AD AD5539S Ultrahigh frequency operational amplifier Datasheet

a
Ultrahigh Frequency
Operational Amplifier
AD5539
FEATURES
Improved Replacement for Signetics SE/NE5539
CONNECTION DIAGRAM
Plastic DIP (N) Package
or Cerdip (Q) Package
AC PERFORMANCE
Gain Bandwidth Product: 1.4 GHz typ
Unity Gain Bandwidth: 220 MHz typ
High Slew Rate: 600 V/ms typ
Full Power Response: 82 MHz typ
Open-Loop Gain: 47 dB min, 52 dB typ
DC PERFORMANCE
All Guaranteed DC Specifications Are 100% Tested
For Each Device Over Its Full Temperature
Range – For All Grades and Packages
VOS: 5 mV max Over Full Temperature Range
(AD5539S)
IB: 20 mA max (AD5539J)
CMRR: 70 dB min, 85 dB typ
PSRR: 100 mV/V typ
MIL-STD-883B Parts Available
PRODUCT DESCRIPTION
The AD5539 is an ultrahigh frequency operational amplifier designed specifically for use in video circuits and RF amplifiers.
Requiring no external compensation for gains greater than 5, it
may be operated at lower gains with the addition of external
compensation.
As a superior replacement for the Signetics NE/SE5539, each
AD5539 is 100% dc tested to meet all of its guaranteed dc
specifications over the full temperature range of the device.
PRODUCT HIGHLIGHTS
1. All guaranteed dc specifications are 100% tested.
2. The AD5539 drives 50 Ω and 75 Ω loads directly.
3. Input voltage noise is less than 4 nV√Hz.
4. Low cost RF and video speed performance.
5. ± 2 volt output range into a 150 Ω load.
6. Low cost.
7. Chips available.
The high slew rate and wide bandwidth of the AD5539 provide
low cost solutions to many otherwise complex and expensive
high frequency circuit design problems.
The AD5539 is available specified to operate over either the
commercial (AD5539JN/JQ) or military (AD5539SQ) temperature range. The commercial grade is available either in 14-pin
plastic or cerdip packages. The military version is supplied in
the cerdip package. Chip versions are also available.
REV. B
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
AD5539–SPECIFICATIONS (@ +258C and V = 68 V dc, unless otherwise noted)
S
Parameter
INPUT OFFSET VOLTAGE
Initial Offset1
TMIN to TMAX
INPUT OFFSET CURRENT
Initial Offset2
TMIN to TMAX
INPUT BIAS CURRENT
Initial2
VCM = 0
Either Input
TMIN to TMAX
FREQUENCY RESPONSE
RL = 150 Ω3
Small Signal Bandwidth
ACL = 24
Gain Bandwidth Product
ACL = 26 dB
Full Power Response
ACL = 24
ACL = 7
ACL = 20
Settling Time (1%)
Slew Rate
Large Signal Propagation Delay
Total Harmonic Distortion
RL = ∞
RL = 100 Ω3
VOUT = 2 V p–p
ACL = 7, f = 1 kHz
INPUT IMPEDANCE
OUTPUT IMPEDANCE (f <10 MHz)
INPUT VOLTAGE RANGE
Differential5
(Max Nondestructive)
Common-Mode Voltage
(Max Nondestructive)
Common-Mode Rejection Ratio
∆VCM = 1.7 V
RS = 100 Ω
TMIN to TMAX
INPUT VOLTAGE NOISE
Wideband RMS Noise (RTI)
BW = 5 MHz; RS = 50 Ω
Spot Noise
F = 1 kHz; RS = 50 Ω
OPEN-LOOP GAIN
VO = +2.3 V, –1.7 V
RL = 150 Ω3
RL = 2 kΩ
TMIN to TMAX –RL = 2 kΩ
Min
AD5539J
Typ
Max
Min
AD5539S
Typ
47
47
43
Units
2
5
6
2
3
5
mV
mV
0.1
2
5
0.1
1
3
µA
µA
6
20
6
13
µA
25
µA
40
70
60
Max
220
220
MHz
1400
1400
MHz
68
82
65
12
600
4
68
82
65
12
600
4
MHz
MHz
MHz
ns
V/µs
ns
0.010
0.016
0.010
0.016
%
%
100
2
100
2
kΩ
Ω
250
250
mV
2.5
2.5
V
85
dB
dB
5
5
FV
4
4
nV√Hz
85
70
60
52
58
58
63
–2–
47
48
46
52
58
57
60
dB
dB
dB
REV. B
AD5539
AD5539J
Parameter
Min
Typ
OUTPUT CHARACTERISTICS
Positive Output Swing
RL = 150 Ω3
+2.3
+2.8
RL = 2 kΩ
+2.3
+3.3
TMIN to TMAX with
RL = 2 kΩ
+2.3
Negative Output Swing
RL = 150 Ω3
–2.2
RL = 2 kΩ
–2.9
TMIN to TMAX with
RL = 2 kΩ
POWER SUPPLY (No Load, No Resistor to –VS)
Rated Performance
±8
Operating Range
64.5
Quiescent Current
Initial ICC+
14
TMIN to TMAX
Initial ICC–
11
TMIN to TMAX
PSRR
Initial
100
TMIN to TMAX
TEMPERATURE RANGE
Operating,
Rated Performance
Commercial (0°C to +70°C)
AD5539JN, AD5539JQ
Military (–55°C to +125°C)
PACKAGE OPTIONS
Plastic (N-14)
AD5539JN
Cerdip (Q-14)
AD5539JQ
J and S Grade Chips Available
Max
Min
AD5539S
Typ
+2.3
+2.5
+2.8
+3.3
–1.7
–1.7
V
–2.2
–2.9
–1.5
610
64.5
–3–
Units
V
V
+2.3
±8
18
20
15
17
14
1000
2000
100
11
–1.7
–2.0
V
V
–1.5
V
610
V
V
17
18
14
15
mA
mA
mA
mA
1000
2000
µV/V
µV/V
AD5539SQ
AD5539SQ, AD5539SQ/883B
NOTES
1
Input Offset Voltage specifications are guaranteed after 5 minutes of operation at T A = +25°C.
2
Bias Current specifications are guaranteed maximum at either input after 5 minutes of operation at T A = +25°C.
3
RX = 470 Ω to –VS.
4
Externally compensated.
5
Defined as voltage between inputs, such that neither exceeds +2.5 V, –5.0 V from ground.
Specifications subject to change without notice.
Specifications in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing
quality levels. All min and max specifications are guaranteed, although only those shown in boldface are tested on all production units.
REV. B
Max
AD5539
ABSOLUTE MAXIMUM RATINGS *
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 10 V
Internal Power Dissipation . . . . . . . . . . . . . . . . . . . . . 550 mW
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . +2.5 V, –5.0 V
Differential Input Voltage . . . . . . . . . . . . . . . . . . . . . . 0.25 V
Storage Temperature Range (Q) . . . . . . . . . –65°C to +150°C
Storage Temperature Range (N) . . . . . . . . . –65°C to +125°C
Operating Temperature Range
AD5539JN . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
AD5539JQ . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
AD5539SQ . . . . . . . . . . . . . . . . . . . . . . . –55°C to +125°C
Lead Temperature Range (Soldering 60 Seconds) . . . +300°C
*
OFFSET NULL CONFlGURATION
Stresses above those listed under “Absolute Maximum Ratings” may cause
permanent damage to the device. This is a stress rating only and functional
operation of the device at these or any other conditions above those indicated in
the operational section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.
METALIZATION PHOTOGRAPH
Dimensions shown in inches and (mm).
Contact factory for latest dimensions.
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD5539 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
–4–
WARNING!
ESD SENSITIVE DEVICE
REV. B
Typical Characteristics—AD5539
Figure 1. Output Voltage Swing
vs. Supply Voltage
Figure 2. Output Voltage Swing
vs. Load Resistance
Figure 3. Maximum CommonMode Voltage vs. Supply Voltage
Figure 4. Positive Supply Current
vs. Supply Voltage
Figure 5. Input Voltage vs. Output
Voltage for Various Temperatures
Figure 6. Low Frequency Input
Noise vs. Frequency
Figure 7. Common-Mode
Rejection Ratio vs. Frequency
REV. B
Figure 8. Harmonic Distortion
vs. Frequency – Low Gain
–5–
Figure 9. Harmonic Distortion
vs. Frequency – High Gain
AD5539
Figure 10. Full Power Response
Figure 11. Deviation from Ideal Gain
vs. Closed-Loop Voltage Gain
Figure 12. AD5539 Circuit
some special precautions are in order. All real-world applications circuits must be built using proper RF techniques: the use
of short interconnect leads, adequate shielding, groundplanes,
and very low profile IC sockets. In addition, very careful bypassing of power supply leads is a must.
FUNCTIONAL DESCRIPTION
The AD5539 is a two-stage, very high frequency amplifier.
Darlington input transistors Q1, Q4–Q2, Q3 form the first
stage—a differential gain amplifier with a voltage gain of approximately 50. The second stage, Q5, is a single-ended amplifier whose input is derived from one phase of the differential
amplifier output; the other phase of the differential output is
then summed with the output of Q5. The all NPN design of the
AD5539 is configured such that the emitter of Q5 is returned,
via a small resistor to ground; this eliminates the need for separate level shifting circuitry.
Low-impedance transmission line is frequently used to carry signals at RF frequencies: 50 Ω line for telecommunications purposes and 75 Ω for video applications. The AD5539 offers a
relatively low output impedance; therefore, some consideration
must be given to impedance matching. A common matching
technique involves simply placing a resistor in series with the
amplifier output that is equal to the characteristic impedance of
the transmission line. This provides a good match (although at a
loss of 6 dB), adequate for many applications.
The output stage, consisting of transistors Q9 and Q10, is a
Darlington voltage follower with a resistive pull-down. The bias
section, consisting of transistors Q6, Q7 and Q8, provides a
stable emitter current for the input section, compensating for
temperature and power supply variations.
All of the circuits here were built and tested in a 50 Ω system.
Care should be taken in adapting these circuits for each particular use. Any system which has been properly matched and terminated in its characteristic impedance should have the same
small signal frequency response as those shown in this
data sheet.
SOME GENERAL PRINCIPLES OF HIGH FREQUENCY
CIRCUIT DESIGN
In designing practical circuits with the AD5539, the user must
remember that whenever very high frequencies are involved,
–6–
REV. B
AD5539
APPLYING THE AD5539
The AD5539 is stable for closed-loop gains of 4 or more as an
inverter and at (noise) gains of 5 or greater as a voltage follower.
This means that whenever the AD5539 is operated at noise
gains below 5, external frequency compensation must be used to
insure stable operation.
The following sections outline specific compensation circuits
which permit stable operation of the AD5539 down to follower
(noise) gains of 3 (inverting gains of 2) with corresponding
–3 dB bandwidths up to 390 MHz. External compensation is
achieved by modifying the frequency response to the AD5539’s
external feedback network (i.e., by adding lead-lag compensation) so that the amplifier operates at a noise gain of 5 (or more)
at frequencies over 44 MHz, independent of signal gain.
when operating at a noise gain of 7. Under these conditions, excess phase shift causes nearly 10 dB of peaking at 150 MHz.
Figure 15 illustrates the use of both lead and lag compensation
to permit stable low-gain operation. The AD5539 is shown connected as an inverting amplifier with the required external components added to provide stability and improve high frequency
response. The stray capacitance between the amplifier summing
junction and ground, CX, represents whatever capacitance is associated with the particular type of op amp package used plus
the stray wiring capacitance at the summing junction.
Evaluating the lead capacitance first (ignoring RLAG and CLAG
for now): the feedback network, consisting of R2 and CLEAD, has
a pole frequency equal to:
1
FA = 2 π C
+
CX
LEAD
(
) ( R1||R2)
(1)
and a zero frequency equal to:
1
FB = 2 π R1 × C
LEAD
(
)
(2)
Usually, frequency FA is made equal to FB; that is, (R1CX) =
(R2 CLEAD), in a manner similar to the compensation used for
an attenuator or scope probe. However, if the pole frequency,
FA, will lie above the unity gain crossover frequency (440 MHz),
then the optimum location of FB will be near the crossover
Figure 13. Small Signal Open-Loop Gain and
Phase vs. Frequency
GENERAL PRINCIPLES OF LEAD AND LAG
COMPENSATION
The AD5539 has its first pole or breakpoint in its open-loop frequency response at about 10 MHz (see Figure 13). At frequencies beyond 100 MHz, phase shift increases such that the output
lags the input by 180°—well before the unity gain crossover frequency. Therefore, severe peaking (and possible oscillation) will
result if the AD5539 is operated at noise gains below 5, unless
external compensation is employed. Figure 14 shows the uncompensated closed-loop frequency response of the AD5539
Figure 15. Inverting Amplifier Model Showing Both Lead
and Lag Compensation
Figure 16. A Model of the Feedback Network of the
Inverting Amplifier
Figure 14. AD5539 Uncompensated Response, ClosedLoop Gain = 7
REV. B
–7–
AD5539
frequency. Both of these circuit techniques add a large amount
of leading phase shift at the crossover frequency, greatly aiding
stability.
The lag network (RLAG, CLAG) increases the feedback attenuation, i.e., the amplifier operates at a higher noise gain, above
some frequency, typically one-tenth of the crossover frequency.
As an example, to achieve a noise gain of 5 at frequencies above
44 MHz, for the circuit of Figure 15, would require a network
of:
RLAG =
and . . .
C LAG =
R1
(4R1 / R2) – 1
1
(
2 π RLAG 44 × 106
(3)
)
(4)
Figure 18. Response of the (Figure 17) Inverter Circuit
without a Lag Compensation Network
It is worth noting that an RLAG resistor may be used alone, to increase the noise gain above 5 at all frequencies. However, this
approach has the disadvantage of also increasing the dc offset
and low frequency noise errors by an amount equal to the increase in gain, in this case, by a factor of 5.
A lag network (Figure 15) can be added to improve the response
of this circuit even further as shown in Figures 19 and 20. In almost all cases, it is imperative to make capacitor CLEAD adjustable; in some cases, CLAG must also be variable. Otherwise,
component and circuit capacitance variations will dominate circuit performance.
SOME PRACTICAL CIRCUITS
The preceding general principles may now be applied to some
actual circuits.
A General Purpose Inverter Circuit
Figure 17 is a general purpose inverter circuit operating at a
gain of –2.
For this circuit, the total capacitance at the inverting input is approximately 3 pF; therefore, CLEAD from Equations 1 and 2
needs to be approximately 1.5 pF. As shown in Figure 17, a
small trimmer is used to optimize the frequency response of this
circuit. Without a lag compensation network, the noise gain of
the circuit is 3.0 and, as shown in Figure 18, the output amplitude remains within ± 0.5 dB to 170 MHz and the –3 dB bandwidth is 200 MHz.
Figure 19. Response of the (Figure 17) Inverter Circuit
with an RLAG Compensation Network Employed
Figure 17. A General Purpose Inverter Circuit
Figure 20. Response of the (Figure 17) Inverter Circuit
with an RLAG and a CLAG Compensation Network
Employed
–8–
REV. B
AD5539
Figures 21 and 22 show the small and large signal pulse responses of the general purpose inverter circuit of Figure 17, with
CLEAD = 1.5 pF, RLAG = 330 Ω and CLAG = 3.5 pF.
Figure 21. Small Signal Pulse Response of the (Figure 17)
Inverter Circuit; Vertical Scale: 50 mV/div; Horizontal
Scale: 5 ns/div
Figure 23. A Gain of 2 Inverter Circuit with the CLEAD
Capacitor Connected to Pin 12
Figure 22. Large Signal Response of the (Figure 17)
Inverter Circuit; Vertical Scale: 200 mV/div, Horizontal
Scale: 5 ns/div
A CLEAD capacitor may be used to limit the circuit bandwidth
and to achieve a single pole response free of overshoot


1
 –3 dB frequency = 2 π R2 C


LEAD 
If this option is selected, it is recommended that a CLEAD be
connected between Pin 12 and the summing junction, as shown
in Figure 23. Pin 12 provides a separately buffered version of
the output signal. Connecting the lead capacitor here avoids the
excess output-stage phase shift and subsequent oscillation problems (at approx. 350 MHz) which would otherwise occur when
using the circuit of Figure 17 with a CLEAD of more than about
2 pF.
Figure 24 shows the response of the circuit of Figure 23 for each
connection of CLEAD. Lag components may also be added to this
circuit to further tailor its response, but, in this case, the results
will be slightly less satisfactory than connecting CLEAD directly
to the output, as was done in Figure 17.
REV. B
Figure 24. Response of the Circuit of Figure 23 with
CLEAD = 10 pF
A General Purpose Voltage Follower Circuit
Noninverting (voltage follower) circuits pose an additional complication, in that when a lag network is used, the source impedance will affect the noise gain. In addition, the slightly greater
bandwidth of the noninverting configuration makes any excess
phase shift due to the output stage more of a problem.
For example, a gain of 3 noninverting circuit with CLEAD connected normally (across the feedback resistor – Figure 25) will
require a source resistance of 200 Ω or greater to prevent UHF
oscillation; the extra source resistance provides some damping
as well as increasing the noise gain. The frequency response plot
of Figure 26 shows that the highest –3 dB frequency of all the
applications circuits can be achieved using this connection, unfortunately, at the expense of a noise gain of 14.2.
–9–
AD5539
Figure 28. Response of the Gain of 3 Follower with CLEAD,
CLAG and RLAG
These same principles may be applied when capacitor CLEAD is
connected to Pin 12 (Figure 29). Figure 30 shows the bandwidth of the gain of 3 amplifier for various values of RLAG. It can
be seen from these response plots that a high noise gain is still
needed to achieve a reasonably flat response (the smaller the
Figure 25. A Gain of 3 Follower with Both Lead and Lag
Compensation
Figure 26. Response of the Gain of 3 Follower Circuit
Adding a lag capacitor (Figure 27) will greatly reduce the
midband and low frequency noise gain of the circuit while sacrificing only a small amount of bandwidth as shown in Figure 28.
Figure 27. A Gain of 3 Follower Circuit with Both CLEAD
and RLAG Compensation
Figure 29. A Gain of 3 Follower Circuit with CLEAD
Compensation Connected to Pin 12
Figure 30. Response of the Gain of 3 Follower Circuit with
CLEAD Connected to Pin 12
–10–
REV. B
AD5539
value of RLAG, the higher the noise gain). For example, with a
220 Ω RLAG and a 50 Ω source resistance, the noise gain will be
12.8, because the source resistance affects the noise gain.
Figures 31 and 32 show the small and large signal responses of
the circuit of Figure 29.
Figure 33. A 20 dB Gain Video Amplifier for 75 Ω Systems
Figure 31. The Small-Signal Pulse Response of the Gain
of 3 Follower Circuit with RLAG and CLEAD Compensation to
Pin 12; Vertical Scale: 50 mV/div; Horizontal Scale:
5 ns/div
Figure 34. Response of the 20 dB Video Amplifier
Figure 32. The Large-Signal Pulse Response of the Gain
of 3 Follower Circuit with RLAG and CLEAD Compensation to
Pin 12; Vertical Scale: 200 mV/div; Horizontal Scale:
5 ns/div
A Video Amplifier Circuit with 20 dB Gain (Terminated)
High gain applications (14 dB and up) require only a small lead
capacitance to obtain flat response. The 26 dB (20 dB terminated) video amplifier circuit of Figure 33 has the response
shown in Figure 34 using only approximately 0.5-1 pF lead capacitance. Again, a small CLEAD can be connected, either to the
output or to Pin 12 with very little difference in response.
REV. B
In color video applications, the quality of differential gain and
differential phase response is very important. Figures 35 and 36
show a circuit and test setup to measure the AD5539’s response
to a modulated ramp signal (0-90 IRE p-p ramp, 40 IRE p-p
modulation, 4.4 MHz).
Figures 37 and 38 show the differential gain and phase response.
–11–
AD5539
Figure 38. Differential Phase vs. Ramp Amplitude
Figure 35. Differential Gain and Phase Measurement
Circuit
MEASURING AD5539 SETTLING TIME
Measuring the very rapid settling times associated with AD5539
can be a real problem for the designer; proper component layout
must be used and appropriate test equipment selected. In addition, both cable dispersion (a function of cable losses) and the
quality of termination (SWR) directly affect the measurement.
The circuit of Figure 39 was used to make a “brute force”
AD5539 settling time measurement. The fixture containing the
circuit was connected directly—using a male BNC connector
(but no cable)—onto the front of a 50 Ω input oscilloscope
preamp. A digital mainframe was then used to capture, average,
and expand the error signal. Most of the small-scale waveform
aberrations shown on the figure were caused by the oscilloscope
itself, especially the glitch at 15 ns. The pulse source used for
this measurement was an EH-SPG2000 pulse generator set for a
1 ns rise-time; it was coupled directly to the circuit using 18" of
microwave 50 Ω hard line.
Figure 36. Differential Gain and Phase Test Setup
Figure 37. Differential Gain vs. Ramp Amplitude
Figure 39. AD5539 Settling Time Test Circuit
–12–
REV. B
AD5539
APPLICATIONS SUMMARY CHART
R1
Gain = –1 to –5
Circuit of Fig. 17
Gain = –1 to –5
Circuit of Fig. 23
Gain = +2 to +53
Circuit of Fig. 27
Gain = +2 to +54
Circuit of Fig. 29
R21
R2
G
2k
R2
G
2k
R2
G –1
2k
CLAG2
RLAG
≤
≤
≤
R1
R1
–1
4
R2
≥
R1
R1
–1
4
R2
≥
R1
R1
–1
10
R2
≥
CLEAD2
(
1
(
1
(
1
2 π 44 × 10
2 π 44 × 10
2 π 44 × 10
6
6
6
)R
)R
)R
GAIN
GAIN
FLATNESS
(TRIMMED)
3 dB
BANDWIDTH
≈
3pF
G
–2
± 0.2 dB
200 MHz
≈
3pF
G
–2
± 1 dB
180 MHz
≈
3 pF
G –1
+3
± 1 dB
390 MHz
≈
3 pF
G –1
+3
± 0.5 dB
340 MHz
LAG
LAG
LAG
R2
G –1
2k
Gain < –5
R2
G
1.5 k
NA
NA
Trimmer5
–20
± 0.2 dB
80 MHz
Gain > +5
R2
G –1
1.5 k
NA
NA
Trimmer5
+20
± 0.2 dB
80 MHz
≤
R1
R1
–1
10
R2
NA
NOTES
G = Gain NA = Not Applicable
1
Values given for specific results summarized here—applications can be adapted for values different than those specified.
2
It is recommended that C LEAD and C LAG be trimmers covering a range that includes the computed value above.
3
RSOURCE ≥ 200 Ω.
4
RSOURCE ≥ 50 Ω.
5
Use Voltronics CPA2 0.1–2.5 pF Teflon Trimmer Capacitor (or equivalent).
The photos of Figures 40 and 41 demonstrate how the AD5539
easily settles to 1% (1 mV) in less than 12 ns; settling to 0.1%
(100 µV) requires less than 25 ns.
Figure 40. Error Signal from AD5539 Settling Time Test
Circuit – Falling Edge. Vertical Scale: 5 ns/div.; Horizontal
Scale: 500 µ V/div
REV. B
Figure 41. Error Signal from AD5539 Settling Time Test
Circuit – Rising Edge. Vertical Scale: 5 ns/div.; Horizontal
Scale: 500 µ V/div
–13–
AD5539
Figure 42 shows the oscilloscope response of the generator
alone, set up to simulate the ideal test circuit error signal
(Figure 43).
Figure 42. The Oscilloscope Response Alone Directly
Driven by the Test Generator. Vertical Scale: 5 ns/div.;
Horizontal Scale: 500 µ V/div
Figure 43. A Simulated Ideal Test Circuit Error Signal
A 50 MHz VOLTAGE-CONTROLLED AMPLIFIER
Figure 44 is a circuit for a 50 MHz voltage-controlled amplifier
(VCA) suitable for use in high quality video-speed applications.
This circuit uses the AD5539 as an output amplifier for the
AD539, a high bandwidth multiplier. The outputs from the two
signal channels of the AD539 are applied to the op amp in a
subtracting configuration. This connection has two main advantages: first, it results in better rejection of the control voltage,
particularly when over-driven (VX < 0 or VX > 3.3 V). Secondly,
it provides a choice of either noninverting or inverting responses,
using either input VY1 or VY2, respectively. In this circuit, the
output of the op amp will equal:
VOUT =
(
V X VY 1 – VY 2
2V
) for V
X
>0
Hence, the gain is unity at VX = +2 V. Since VX can overrange
to +3.3 V, the maximum gain in this configuration is about
4.3 dB. (Note: If Pin 9 of the AD539 is grounded, rather than
connected to the output of the 5539N, the maximum gain becomes 10 dB.)
The bandwidth of this circuit is over 50 MHz at full gain, and is
not substantially affected at lower gains. Of course, when VX is
zero (or slightly negative, to override the residual input offset)
there is still a small amount of capacitive feedthrough at high
frequencies; therefore, extreme care is needed in laying out the
PC board to minimize this effect. Also, for small values of VX,
the combination of this feedthrough with the multiplier output
can cause a dip in the response where they are out of phase.
Figure 45 shows the ac response from the noninverting input,
with the response from the inverting input, VY2, essentially identical. Test conditions: VY1 = 0.5 V rms for values of VX from
+10 mV to +3.16 V; this is with a 75 Ω load on the output. The
feedthrough at VX = –10 mV is also shown.
Figure 45. AC Response of the VCA at Different Gains
VY = 0.5 V RMS
Figure 44. A Wide Bandwidth Voltage-Controlled Amplifier
–14–
REV. B
AD5539
The transient response of the signal channel at VX = +2 V,
VY = VOUT = + or –1 V is shown in Figure 46; with the VCA
driving a 75 Ω load. The rise and fall times are both approximately 7 ns.
A few final circuit details: in general, the control amplifier compensation capacitor for Pin 2, CC, must have a minimum value
of 3000 pF (3 nF) to provide both circuit stability and maximum
control bandwidth. However, if the maximum control bandwidth
is not needed, then it is advisable to use a larger value of CC,
with typical values between 0.01 and 0.1 µF. Like many aspects
of design, the value of CC will be a tradeoff: higher values of CC
will lower the high frequency distortion, reduce the high frequency crosstalk and improve the signal channel phase response.
Conversely, lower values of CC will provide a higher control
channel bandwidth at the expense of degraded linearity in the
output response when amplitude modulating a carrier signal.
The control channel bandwidth will vary in inverse proportion to
the value of CC, providing a typical bandwidth of 2 MHz with a
CC of 0.01 µF and a VX voltage of +1.7 volts.
Both the bandwidth and pulse response of the control channel
can be further increased by using a feedforward capacitor, Cff,
with a value between 5 and 20 percent of CC. Cff should be carefully adjusted to give the best pulse response for a particular step
input applied to the control channel. Note that since Cff is connected between a linear control input (Pin 1) and a logarith-
Figure 46. Transient Response of the Voltage-Controlled
Amplifier VX = +2 Volts, VY = ± 1 Volt
mic node, the settling time of the control channel with a pulse
input will vary with different control input step levels.
Diode D1 clamps the logarithmic control node at Pin 2 of the
AD539, (preventing this point from going too negative); this
diode helps decrease the circuit recovery time when the control
input goes below ground potential.
THE AD539/5539 COMBINATION AS A FAST, LOW
FEEDTHROUGH, VIDEO SWITCH
Figure 47 shows how the AD539/5539 combination can be used
to create a fast video speed switch suitable for many high fre-
Figure 47. An Analog Multiplier Video Switch
REV. B
–15–
quency applications including color key switching. It features
both inverting and noninverting inputs and can provide an output of ± 1 V into a reverse-terminated 75 Ω load (or ± 2 V into
150 Ω). An optional output offset adjustment is provided. The
input range of the video switch is the same as the output range:
± 1 V at either input generates ± 1 V (noninverting) or 71 V
(inverting) across the 75 Ω load. The circuit provides a gain of
about 1, when “ON,” or zero when “OFF.”
The differential-gain and differential-phase characteristics of
this switch are compatible with video applications. The incremental gain changes less than 0.05 dB over a signal window of 0
to +1 V, with a phase variation of less than 0.5 degree at the
subcarrier frequency of 3.58 MHz. The noise level of this circuit measured at the 75 Ω load is typically 200 µV in a 0 MHz
to 5 MHz bandwidth or approximately 100 nV per root hertz.
The noise spectral density is essentially flat to 40 MHz.
The differential configuration uses both channels of the AD539
not only to provide alternative input phases, but also to eliminate the switching pedestal due to step changes in the output
current as the AD539 is gated on or off.
The waveforms shown in Figures 48 and 49 were taken across a
75 Ω termination; in both photos, the signal of 0 to +1 V (in
this case, an offset sine wave at 1 MHz) was applied to the
noninverting input. In Figure 48, the envelope response shows
the output being fully switched in about 50 ns. Note that the
output is ON when the control input is zero (or more negative)
and OFF for a control input of +1 V or more. There is very
little control-signal breakthrough.
Figure 49 shows the response to a pulse of 0 to +1 V on the
signal channel. With the control input held at zero, the rise
time is under 10 ns. The response from the inverting input
is similar.
C1044a–10–2/88
AD5539
Figure 49. The Signal Response of the Video Switcher
Figure 48. The Control Response of the Video Switcher
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
14-Pin Plastic DIP Package
(N-14)
PRINTED IN U.S.A.
14-Pin Cerdip Package
(Q-14)
–16–
REV. B