Dec 2002 2500V/µs Slew Rate Op Amps Process Large Signals with Low Distortion at High Frequencies

DESIGN FEATURES
2500V/µs Slew Rate Op Amps Process
Large Signals with Low Distortion at
High Frequencies
by Kris Lokere and Glen Brisebois
Introduction
The LT1818 and LT1819 are low distortion single and dual operational
amplifiers that offer 400MHz gain
bandwidth product and 2500V/µs
slew rate. The parts operate with supplies from ±2V to ±6V and draw a
typical supply current of only 9mA
per amplifier.
The amplifiers can drive 100Ω loads
with a low distortion of –85dBc relative to a 5MHz, 2VP–P signal. In single
5V supply applications, the output
swings to 0.8V from either supply rail
with a 500Ω load (to 2.5V), and to
1.0V with a 100Ω load. The output
current drive capability is typically
70mA. The low distortion and good
output drive capability, combined with
the 6nV/√Hz input voltage noise,
make the LT1818/LT1819 an ideal
choice for a wide variety of applications.
The LT1818 and LT1819 are available in space saving packages: the
LT1818 single op amp in an SOT23-5;
the LT1819 dual op amp in an 8-lead
MSOP. Both parts are also available
in an easy to use 8-lead SOIC. All
parts are fully specified at ±5V and
single 5V supplies, and are available
in commercial and industrial temperature grades.
Table 1 summarizes the performance specifications of the LT1818
and LT1819.
499Ω
–
499Ω
VIN
VOUT
LT1818
499Ω
+
Figure 1. Test circuit for LT1818 (or
competitor’s part) configured in gain of –1.
Linear Technology Magazine • December 2002
2V/DIV
2V/DIV
5ns/DIV
5ns/DIV
Figure 2a. The LT1818 responds to a
20MHz, ±4V input pulse with a fast
enough slew rate to retain the shape
of the pulse
Fast Slew Rate Preserves
High Bandwidth for Large
Signals
The LT1818/LT1819 amplifiers exhibit an ultrafast slew rate of 2500V/
µs in a gain of +1 and 1800V/µs in a
gain of –1. The importance of a high
slew rate is demonstrated by comparing the large signal dynamic response
of amplifiers with similar bandwidths
but differing slew rates. We compared
the LT1818 to a competitor’s ampli-
Figure 2b. Competitor’s 625MHz op
amp responds to the same input with
inadequate slew rate to maintain
pulse integrity
fier, which has a wider, 625MHz bandwidth, but only a 400V/µs slew rate.
Figure 1 shows a test circuit for a
device configured in an inverting gain
(AV = –1). Figures 2a and 2b show the
step response to a 20MHz, ±4V input
pulse. The 3.5ns rise/fall time of the
LT1818 (consistent with a 1800V/µs
slew rate) preserves the 20MHz step
very well. The lower slew rate amplifier, in contrast, is only barely fast
enough to transmit the waveform,
Table 1: LT1818/LT1819 performance summary
(all specifications are typical with ±5V supplies and 25°C unless otherwise noted)
Parameter
Value
Gain Bandwidth Product
400MHz
Slew Rate
2500V/µs
Supply Current (per Amplifier)
9mA
Harmonic Distortion (5MHz, 2VP–P, RL=100Ω)
–85dBc
Input Noise Voltage
6nV/√Hz
Input Noise Current
1.2pA /√Hz
Input Offset Voltage (Max)
1.5mV
Input Bias Current (Max)
±8µA
Input Common Mode Range
±4.2V
Output Voltage Swing (RL=500Ω)
±4.1V
Settling Time (5V, ±0.1%)
9ns
11
DESIGN FEATURES
Low Distortion ADC Driver
1V/DIV
1V/DIV
5ns/DIV
5ns/DIV
Figure 3a. LT1818 response to 50MHz,
4VP–P sine wave maintains good
fidelity.
turning it into a triangle wave. The
difference in slew rate also has important ramifications for distortion of
sine waves. Figures 3a and 3b show
the response of both amplifiers to a
50MHz, 4VP–P sine wave. The LT1818
transmits the waveform with minimal
distortion, but the competitor’s part
turns the sine wave into a triangle
wave, a result of the lower slew rate.
This demonstrates that, to accurately
process real world signals, slew rate
is often the limiting parameter rather
than bandwidth.
+
VIN
+
1/2 LT1819
–
1/2 LT1819
–
432Ω
432Ω
200Ω
200Ω
3pF
Figure 4. Dual op amp cascaded to form a
gain of 10 (20dB).
VOUT
Figure 3b. Competitor’s 625MHz
response to the same sine wave is
distorted.
The application shown in Figure 4
demonstrates the excellent high-frequency response of the LT1819 dual
amplifier, and how two op amps can
be combined to further increase bandwidth. The circuit shows the dual
LT1819 op amp configured as two
cascaded gain stages that together
form a gain of 10 (= 20dB). The feedback capacitor on the second stage
serves to cancel a pole formed by the
feedback resistors and the input capacitance, to reduce peaking and
ringing. The frequency-domain response (Figure 5) shows the –3dB
frequency at 80MHz, which represents a gain-bandwidth product of
800MHz, consistent with two 400MHz
amplifiers in series. Figure 6 shows
the transient response to a small step.
The 3.5ns rise/fall time is consistent
with an 80MHz –3dB bandwidth. Figure 7 shows that the step response is
only a little slower making a transition to a full 6VP–P, showing the merits
of a high slew rate.
Figure 8 illustrates the use of the
LT1818 as a buffer for the LTC1744
14-Bit 50Msps ADC. The amplifier
must provide low noise, low distortion
as well as fast settling characteristics
in order to recover quickly from the
sampling loading effects of the ADC.
The LTC1744 signal-to-noise ratio
(SNR) of 73.5dB at 2VP–P implies an
input referred noise of 149µVRMS. The
6nV/√Hz input voltage noise of the
LT1818, integrated over a 94MHz
bandwidth (formed by the 51.1Ω, 18pF
and the CIN of the ADC) results in only
91µVRMS of input noise. The contribution of the input referred current
noise of the LT1818 depends on the
source resistance of the circuit that
drives the LT1818. For a 1k source
resistance, the 1.2pA/√Hz input current noise results in 18µVRMS over the
same bandwidth. Both noise sources
taken together still do not degrade the
noise performance of the 14-bit ADC.
Figure 9 shows the 4096 bin FFT of
the converter output. This implies a
full scale SNR of 76dB, just slightly
better than the ADC’s typical 73.5dB
specification. More readily seen in
the FFT is the SFDR of 78dB.
You can further improve the performance of the circuit by using the
two amplifiers of the LT1819 to convert the single-ended input signal to a
differential signal and drive both inputs of the LTC1744, as shown in
Figure 10. An advantage of driving
the ADC differentially is that the signal swing at each input can be
reduced, which reduces the distortion of both the ADC and the amplifier.
Typical performance is shown in the
25
20
GAIN (dB)
15
10
5
20mV/
DIV
1V/DIV
0
–5
VS = ±5V
TA = 25°C
–10
100k
1M
10M
FREQUENCY (Hz)
100M
Figure 5. Frequency-domain
response of 20dB gain block
12
10ns/DIV
Figure 6. Small-signal transient
response of 20dB gain block
10ns/DIV
Figure 7. Large-signal transient
response of 20dB gain block
Linear Technology Magazine • December 2002
DESIGN FEATURES
0
–20
+
51.1Ω
AIN+
LT1818
–
18pF
2.5V
AIN–
–30
LTC1744
14 BITS
50Msps
(SET FOR 2VP-P
FULL SCALE)
AMPLITUDE (dBc)
2.5VDC
±1VAC
fIN = 5.102539MHz
fS = 50Msps
VIN = 300mVP-P
SFDR = 78dB
8192 SAMPLES
NO WINDOWING
OR AVERAGING
–10
5V
5V
–40
–50
–60
–70
2
3
–80
–90
Figure 8. Single ended ADC driver
–100
–110
VIN
±0.5VAC
18pF
+
5M
0
+
18pF
536Ω
AIN–
4.99k
–20
–30
51.1Ω
1/2 LT1819
+
fIN = 5.023193MHz
fS = 50Msps
VIN = 750mVP-P
SFDR = 81dB
8192 SAMPLES
NO WINDOWING
NO AVERAGING
–10
LTC1744
14 BITS
50Msps
(SET FOR 2VP-P
FULL SCALE)
AMPLITUDE (dBc)
AIN
–
25M
5V
–
536Ω
10M
15M
20M
FREQUENCY (Hz)
Figure 9. FFT of single ended ADC driver
51.1Ω
1/2 LT1819
5V
0
5V
10µF
18pF
–40
–50
–60
–70
–80
–90
–100
4.99k
–110
0.1µF
–120
0
Fast Edge Generation to
Measure Slew Rate
A 2500V/µs slew rate implies that the
transition between ±2V occurs in
1.6ns. In order to accurately measure
slew rates this fast, it is necessary to
generate an input step that is faster
than the device-under-test. Many offthe-shelf function generators fail in
this regard, in which case custommade circuitry may be necessary. The
widely used HP8110A 100MHz Pulse
Generator, for example, has a minimum rise/fall time of 1.8ns, which
fails to provide a fast enough stimu-
10M
15M
20M
FREQUENCY (Hz)
25M
Figure 11. FFT of single-todifferential ADC driver
Figure 10. Single-to-differential ADC driver
FFT of Figure 11, again consisting of
4096 bins derived from 8192 samples.
5M
lus for this test. The older HP8082A
Pulse Generator provides 1ns transitions, but its output amplitude is only
5V into 50Ω, which limits its flexibility to drive the amplifier on ±5V
supplies.
The simple circuit of Figure 12
uses a high-speed inverter such as
the NC7SZU04. Since the inverter
has high gain, it sharpens the input
edge of whatever pulse generator is
used to drive it. The output is AC
coupled to level shift the single sup-
ply inverter output to drive a split
supply biased op amp. This inverter
can provide rise and fall times as fast
as 0.8ns, but the maximum supply
voltage (and hence the maximum
swing) is 5.5V, which is just short of
the ±3V or ±4V desired for full range
slew rate testing of the LT1818/
LT1819. In addition, the 16mA output current drive of the inverter
precludes a 50Ω termination, which
limits the universal applicability of
this circuit.
+V1
750Ω
D1
750Ω
D2
15V
5.5V
–
VIN
VOUT
D3
D4
750Ω
Figure 12. A simple inverter used to
generate a sharp pulse—fast but too little
signal swing and output drive
Linear Technology Magazine • December 2002
±V1 = ±14V
–V1
51Ω
1µF
1µF
VOUT
LT1210
+
VIN
D5
D6
–15V
–V2
+V2
D1–D4: IN5711
D5, D6: METELICS MMD-0840 (408) 737-8181
Figure 13. Improved pulse-sharpener circuit
13
DESIGN FEATURES
1V/DIV
500ps/DIV
Figure 14. Output of pulse-sharpener
circuit
Figure 13 shows a more involved,
but very flexible and fast solution.
The diode bridge D1-D4 switches the
LT1210 current-feedback amplifier,
which runs off ±15V supplies. This
way, the input edge does not need to
swing more than ±1V, and the current into the inverting node of the
LT1210 switches very quickly. The
already fast waveform at the output
of the amplifier is then AC coupled
into the D5–D6 Step Recovery Diodes
(SRD).
An SRD is a two terminal p-i-n
junction whose DC characteristics are
similar to the usual p-n junction diode,
but whose switching characteristics
are quite different. The most distinguishing feature of the SRD is the
very abrupt dependence of its junction impedance upon its internal
charge storage. If a forward biased
SRD is suddenly reverse-current biased, it will first appear as a very low
impedance until the stored charge is
depleted. Then the impedance will
suddenly increase to its normal high
reverse value, thereby stopping the
flow of reverse current. This impedance transition generally takes only a
few hundred picoseconds. The circuit
in Figure 13 uses this SRD property
as a pulse sharpener, generating a
sub-one-nanosecond edge at the output. You can set the voltage levels of
the output waveform by adjusting
±V2. Figure 14 shows a 5V, 0.8ns
output waveform generated by this
circuit.
Finally, an important consideration
in these measurements is the bandwidth of the oscilloscope used. The
photograph in Figure 14 is taken with
a Tektronix TDS820 6GHz digitizing
oscilloscope. For a detailed description of the effect of slower oscilloscopes
on measuring fast edges, refer to Linear Technology Application Note 47.
LT1818 Circuit Design
A simplified schematic of the LT1818/
LT1819 is shown in Figure 15. Both
inputs are high impedance, classifying the amplifier as a voltage feedback
topology. Complementary NPN and
PNP emitter followers Q1-Q8 buffer
each input and present the differential input signal across the internal
resistor R1. The input common mode
range extends to typically 0.8V from
either supply, and is limited by a VBE
of Q10/Q14 plus a VSAT of Q5/Q6.
NPN and PNP current mirrors Q10–
Q11 and Q14–Q15 mirror the current
generated through R1 into the high
impedance node. Cascode devices Q9
and Q13 raise the output impedance
of the mirror, improving the open loop
gain.
Resistor R1, the transconductances
of Q5–Q8, and the compensation capacitor C1 set the 400MHz gain
bandwidth product of the amplifier.
The RC, CC network between the high
impedance node and the output provides extra compensation when the
output drives a capacitive load. This
keeps the LT1818/LT1819 unity-gain
stable with a CLOAD up to 20pF. The
amplifier can drive larger capacitive
loads when configured in higher noise
gains, or with an isolation resistor of
10Ω to 50 Ω in series with the load.
The R2, C2 networks on the current
mirrors provide a pole and zero at a
frequency below the unity-gain frequency. This lowers the frequency
where the open loop gain crosses
0dB, which improves the phase margin of the amplifier while maintaining
a high open loop gain at lower frequencies.
The current generated across R1,
divided by the capacitor C1, determines the slew rate. Note that this
current, and hence the slew rate, are
proportional to the magnitude of the
input step. The input step equals the
output step divided by the closed loop
gain. The highest slew rates are there-
V+
C2
Q11
Q27
Q10
Q12
Q26
Q25
Q24
R2
Q19
Q9
Q17
RC
Q3
–
IN
CC
Q5
Q7
OUT
R1
Q2
Q4
Q8
Q1
+IN
Q18
Q20
Q13
R2
Q28
C2
Q21
C1
Q6
Q14
Q15
Q16 Q22
Q23
V–
Figure 15. Simplified schematic of LT1818/LT1819 amplifier
14
Linear Technology Magazine • December 2002
DESIGN FEATURES
fore obtained in the lowest gain configurations. The 2500V/µs slew rate
specified on the data sheet is measured in a noninverting unity gain
configuration. The 1800V/µs production tested slew rate is measured in
an (inverting) gain of –1, which is
equivalent to a noninverting gain of 2.
The internal current generated
across the input resistor can be much
higher than the quiescent supply current (up to 80mA). In normal transient
closed loop operation this does not
present a problem, since after a few
nanoseconds the feedback brings the
differential input signal back to zero.
However, sustained (i.e. open loop)
differential input voltages may result
in excessive power dissipation and
therefore this amplifier should not be
used as a comparator.
The output stage buffers the high
impedance node from the load by
providing current gain. Emitter followers Q17–Q20 provide a current
gain equal to BetaNPN × BetaPNP, but
the effective current gain is greatly
enhanced by the dynamic base current compensation provided by
Q24–Q26 and Q21–Q23. Q24 measures a fraction of the output current
that flows through Q19, and mirror
Q25–Q26 injects the appropriate current back into the base of Q19. This
signal-dependent boost improves the
linearity of the amplifier by reducing
the amount of differential input signal required for a given output current.
An additional advantage is that the
output devices can be smaller, which
requires less quiescent current for a
given amplifier speed.
LTC4412, continued from page 4
High Side Power Switch
voltage, therefore denying power to
the load. The MOSFET is connected
with its source connected to the power
source. This prevents the drain-source
diode from supplying voltage to the
load when the MOSFET is off.
no load current is drawn from the
batteries. The STAT pins provide information as to which input is
supplying the load current. This concept can be applied to as many power
inputs as are needed.
Multiple Battery Charging
Figure 6 shows an application circuit
for automatically charging two batteries from a single charger. Whichever
battery has the lower voltage receives
the charging current until both battery voltages are equal then both will
be charged. When both are charged
simultaneously the higher capacity
battery receives proportionally higher
current from the charger. For Li-Ion
batteries both batteries achieve the
float voltage of the battery charger
minus the forward regulation voltage
of 20mV. This concept can apply to
more than two batteries. The STAT
pins provide information as to which
batteries are being charged.
Figure 7 illustrates an application
circuit for a logic controlled high side
power switch using the control input
pin. When the CTL pin is a logical low
the LTC4412 turns on the MOSFET.
Because the SENSE pin is grounded
the LTC4412’s internal controller
functions as an open-loop comparator and applies maximum gate drive
voltage to the MOSFET. When the
CTL pin is a logical high the LTC4412
turns off the MOSFET by pulling its
gate voltage up to the supply input
Q1
*
SUPPLY
INPUT
LOGIC
INPUT
TO LOAD
LTC4412
6
1
VIN SENSE
5
2
GND GATE
4
3
CTL STAT
COUT
Conclusion
The ultrafast slew rate and high bandwidth allow the LT1818 and LT1819
op amps to process large signals at
high frequencies with low distortion.
Combined with the low noise and
moderate supply current, these amplifiers are a good choice for receivers,
filters, or drivers of cables and ADCs
in high-speed communication or data
acquisition systems.
Conclusion
The LTC4412 provides a simple and
efficient way to implement a low loss
ideal diode controller that extends
battery life and significantly reduces
self-heating. The low external parts
count translates directly to low overall system cost and its ThinSOT 6-pin
package makes for compact design
solutions. It’s versatile enough to be
used in a variety of diode OR-ing
applications covering a wide range of
supply voltages.
4412 F07
*PARASITIC DRAIN-SOURCE DIODE OF MOSFET
Q1: FAIRCHILD SEMICONDUCTOR FDN306P (408) 822-2126
Figure 7. Logic controlled
high side power switch
for
the latest information
on LTC products,
visit
www.linear.com
Linear Technology Magazine • December 2002
15
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