AD AD797 Ultralow distortion, ultralow noise op amp Datasheet

Ultralow Distortion,
Ultralow Noise Op Amp
AD797
PIN CONFIGURATION
Low noise
0.9 nV/√Hz typical (1.2 nV/√Hz maximum) input voltage
noise at 1 kHz
50 nV p-p input voltage noise, 0.1 Hz to 10 Hz
Low distortion
−120 dB total harmonic distortion at 20 kHz
Excellent ac characteristics
800 ns settling time to 16 bits (10 V step)
110 MHz gain bandwidth (G = 1000)
8 MHz bandwidth (G = 10)
280 kHz full power bandwidth at 20 V p-p
20 V/μs slew rate
Excellent dc precision
80 μV maximum input offset voltage
1.0 μV/°C VOS drift
Specified for ±5 V and ±15 V power supplies
High output drive current of 50 mA
+IN 3
6
OUTPUT
–VS 4
5
OFFSET NULL
AD797
TOP VIEW
8
Figure 1. 8-Lead Plastic Dual In-Line Package [PDIP] and
8-Lead Standard Small Outline Package [SOIC]
GENERAL DESCRIPTION
The AD797 is a very low noise, low distortion operational
amplifier ideal for use as a preamplifier. The low noise of
0.9 nV/√Hz and low total harmonic distortion of −120 dB at
audio bandwidths give the AD797 the wide dynamic range
necessary for preamps in microphones and mixing consoles.
Furthermore, the AD797’s excellent slew rate of 20 V/μs and
110 MHz gain bandwidth make it highly suitable for low
frequency ultrasound applications.
The AD797 is also useful in infrared (IR) and sonar imaging
applications, where the widest dynamic range is necessary. The
low distortion and 16-bit settling time of the AD797 make it
ideal for buffering the inputs to Σ-Δ ADCs or the outputs of
high resolution DACs, especially when the device is used in
critical applications such as seismic detection or in spectrum
analyzers. Key features such as a 50 mA output current drive
and the specified power supply voltage range of ±5 V to ±15 V
make the AD797 an excellent general-purpose amplifier.
APPLICATIONS
Professional audio preamplifiers
IR, CCD, and sonar imaging systems
Spectrum analyzers
Ultrasound preamplifiers
Seismic detectors
Σ-Δ ADC/DAC buffers
4
THD (dB)
3
2
–100
0.001
–110
0.0003
–120
0.0001
THD (%)
–90
5
1
0
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
10M
–130
100
300
1k
3k
10k
FREQUENCY (Hz)
30k
100k
300k
00846-003
MEASUREMENT
LIMIT
00846-002
INPUT VOLTAGE NOISE (nV/√Hz)
–IN 2
DECOMPENSATION
AND DISTORTION
NEUTRALIZATION
7 +VS
OFFSET NULL 1
00846-001
FEATURES
Figure 2. AD797 Voltage Noise Spectral Density
Figure 3. THD vs. Frequency
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 that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113
©2008 Analog Devices, Inc. All rights reserved.
Rev. F
AD797
TABLE OF CONTENTS
Features .............................................................................................. 1
Low Frequency Noise ................................................................ 12
Applications....................................................................................... 1
Wideband Noise ......................................................................... 12
Pin Configuration............................................................................. 1
Bypassing Considerations ......................................................... 12
General Description ......................................................................... 1
The Noninverting Configuration............................................. 13
Revision History ............................................................................... 2
The Inverting Configuration .................................................... 14
Specifications..................................................................................... 3
Driving Capacitive Loads.......................................................... 14
Absolute Maximum Ratings............................................................ 5
Settling Time............................................................................... 14
ESD Caution.................................................................................. 5
Distortion Reduction ................................................................. 15
Typical Performance Characteristics ............................................. 6
Outline Dimensions ....................................................................... 18
Theory of Operation ...................................................................... 11
Ordering Guide .......................................................................... 19
Noise and Source Impedance Considerations........................ 11
REVISION HISTORY
1/08—Rev. E to Rev. F
Changes to Absolute Maximum Ratings ....................................... 5
Change to Equation 1..................................................................... 12
Changes to the Noninverting Configuration Section................ 13
Updated Outline Dimensions ....................................................... 19
Changes to Ordering Guide .......................................................... 20
7/05—Rev. D to Rev. E
Updated Figure 1 Caption ............................................................... 1
Deleted Metallization Photo ........................................................... 6
Changes to Equation 1 ................................................................... 12
Updated Outline Dimensions ....................................................... 19
Changes to Ordering Guide .......................................................... 20
10/02—Rev. C to Rev. D
Deleted 8-Lead CERDIP Package (Q-8)..........................Universal
Edits to Specifications ...................................................................... 2
Edits to Absolute Maximum Ratings ............................................. 3
Edits to Ordering Guide .................................................................. 3
Edits to Table I .................................................................................. 9
Deleted Operational Amplifiers Graphic .................................... 15
Updated Outline Dimensions ....................................................... 15
Rev. F | Page 2 of 20
AD797
SPECIFICATIONS
TA = 25°C and VS = ±15 V dc, unless otherwise noted.
Table 1.
Parameter
INPUT OFFSET VOLTAGE
Supply
Voltage (V)
±5 V, ±15 V
Conditions
Min
TMIN to TMAX
Offset Voltage Drift
INPUT BIAS CURRENT
±5 V, ±15 V
±5 V, ±15 V
TMIN to TMAX
INPUT OFFSET CURRENT
OPEN-LOOP GAIN
DYNAMIC PERFORMANCE
Gain Bandwidth Product
–3 dB Bandwidth
Full Power Bandwidth1
Slew Rate
Settling Time to 0.0015%
COMMON-MODE REJECTION
POWER SUPPLY REJECTION
INPUT VOLTAGE NOISE
INPUT CURRENT NOISE
INPUT COMMON-MODE VOLTAGE RANGE
OUTPUT VOLTAGE SWING
Short-Circuit Current
Output Current 3
TOTAL HARMONIC DISTORTION
±5 V, ±15 V
TMIN to TMAX
VOUT = ±10 V
RLOAD = 2 kΩ
TMIN to TMAX
RLOAD = 600 Ω
TMIN to TMAX
@ 20 kHz 1
±15 V
G = 1000
G = 1000 2
G = 10
VOUT = 20 V p-p,
RLOAD = 1 kΩ
RLOAD = 1 kΩ
10 V step
VCM = CMVR
TMIN to TMAX
VS = ±5 V to ±18 V
TMIN to TMAX
f = 0.1 Hz to 10 Hz
f = 10 Hz
f = 1 kHz
f = 10 Hz to 1 MHz
f = 1 kHz
1
1
1
1
14,000
±15 V
15 V
±15 V
±15 V
±15 V
±15 V
±5 V, ±15 V
RLOAD = 2 kΩ
RLOAD = 600 Ω
RLOAD = 600 Ω
RLOAD = 1 kΩ, CN = 50 pF,
f = 250 kHz, 3 V rms
RLOAD = 1 kΩ,
f = 20 kHz, 3 V rms
±15 V
±15 V
±15 V
±15 V
±15 V
±15 V
±5 V
±15 V
±15 V
±5 V
±5 V, ±15 V
±5 V, ±15 V
±15 V
±15 V
INPUT CHARACTERISTICS
Input Resistance
Differential
Common Mode
Input Capacitance
Differential 4
Common Mode
Rev. F | Page 3 of 20
AD797A
Typ
25
50
0.2
0.25
0.5
100
120
20
6
15
5
20,000
Max
80
125/180
1.0
1.5
3.0
400
600/700
110
450
8
280
12.5
114
110
114
110
±11
±2.5
±12
±11
±2.5
30
20
800
130
120
130
120
50
1.7
0.9
1.0
2.0
±12
±3
±13
±13
±3
80
50
−98
−120
AD797B
Typ
10
30
0.2
0.25
0.25
80
120
2
20
2
10
2
15
2
7
14,000 20,000
Min
Max
40
60
0.6
0.9
2.0
200
300
110
450
8
MHz
MHz
MHz
kHz
280
12.5
−90
20
800
130
120
114
120
50
1.7
0.9
1.0
2.0
±11
±2.5
±13
±13
±3
80
50
−98
−110
−120
1200
120
114
120
130
1.2
1.3
±12
±11
±2.5
30
Unit
μV
μV
μV/°C
μA
μA
nA
nA
V/μV
V/μV
V/μV
V/μV
V/V
−90
V/μs
ns
dB
dB
dB
dB
nV p-p
nV/√Hz
nV/√Hz
μV rms
pA/√Hz
V
V
V
V
V
mA
mA
dB
−110
dB
1200
2.5
1.2
1.2
±12
±3
7.5
100
7.5
100
kΩ
MΩ
20
5
20
5
pF
pF
AD797
Parameter
OUTPUT RESISTANCE
POWER SUPPLY
Operating Range
Quiescent Current
Conditions
AV = 1, f = 1 kHz
Supply
Voltage (V)
Min
AD797A
Typ
Max
3
±5
±5 V, ±15 V
1
8.2
Full power bandwidth = slew rate/2π VPEAK.
Specified using external decompensation capacitor.
3
Output current for |VS – VOUT| > 4 V, AOL > 200 kΩ.
4
Differential input capacitance consists of 1.5 pF package capacitance and 18.5 pF from the input differential pair.
2
Rev. F | Page 4 of 20
±18
10.5
Min
AD797B
Typ
±5
8.2
Max
3
Unit
mΩ
±18
10.5
V
mA
AD797
ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter
Supply Voltage
Internal Power Dissipation @ 25°C1
PDIP
SOIC
Input Voltage
Differential Input Voltage2
Output Short-Circuit Duration
Storage Temperature Range
(N, R Suffix)
Operating Temperature Range
Lead Temperature Range
(Soldering 60 sec)
1
2
Ratings
±18 V
1.3 W − (TA − 25°C)/θJA
0.9 W (TA − 25°C)/θJA
±VS
±0.7 V
Indefinite within
maximum internal
power dissipation
−65°C to +125°C
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; 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.
ESD CAUTION
−40°C to +85°C
300°C
θJA = 95°C/W for the 8-lead PDIP; 155°C/W for the 8-lead SOIC.
The AD797 inputs are protected by back-to-back diodes. To achieve low
noise, internal current-limiting resistors are not incorporated into the design
of this amplifier. If the differential input voltage exceeds ±0.7 V, the input
current should be limited to less than 25 mA by series protection resistors.
Note, however, that this degrades the low noise performance of the device.
Rev. F | Page 5 of 20
AD797
TYPICAL PERFORMANCE CHARACTERISTICS
10
5
0
5
10
15
20
SUPPLY VOLTAGE (±V)
HORIZONTAL SCALE (5sec/DIV)
Figure 7. 0.1 Hz to 10 Hz Noise
Figure 4. Input Common-Mode Voltage Range vs. Supply Voltage
0
INPUT BIAS CURRENT (µA)
15
10
+VOUT
–VOUT
5
0
0
5
10
15
20
SUPPLY VOLTAGE (±V)
–0.5
–1.0
–1.5
–2.0
–60
00846-005
–20
0
20
40
60
80
100
120
140
TEMPERATURE (°C)
Figure 5. Output Voltage Swing vs. Supply Voltage
Figure 8. Input Bias Current vs. Temperature
30
140
SHORT-CIRCUIT CURRENT (mA)
VS = ± 15V
20
10
VS = ±5
120
100
SOURCE CURRENT
SINK CURRENT
80
60
0
10
100
1k
LOAD RESISTANCE (Ω)
10k
40
–60
00846-006
OUTPUT VOLTAGE SWING (V p-p)
–40
–40
–20
0
20
40
60
80
100
120
TEMPERATURE (°C)
Figure 9. Short-Circuit Current vs. Temperature
Figure 6. Output Voltage Swing vs. Load Resistance
Rev. F | Page 6 of 20
140
00846-009
OUTPUT VOLTAGE SWING (±V)
20
00846-008
0
00846-007
VERTICAL SCALE (0.01µV/DIV)
15
00846-004
INPUT COMMON-MODE RANGE (±V)
20
AD797
+125°C
9
+25°C
8
7
5
15
10
20
SUPPLY VOLTAGE (±V)
Figure 10. Quiescent Supply Current vs. Supply Voltage
125
CMR
60
100
40
75
20
1
10
100
1k
10k
100k
FREQUENCY (Hz)
–60
f = 1kHz
RL = 600Ω
G = +10
RL = 600Ω
G = +10
f = 10kHz
NOISE BW = 100kHz
THD + NOISE (dB)
9
6
–80
VS = ±5V
–100
3
0
±5
±10
±15
±20
SUPPLY VOLTAGE (±V)
–120
0.01
0.1
1
10
OUTPUT LEVEL (V)
Figure 11. Output Voltage vs. Supply Voltage for 0.01% Distortion
00846-014
VS = ±15V
00846-011
0
50
1M
Figure 13. Power Supply and Common-Mode Rejection vs. Frequency
12
OUTPUT VOLTAGE (V rms)
150
80
00846-010
0
PSR
+SUPPLY
PSR
–SUPPLY
100
–55°C
6
175
120
00846-013
10
COMMON MODE REJECTION (dB)
200
140
POWER SUPPLY REJECTION (dB)
QUIESCENT SUPPLY CURRENT (mA)
11
Figure 14. Total Harmonic Distortion (THD) + Noise vs. Output Level
1.0
30
OUTPUT VOLTAGE (V p-p)
±15V SUPPLIES
0.0015%
0.6
0.01%
0.4
RL = 600Ω
20
10
0
0
2
4
6
8
STEP SIZE (V)
10
0
10k
100k
1M
FREQUENCY (Hz)
Figure 12. Settling Time vs. Step Size (±)
Figure 15. Large-Signal Frequency Response
Rev. F | Page 7 of 20
10M
00846-015
±5V SUPPLIES
0.2
00846-012
SETTLING TIME (µs)
0.8
AD797
110
SLEW RATE (V/µs)
30
3
2
SLEW RATE
RISING EDGE
25
100
SLEW RATE
FALLING EDGE
20
1
0
10
100
1k
10k
100k
1M
10M
FREQUENCY (Hz)
90
15
–60
–40
–20
0
20
40
60
80
100
120
80
140
TEMPERATURE (°C)
00846-019
GAIN/BANDWIDTH PRODUCT
4
GAIN/BANDWIDTH PRODUCT (MHz (G = 1000))
120
35
00846-016
INPUT VOLTAGE NOISE (nV/√Hz)
5
Figure 19. Slew Rate and Gain/Bandwidth Product vs. Temperature
Figure 16. Input Voltage Noise Spectral Density
120
100
160
PHASE MARGIN
60
80
40
60
GAIN
20
40
*RS = 100
WITHOUT
RS*
20
*SEE FIGURE 25.
120
0
WITH RS*
10k
100k
1M
FREQUENCY (Hz)
10M
100M
100
100
Figure 17. Open-Loop Gain and Phase Margin vs. Frequency
Figure 20. Open-Loop Gain vs. Load Resistance
100
MAGNITUDE OF OUTPUT IMPEDANCE (Ω)
OVERCOMPENSATED
150
0
–150
UNDER COMPENSATED
–40
–20
0
20
40
60
80
100
TEMPERATURE (°C)
120
140
10
1
WITHOUT CN*
0.1
WITH CN*
*SEE FIGURE 32.
0.01
00846-018
INPUT OFFSET CURRENT (nA)
300
–300
–60
10k
1k
LOAD RESISTANCE (Ω)
00846-020
1k
140
10
100
1k
10k
100k
FREQUENCY (Hz)
Figure 21. Magnitude of Output Impedance vs. Frequency
Figure 18. Input Offset Current vs. Temperature
Rev. F | Page 8 of 20
1M
00846-021
0
100
OPEN-LOOP GAIN (dB)
80
PHASE MARGIN (Degrees)
WITHOUT
RS*
WITH RS*
00846-017
OPEN-LOOP GAIN (dB)
100
AD797
20pF
100Ω
+VS
1kΩ
2
7
AD797
3
VIN
VOUT
6
RS*
AD797
3
VOUT
6
600Ω
4
**
4
–VS
*
00846-022
*VALUE OF SOURCE RESISTANCE
(SEE THE NOISE AND SOURCE IMPEDANCE
CONSIDERATIONS SECTION).
**SEE FIGURE 35.
–VS
*SEE FIGURE 35.
Figure 22. Inverter Connection
Figure 25. Follower Connection
1µs
5V
100
100
90
90
10
0%
00846-023
10
0%
5V
Figure 23. Inverter Large-Signal Pulse Response
50mV
1µs
00846-026
1kΩ
7
Figure 26. Follower Large-Signal Pulse Response
100ns
50mV
100
100
90
90
10
0%
00846-024
10
0%
100ns
00846-027
VIN
2
*
**
00846-025
+VS
Figure 27. Follower Small-Signal Pulse Response
Figure 24. Inverter Small-Signal Pulse Response
Rev. F | Page 9 of 20
AD797
500ns
50mV
100
100
90
90
10
0%
00846-028
10
0%
Figure 28. 16-Bit Settling Time Positive Input Pulse
500ns
00846-029
50mV
Figure 29. 16-Bit Settling Time Negative Input Pulse
Rev. F | Page 10 of 20
AD797
THEORY OF OPERATION
The architecture of the AD797 was developed to overcome
inherent limitations in previous amplifier designs. Previous
precision amplifiers used three stages to ensure high open-loop
gain (see Figure 30) at the expense of additional frequency compensation components. Slew rate and settling performance are
usually compromised, and dynamic performance is not adequate
beyond audio frequencies. As can be seen in Figure 30, the first
stage gain is rolled off at high frequencies by the compensation
network. Second stage noise and distortion then appears at the
input and degrade performance. The AD797, on the other hand,
uses a single ultrahigh gain stage to achieve dc as well as dynamic
precision. As shown in the simplified schematic (Figure 31),
Node A, Node B, and Node C track the input voltage, forcing
the operating points of all pairs of devices in the signal path to
match. By exploiting the inherent matching of devices fabricated on
the same IC chip, high open-loop gain, CMRR, PSRR, and low
VOS are guaranteed by pairwise device matching (that is, NPN
to NPN and PNP to PNP), not by an absolute parameter such as
beta and the early voltage.
VOUT
BUFFER
gm
RL
C1
R1
benefit of making the low noise of the AD797 (<0.9 nV/√Hz)
extend to beyond 1 MHz. This means new levels of performance for sampled data and imaging systems. All of this
performance as well as load drive in excess of 30 mA are made
possible by the Analog Devices, Inc., advanced complementary
bipolar (CB) process.
Another unique feature of this circuit is that the addition of a
single capacitor, CN (see Figure 31), enables cancellation of
distortion due to the output stage. This can best be explained by
referring to a simplified representation of the AD797 using
idealized blocks for the different circuit elements (Figure 32).
A single equation yields the open-loop transfer function of this
amplifier; solving it at Node B yields
VOUT
V IN
a.
where:
gm is the transconductance of Q1 and Q2.
A is the gain of the output stage (~1).
VOUT is voltage at the output.
VIN is differential input voltage.
VOUT
C2
V IN
A2
R1
gm
C
CN
jω − C N jω − C jω
A
A
When CN is equal to CC, the ideal single-pole op amp response
is attained:
GAIN = gm × R1 × 5 × 106
gm
=
BUFFER
A3
VOUT
C1
00846-030
GAIN = gm × R1 × A2 × A3
b.
gm
jωC
In Figure 32, the terms of Node A, which include the properties of
the output stage, such as output impedance and distortion, cancel
by simple subtraction. Therefore, the distortion cancellation does
not affect the stability or frequency response of the amplifier. With
only 500 μA of output stage bias, the AD797 delivers a 1 kHz
sine wave into 60 Ω at 7 V rms with only 1 ppm of distortion.
RL
R2
=
Figure 30. Model of AD797 vs. That of a Typical Three-Stage Amplifier
VCC
R2
R3
I1
CN
R1
I2
CN
I5
Q4
Q10
Q7
A
Q9
Q1
Q2
I1
–IN
Q5
C
Q6
CC
Q12
A
VOUT
+IN
Q8
–IN
Q1
Q11
CURRENT
MIRROR
Q2
VOUT
CC
1
I6
I7
I4
VSS
00846-031
+IN
B
A
B
I3
C
I4
00846-032
Q3
Figure 32. AD797 Block Diagram
Figure 31. AD797 Simplified Schematic
This matching benefits not just dc precision, but, because it holds
up dynamically, both distortion and settling time are also reduced.
This single stage has a voltage gain of >5 × 106 and VOS < 80 μV,
while at the same time providing a THD + noise of less than
−120 dB and true 16-bit settling in less than 800 ns. The
elimination of second-stage noise effects has the additional
NOISE AND SOURCE IMPEDANCE
CONSIDERATIONS
The AD797 ultralow voltage noise of 0.9 nV/√Hz is achieved
with special input transistors running at nearly 1 mA of
collector current. Therefore, it is important to consider the total
input-referred noise (eNtotal), which includes contributions
Rev. F | Page 11 of 20
AD797
from voltage noise (eN), current noise (iN), and resistor noise
(√4 kTRS).
e N total = [e N 2 + 4 kTR S + (i N × R S ) 2 ]1 / 2
(1)
where RS is the total input source resistance.
The plot in Figure 7 uses a slightly different technique: an
FFT-based instrument (Figure 34) is used to generate a 10 Hz
“brickwall” filter. A low frequency pole at 0.1 Hz is generated
with an external ac coupling capacitor, which is also the
instrument being dc coupled.
This equation is plotted for the AD797 in Figure 33. Because
optimum dc performance is obtained with matched source
resistances, this case is considered even though it is clear from
Equation 1 that eliminating the balancing source resistance
lowers the total noise by reducing the total RS by a factor of 2.
Several precautions are necessary to attain optimum low
frequency noise performance:
At very low source resistance (RS < 50 Ω), the voltage noise of the
amplifier dominates. As source resistance increases, the Johnson
noise of RS dominates until a higher resistance of RS > 2 kΩ is
achieved; the current noise component is larger than the
resistor noise.
•
•
•
100
TOTAL NOISE
•
100kΩ
+VS
RESISTOR
NOISE
ONLY
1
*
1Ω
2
7
AD797
3
10
100
1000
10000
SOURCE RESISTANCE (Ω)
4
HP 3465
DYNAMIC SIGNAL
ANALYZER
(10Hz)
–VS
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 33. Noise vs. Source Resistance
The AD797 is the optimum choice for low noise performance if
the source resistance is kept <1 kΩ. At higher values of source
resistance, optimum performance with respect to only noise is
obtained with other amplifiers from Analog Devices (Table 3).
Table 3. Recommended Amplifiers for Different Source
Impedances
RS (kΩ)
0 to <1
1 to <10
10 to <100
>100
VOUT
*
00846-033
0.1
1.5µF
6
Recommended Amplifier
AD797
AD743/AD745, OP27/OP37, OP07
AD743/AD745, OP07
AD548, AD549, AD711, AD743/AD745
00846-034
NOISE (nV/√Hz)
10
Care must be used to account for the effects of RS. Even
a 10 Ω resistor has 0.4 nV/√Hz of noise (an error of 9%
when root sum squared with 0.9 nV/√Hz).
The test setup must be fully warmed up to prevent eOS drift
from erroneously contributing to input noise.
Circuitry must be shielded from air currents. Heat flow out
of the package through its leads creates the opportunity for
a thermoelectric potential at every junction of different
metals. Selective heating and cooling of these by random
air currents appears as 1/f noise and obscures the true
device noise.
The results must be interpreted using valid statistical
techniques.
Figure 34. Test Setup for Measuring 0.1 Hz to 10 Hz Noise
WIDEBAND NOISE
Due to its single-stage design, the noise of the AD797 is flat
over frequencies from less than 10 Hz to beyond 1 MHz. This
is not true of most dc precision amplifiers, where second-stage
noise contributes to input-referred noise beyond the audio
frequency range. The AD797 offers new levels of performance in
wideband imaging applications. In sampled data systems, where
aliasing of out-of-band noise into the signal band is a problem,
the AD797 outperforms all previously available IC op amps.
BYPASSING CONSIDERATIONS
LOW FREQUENCY NOISE
Analog Devices specifies low frequency noise as a peak-to-peak
quantity in a 0.1 Hz to 10 Hz bandwidth. Several techniques can
be used to make this measurement. The usual technique involves
amplifying, filtering, and measuring the amplifier noise for a
predetermined test time. The noise bandwidth of the filter is
corrected for, and the test time is carefully controlled because
the measurement time acts as an additional low frequency roll-off.
Taking full advantage of the very wide bandwidth and dynamic
range capabilities of the AD797 requires some precautions.
First, multiple bypassing is recommended in any precision
application. A 1.0 μF to 4.7 μF tantalum in parallel with 0.1 μF
ceramic bypass capacitors are sufficient in most applications.
When driving heavy loads, a larger demand is placed on the
supply bypassing. In this case, selective use of larger values of
tantalum capacitors and damping of their lead inductance with
small-value (1.1 Ω to 4.7 Ω) carbon resistors can achieve an
improvement. Figure 35 summarizes power supply bypassing
recommendations.
Rev. F | Page 12 of 20
AD797
VS
OR
0.1µF
Low noise preamplification is usually performed in the noninverting mode (Figure 38). For lowest noise, the equivalent
resistance of the feedback network should be as low as possible.
The 30 mA minimum drive current of the AD797 makes it easier
to achieve this. The feedback resistors can be made as low as
possible, with consideration to load drive and power consumption.
4.7µF TO 22.0µF
4.7µF
0.1µF
1.1Ω TO 4.7Ω
KELVIN RETURN
USE SHORT
LEAD LENGTHS
(<5mm)
LOAD
CURRENT
LOAD
CURRENT
CL
00846-035
KELVIN RETURN
USE SHORT
LEAD LENGTHS
(<5mm)
R2
Figure 35. Recommended Power Supply Bypassing
THE NONINVERTING CONFIGURATION
+VS
Ultralow noise requires very low values of the internal parasitic
resistance (rBB) for the input transistors (≈6 Ω). This implies
very little damping of input and output reactive interactions.
With the AD797, additional input series damping is required
for stability with direct output to input feedback. A 100 Ω
resistor (R1) in the inverting input (Figure 36) is sufficient; the
100 Ω balancing resistor (R2) is recommended but is not
required for stability. The noise penalty is minimal (eNtotal ≈
2.1 nV/√Hz), which is usually insignificant.
R1
100Ω
+VS
*
2
VIN
R2
100Ω
7
AD797
3
6
RL
600Ω
4
VOUT
–VS
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
00846-036
*
Figure 36. Voltage Follower Connection
Best response flatness is obtained with the addition of a small
capacitor (CL < 33 pF) in parallel with the 100 Ω resistor
(Figure 37). The input source resistance and capacitance also
affect the response slightly, and experimentation may be
necessary for best results.
CL
+VS
*
RS
AD797
3
600Ω
4
*
CS
7
3
VOUT
6
RL
4
*
–VS
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 38. Low Noise Preamplifier
Table 4 provides some representative values for the AD797
when used as a low noise follower. Operation on 5 V supplies
allows the use of a 100 Ω or less feedback network (R1 + R2).
Because the AD797 shows no unusual behavior when operating
near its maximum rated current, it is suitable for driving the
AD600/AD602 (see Figure 50) while preserving low noise
performance.
Optimum flatness and stability at noise gains >1 sometimes
require a small capacitor (CL) connected across the feedback
resistor (R1 of Figure 38). Table 4 includes recommended values
of CL for several gains. In general, when R2 is greater than
100 Ω and CL is greater than 33 pF, a 100 Ω resistor should be
placed in series with CL. Source resistance matching is assumed,
and the AD797 should not be operated with unbalanced source
resistance >200 kΩ/G.
Table 4. Values for Follower with Gain Circuit
R1
1 kΩ
300 Ω
33.2 Ω
16.5 Ω
10 Ω
R2
1 kΩ
300 Ω
300 Ω
316 Ω
(G − 1) × 10 Ω
CL
≈20 pF
≈10 pF
≈5 pF
Noise
(Excluding RS)
3.0 nV/√Hz
1.8 nV/√Hz
1.2 nV/√Hz
1.0 nV/√Hz
0.98 nV/√Hz
VOUT
6
–VS
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 37. Alternative Voltage Follower Connection
00846-037
VIN
7
2
AD797
VIN
Gain
2
2
10
20
>35
100Ω
2
*
R1
00846-038
VS
The I-to-V converter is a special case of the follower configuration. When the AD797 is used in an I-to-V converter, for
example as a DAC buffer, the circuit shown in Figure 39 should
be used. The value of CL depends on the DAC, and if CL is greater
than 33 pF, a 100 Ω series resistor is required. A bypassed balancing
resistor (RS and CS) can be included to minimize dc errors.
Rev. F | Page 13 of 20
AD797
100Ω
DRIVING CAPACITIVE LOADS
R1
The capacitive load driving capabilities of the AD797 are
displayed in Figure 41. At gains greater than 10, usually no
special precautions are necessary. If more drive is desirable,
however, the circuit shown in Figure 42 should be used. For
example, this circuit allows a 5000 pF load to be driven cleanly
at a noise gain ≥2.
+VS
*
IIN
2
7
AD797
3
600Ω
4
100nF
–VS
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 39. I-to-V Converter Connection
THE INVERTING CONFIGURATION
The inverting configuration (see Figure 40) presents a low input
impedance, R1, to the source. For this reason, the goals of both
low noise and input buffering are at odds with one another.
Nonetheless, the excellent dynamics of the AD797 makes
it the preferred choice in many inverting applications, and
with careful selection of feedback resistors, the noise penalties
are minimal. Some examples are presented in Table 5 and
Figure 40.
CAPACITIVE LOAD DRIVE CAPABILITY
*
RS
00846-039
CS
VOUT
6
10nF
1nF
100pF
10pF
1pF
10
1
100
1k
CLOSED-LOOP GAIN
00846-041
20pF TO 120pF
Figure 41. Capacitive Load Drive Capability vs. Closed-Loop Gain
CL
20pF
R2
1kΩ
+VS
200pF
100Ω
*
R1
+VS
7
AD797
3
*
VOUT
6
1kΩ
RL
4
2
VIN
AD797
*
–VS
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
3
00846-040
RS
7
33Ω
VOUT
6
C1
4
*
–VS
Figure 40. Inverting Amplifier Connection
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Table 5. Values for Inverting Circuit
Gain
−1
−1
−10
R1
1 kΩ
300 Ω
150 Ω
R2
1 kΩ
300 Ω
1500 Ω
CL
≈20 pF
≈10 pF
≈5 pF
00846-042
2
VIN
Figure 42. Recommended Circuit for Driving a High Capacitance Load
Noise
(Excluding RS)
3.0 nV/√Hz
1.8 nV/√Hz
1.8 nV/√Hz
SETTLING TIME
The AD797 is unique among ultralow noise amplifiers in that it
settles to 16 bits (<150 μV) in less than 800 ns. Measuring this
performance presents a challenge. A special test circuit (see
Figure 43) was developed for this purpose. The input signal was
obtained from a resonant reed switch pulse generator, available
from Tektronix as calibration Fixture No. 067-0608-00. When
open, the switch is simply 50 Ω to ground and settling is purely
a passive pulse decay and inherently flat. The low repetition rate
signal was captured on a digital oscilloscope after being
amplified and clamped twice. The selection of plug-in for the
oscilloscope was made for minimum overload recovery.
Rev. F | Page 14 of 20
AD797
TO TEKTRONIX
7A26
OSCILLOSCOPE 1MΩ
PREAMP INPUT
SECTION
The benefits of adding C1 are evident for closed-loop gains
of ≥100. A maximum value of ≈33 pF at gains of ≥1000 is
recommended. At a gain of 1000, the bandwidth is 450 kHz.
20pF
Table 6 and Figure 45 summarize the performance of the
AD797 with distortion cancellation and decompensation.
4.26kΩ
226Ω
(VIA LESS THAN 1FT
50Ω COAXIAL CABLE)
2
– A2
AD829
3
250Ω
6
7
+
50pF
2×
HP2835
4
2×
HP2835
R1
VERROR × 5
R2
0.47µF
2
0.47µF
AD797
+VS
VIN
–VS
1kΩ
6
3
1kΩ
a.
100Ω
1kΩ
R1
20pF
1kΩ
–
C2
A1
AD797
3
6
7
+
C1
R2
2
51pF
4
8
AD797
0.1µF
+VS
0.1µF
VIN
NOTES
USE CIRCUIT BOARD WITH GROUND PLANE.
VOUT
6
3
–VS
C1, SEE TABLE
C2 = 50pF – C1
b.
00846-043
1µF
1µF
Figure 44. Recommended Connections for Distortion Cancellation
and Bandwidth Enhancement
Figure 43. Settling Time Test Circuit
Table 6. Recommended External Compensation for
Distortion Cancellation and Bandwidth Enhancement
The AD797 has distortion performance (THD < −120 dB,
@ 20 kHz, 3 V rms, RL = 600 Ω) unequaled by most voltage
feedback amplifiers.
At higher gains and higher frequencies, THD increases due to a
reduction in loop gain. However, in contrast to most conventional voltage feedback amplifiers, the AD797 provides two
effective means of reducing distortion as gain and frequency
are increased: cancellation of the output stage’s distortion and
gain bandwidth enhancement by decompensation. By applying
these techniques, gain bandwidth can be increased to 450 MHz
at G = 1000, and distortion can be held to −100 dB at 20 kHz for
G = 100.
The unique design of the AD797 provides cancellation of the
output stage’s distortion. To achieve this, a capacitance equal to
the effective compensation capacitance, usually 50 pF, is
connected between Pin 8 and the output (see C2 in Figure 44).
Use of this feature improves distortion performance when the
closed-loop gain is more than 10 or when frequencies of interest
are greater than 30 kHz.
Bandwidth enhancement via decompensation is achieved by
connecting a capacitor from Pin 8 to ground (see C1 in Figure 44).
Adding C1 results in subtracting from the value of the internal
compensation capacitance (50 pF), yielding a smaller effective
compensation capacitance and therefore a larger bandwidth.
A/B
R1
R2
(Ω)
(Ω)
909 100
1k
10
10 k 10
Gain
10
100
1000
A
C1
(pF)
0
0
0
C2
(pF)
50
50
50
B
3 dB
BW
6 MHz
1 MHz
110 kHz
C1
(pF)
0
15
33
C2
(pF)
50
33
15
3 dB
BW
6 MHz
1.5 MHz
450 kHz
0.01
–80
G = +1000
RL = 600Ω
–90
THD (dB)
DISTORTION REDUCTION
0.003
NOISE LIMIT, G = +1000
G = +1000
RL = 10kΩ
–100
0.001
G = +100
RL = 600Ω
NOISE LIMIT, G = +100
0.0003
–110
G = +10
RL = 600Ω
–120
100
300
1k
THD (%)
2
3k
10k
30k
100k
0.0001
300k
FREQUENCY (Hz)
Figure 45. Total Harmonic Distortion (THD) vs. Frequency @ 3 V rms
for Figure 44b
Differential Line Receiver
The differential receiver circuit of Figure 46 is useful for many
applications, from audio to MRI imaging. The circuit allows
Rev. F | Page 15 of 20
00846-045
VIN
00846-044
TEKTRONIX
CALIBRATION
FIXTURE
8
AD797
extraction of a low level signal in the presence of commonmode noise. As shown in Figure 47, the AD797 provides this
function with only 9 nV/√Hz noise at the output. Figure 48
shows the AD797 20-bit THD performance over the audio band
and the 16-bit accuracy to 250 kHz.
20pF
1kΩ
1kΩ
+VS
**
50pF*
DIFFERENTIAL
INPUT
7
2
8
AD797
VOUT
6
4
3
A General-Purpose ATE/Instrumentation I/O Driver
The ultralow noise and distortion of the AD797 can be
combined with the wide bandwidth, slew rate, and load drive
of a current feedback amplifier to yield a very wide dynamic
range general-purpose driver. The circuit shown in Figure 49
combines the AD797 with the AD811 in just such an
application. Using the component values shown, this circuit is
capable of better than −90 dB THD with a ±5 V, 500 kHz output
signal. The circuit is, therefore, suitable for driving a high
resolution ADC as an output driver in automatic test equipment
(ATE) systems. Using a 100 kHz sine wave, the circuit drives a
600 Ω load to a level of 7 V rms with less than −109 dB THD
and a 10 kΩ load at less than −117 dB THD.
22pF
**
1kΩ
–VS
R2
1kΩ
2kΩ
+VS
*
20pF
+VS
2
00846-046
* OPTIONAL
** USE THE POWER SUPPLY BYPASSING
SHOWN IN FIGURE 35.
7
AD797
Figure 46. Differential Line Receiver
VIN
1kΩ
3
6
3
7
AD811
4
*
16
2
4
649Ω
VOUT
*
–VS
649Ω
12
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
10
Figure 49. A General-Purpose ATE/Instrumentation I/O Driver
Ultrasound/Sonar Imaging Preamp
8
The AD600 variable gain amplifier provides the time-controlled
gain (TCG) function necessary for very wide dynamic range
sonar and low frequency ultrasound applications. Under some
circumstances, it is necessary to buffer the input of the AD600
to preserve its low noise performance. To optimize dynamic
range, this buffer should have a maximum of 6 dB of gain. The
combination of low noise and low gain is difficult to achieve.
The input buffer circuit shown in Figure 50 provides 1 nV/√Hz
noise performance at a gain of 2 (dc to 1 MHz) by using
26.1 Ω resistors in its feedback path. Distortion is only −50 dBc
at 1 MHz for a 2 V p-p output level and drops rapidly to better
than −70 dBc at an output level of 200 mV p-p.
10
100
1k
10k
100k
1M
10M
FREQUENCY (Hz)
00846-047
6
Figure 47. Output Voltage Noise Spectral Density
for Differential Line Receiver
0.003
WITHOUT
OPTIONAL
50pF CN
–100
MEASUREMENT
LIMIT
–110
0.001
0.0003
THD (%)
–90
0.0001
–120
–130
100
WITH
OPTIONAL
50pF CN
300
1k
3k
10k
30k
100k
300k
FREQUENCY (Hz)
00846-048
THD (dB)
6
00846-049
OUTPUT VOLTAGE NOISE (nV/√Hz)
–VS
14
*
Figure 48. Total Harmonic Distortion (THD) vs. Frequency
for Differential Line Receiver
Rev. F | Page 16 of 20
AD797
26.1Ω
Professional Audio Signal Processing—DAC Buffers
The low noise and low distortion of the AD797 make it an ideal
choice for professional audio signal processing. An ideal I-to-V
converter for a current output DAC would simply be a resistor
to ground, were it not for the fact that most DACs do not operate
linearly with voltage on their output. Standard practice is to
operate an op amp as an I-to-V converter, creating a virtual
ground at its inverting input. Normally, clock energy and current
steps must be absorbed by the op amp output stage. However, in
the configuration shown in Figure 53, Capacitor CF shunts high
frequency energy to ground while correctly reproducing the
desired output with extremely low THD and IMD.
+VS
*
*
26.1Ω
7
AD797
3
VOUT
4
*
*
–VS
00846-050
VIN
AD600
6
VS = ±6Vdc
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 50. An Ultrasound Preamplifier Circuit
Amorphous (Photodiode) Detector
Large area photodiodes (CS ≥ 500 pF) and certain image
detectors (amorphous Si) have optimum performance when used
in conjunction with amplifiers with very low voltage (rather than
very low current noise). Figure 51 shows the AD797 used with
an amorphous Si (CS = 1000 pF) detector. The response is adjusted
for flatness using capacitor CL, and the noise is dominated by
voltage noise amplified by the ac noise gain. The AD797’s excellent
input noise performance gives 27 μV rms total noise in a 1 MHz
bandwidth, as shown by Figure 52.
CF
82pF
100Ω
3kΩ
+VS
*
AD1862
DAC
2
C1
2000pF
7
AD797
3
4
*
CL
50pF
100Ω
VOUT
6
–VS
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
10kΩ
Figure 53. A Professional Audio DAC Buffer
+VS
+VS
*
IS
CS
1000pF
7
AD797
3
–IN
+IN
–50
60
–60
40
–70
20
–80
100k
1M
FREQUENCY (Hz)
10M
0
100M
Figure 54. Offset Null Configuration
00846-052
VOUT (dB Re 1V/µA)
80
NOISE
20kΩ
VOS ADJUST
VOLTAGE NOISE (mV rms (0.1Hz FREQUENCY))
100
10k
VOUT
–VS
Figure 51. Amorphous Detector Preamp
–30
1k
3
4
00846-051
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
100
6
5
1
–VS
VOUT
7
AD797
4
*
–40
2
VOUT
6
Figure 52. Total Integrated Voltage Noise and VOUT
of Amorphous Detector Preamp
Rev. F | Page 17 of 20
00846-054
2
00846-053
2
AD797
OUTLINE DIMENSIONS
0.400 (10.16)
0.365 (9.27)
0.355 (9.02)
8
5
1
4
0.280 (7.11)
0.250 (6.35)
0.240 (6.10)
0.100 (2.54)
BSC
0.325 (8.26)
0.310 (7.87)
0.300 (7.62)
0.060 (1.52)
MAX
0.210 (5.33)
MAX
0.015
(0.38)
MIN
0.150 (3.81)
0.130 (3.30)
0.115 (2.92)
SEATING
PLANE
0.022 (0.56)
0.018 (0.46)
0.014 (0.36)
0.195 (4.95)
0.130 (3.30)
0.115 (2.92)
0.015 (0.38)
GAUGE
PLANE
0.430 (10.92)
MAX
0.005 (0.13)
MIN
0.014 (0.36)
0.010 (0.25)
0.008 (0.20)
0.070 (1.78)
0.060 (1.52)
0.045 (1.14)
070606-A
COMPLIANT TO JEDEC STANDARDS MS-001
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.
CORNER LEADS MAY BE CONFIGURED AS WHOLE OR HALF LEADS.
Figure 55. 8-Lead Plastic Dual In-Line Package [PDIP]
Narrow Body (N-8)
Dimensions shown in inches and (millimeters)
5.00 (0.1968)
4.80 (0.1890)
8
1
5
6.20 (0.2441)
5.80 (0.2284)
4
1.27 (0.0500)
BSC
0.25 (0.0098)
0.10 (0.0040)
COPLANARITY
0.10
SEATING
PLANE
1.75 (0.0688)
1.35 (0.0532)
0.51 (0.0201)
0.31 (0.0122)
0.50 (0.0196)
0.25 (0.0099)
45°
8°
0°
0.25 (0.0098)
0.17 (0.0067)
1.27 (0.0500)
0.40 (0.0157)
COMPLIANT TO JEDEC STANDARDS MS-012-A A
CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.
Figure 56. 8-Lead Standard Small Outline Package [SOIC_N]
Narrow Body (R-8)
Dimensions shown in millimeters and (inches)
Rev. F | Page 18 of 20
012407-A
4.00 (0.1574)
3.80 (0.1497)
AD797
ORDERING GUIDE
Model
AD797AN
AD797ANZ 1
AD797AR
AD797AR-REEL
AD797AR-REEL7
AD797ARZ1
AD797ARZ-REEL1
AD797ARZ-REEL71
AD797BR
AD797BR-REEL
AD797BR-REEL7
AD797BRZ1
AD797BRZ-REEL1
AD797BRZ-REEL71
1
Temperature Range
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
Package Description
8-Lead Plastic Dual In-Line Package [PDIP]
8-Lead Plastic Dual In-Line Package [PDIP]
8-Lead Standard Small Outline Package [SOIC_N]
8-Lead Standard Small Outline Package [SOIC_N]
8-Lead Standard Small Outline Package [SOIC_N]
8-Lead Standard Small Outline Package [SOIC_N]
8-Lead Standard Small Outline Package [SOIC_N]
8-Lead Standard Small Outline Package [SOIC_N]
8-Lead Standard Small Outline Package [SOIC_N]
8-Lead Standard Small Outline Package [SOIC_N]
8-Lead Standard Small Outline Package [SOIC_N]
8-Lead Standard Small Outline Package [SOIC_N]
8-Lead Standard Small Outline Package [SOIC_N]
8-Lead Standard Small Outline Package [SOIC_N]
Z = RoHS Compliant Part.
Rev. F | Page 19 of 20
Package Option
N-8
N-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
R-8
AD797
NOTES
©2008 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D00846-0-1/08(F)
Rev. F | Page 20 of 20