while it was still functional, the major harmonic that
remained in the output was the second. This made it
look like a sine wave at 600 MHz. Figure 13 is an oscilloscope plot of the output when driven by a 100 MHz,
2.5 V p-p input.
Figure 12 shows the configuration of such a circuit. Each
of the AD813x outputs drives the anode of an HP2835
Schottky diode. These Schottky diodes were chosen for
their high-speed operation. At lower frequencies (approximately lower than 10 MHz), a silicon signal diode such as
REV. 0
CR1
–7–
Figure 14 is a schematic of a low-pass, multiple feedback
filter. The active section contains two poles, and an
additional pole is added at the output. The filter was
designed to have a –3 dB frequency of 1 MHz. The actual
–3 dB frequency was measured to be 1.12 MHz as shown
in Figure 15.
Sometimes a second harmonic generator is actually
useful, such as creating a clock to oversample a DAC by
a factor of two. If the output of this circuit is run
through a low-pass filter, it can be used as a second harmonic generator.
1V
2.15k⍀
33pF
2k⍀
VIN
49.9⍀
549⍀
953⍀
100pF
100pF
200pF
VOUT
953⍀
2k⍀
200pF
33pF
24.9⍀
E02656–0–2/02(0)
AN-584
549⍀
2.15k⍀
Figure 14. 1 MHz, 3-Pole Differential Output Low-Pass
Multiple Feedback Filter
100mV
2ns
10
Figure 13. Full-Wave Rectifier Response with
100 MHz Input
0
–10
Differential Filtering Applications
Similar to an op amp, various types of active filters can
be created with the AD813x. These can have singleended inputs and differential outputs, which can
provide an antialias function when driving a differential
A/D converter.
VOUT/VIN – dB
–20
–30
–40
–50
–60
–70
–80
–90
10k
100k
1M
FREQUENCY – Hz
10M
100M
PRINTED IN U.S.A.
Figure 15. Frequency Response of 1 MHz Low-Pass Filter
–8–
REV. 0
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