a
Dual, Low Noise, Wideband
Variable Gain Amplifiers
AD600/AD602*
FEATURES
Two Channels with Independent Gain Control
“Linear in dB” Gain Response
Two Gain Ranges:
AD600: 0 dB to 40 dB
AD602: –10 dB to +30 dB
Accurate Absolute Gain: 0.3 dB
Low Input Noise: 1.4 nV/√Hz
Low Distortion: –60 dBc THD at 1 V Output
High Bandwidth: DC to 35 MHz (–3 dB)
Stable Group Delay: 2 ns
Low Power: 125 mW (Max) per Amplifier
Signal Gating Function for Each Amplifier
Drives High-Speed A/D Converters
MIL-STD-883-Compliant and DESC Versions Available
APPLICATIONS
Ultrasound and Sonar Time-Gain Control
High-Performance Audio and RF AGC Systems
Signal Measurement
PRODUCT DESCRIPTION
The AD600 and AD602 dual channel, low noise variable gain
amplifiers are optimized for use in ultrasound imaging systems,
but are applicable to any application requiring very precise gain,
low noise and distortion, and wide bandwidth. Each independent channel provides a gain of 0 dB to +40 dB in the AD600
and –10 dB to +30 dB in the AD602. The lower gain of the
AD602 results in an improved signal-to-noise ratio at the output. However, both products have the same 1.4 nV/√Hz input
noise spectral density. The decibel gain is directly proportional
to the control voltage, is accurately calibrated, and is supplyand temperature-stable.
To achieve the difficult performance objectives, a proprietary
circuit form—the X-AMP®—has been developed. Each channel of the X-AMP comprises a variable attenuator of 0 dB to
–42.14 dB followed by a high speed fixed gain amplifier. In this
way, the amplifier never has to cope with large inputs, and can
benefit from the use of negative feedback to precisely define the
gain and dynamics. The attenuator is realized as a seven-stage
R-2R ladder network having an input resistance of 100 Ω, lasertrimmed to ± 2%. The attenuation between tap points is 6.02 dB;
the gain-control circuit provides continuous interpolation between
these taps. The resulting control function is linear in dB.
FUNCTIONAL BLOCK DIAGRAM
GAT1
SCALING
REFERENCE
PRECISION PASSIVE
INPUT ATTENUATOR
GATING
INTERFACE
C1HI
VG
A1OP
GAIN CONTROL
INTERFACE
A1CM
C1LO
0dB
–12.04dB
–6.02dB
–18.06dB
–22.08dB
–36.12dB
–30.1dB
–42.14dB
A1HI
A1LO
500
R – 2R LADDER NETWORK
62.5
RF2
2.24k(AD600)
694(AD602)
RF1
20
FIXED-GAIN
AMPLIFIER
41.07dB(AD600)
31.07(AD602)
The gain-control interfaces are fully differential, providing an
input resistance of ~15 MΩ and a scale factor of 32 dB/V (that
is, 31.25 mV/dB) defined by an internal voltage reference. The
response time of this interface is less than 1 µs. Each channel
also has an independent gating facility that optionally blocks
signal transmission and sets the dc output level to within a few
millivolts of the output ground. The gating control input is TTL
and CMOS compatible.
The maximum gain of the AD600 is 41.07 dB, and that of the
AD602 is 31.07 dB; the –3 dB bandwidth of both models is
nominally 35 MHz, essentially independent of the gain. The
signal-to-noise ratio (SNR) for a 1 V rms output and a 1 MHz
noise bandwidth is typically 76 dB for the AD600 and 86 dB for
the AD602. The amplitude response is flat within ± 0.5 dB from
100 kHz to 10 MHz; over this frequency range the group delay
varies by less than ± 2 ns at all gain settings.
Each amplifier channel can drive 100 Ω load impedances with
low distortion. For example, the peak specified output is ± 2.5 V
minimum into a 500 Ω load, or ± 1 V into a 100 Ω load. For a
200 Ω load in shunt with 5 pF, the total harmonic distortion for
a ± 1 V sinusoidal output at 10 MHz is typically –60 dBc.
The AD600J and AD602J are specified for operation from 0°C
to 70°C, and are available in both 16-lead plastic DIP (N) and
16-lead SOIC (R). The AD600A and AD602A are specified for
operation from –40°C to +85°C and are available in both 16-lead
cerdip (Q) and 16-lead SOIC (R).
The AD600S and AD602S are specified for operation from
–55°C to +125°C and are available in a 16-lead cerdip (Q)
package and are MIL-STD-883 compliant. The AD600S and
AD602S are also available under DESC SMD 5962-94572.
X-AMP is a registered trademark of Analog Devices, Inc.
*Patented.
REV. C
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. 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: 781/329-4700
www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 2002
AD600/AD602–SPECIFICATIONS (Each amplifier section, at T = 25C, V = 5 V, –625 mV ≤ V ≤
A
S
G
+625 mV, RL = 500 , and CL = 5 pF, unless otherwise noted. Specifications for AD600 and AD602 are identical unless otherwise noted.)
Parameter
INPUT CHARACTERISTICS
Input Resistance
Input Capacitance
Input Noise Spectral Density1
Noise Figure
Common-Mode Rejection Ratio
OUTPUT CHARACTERISTICS
–3 dB Bandwidth
Slew Rate
Peak Output2
Output Impedance
Output Short-Circuit Current
Group Delay Change vs. Gain
Group Delay Change vs. Frequency
Total Harmonic Distortion
ACCURACY
AD600
Gain Error
Maximum Output Offset Voltage3
Output Offset Variation
AD602
Gain Error
Maximum Output Offset Voltage3
Output Offset Variation
GAIN CONTROL INTERFACE
Gain Scaling Factor
Common-Mode Range
Input Bias Current
Input Offset Current
Differential Input Resistance
Response Rate
SIGNAL GATING INTERFACE
Logic Input “LO” (Output ON)
Logic Input “HI” (Output OFF)
Response Time
Input Resistance
Output Gated OFF
Output Offset Voltage
Output Noise Spectral Density
Signal Feedthrough @ 1 MHz
AD600
AD602
Conditions
AD600J/AD602J
Min
Typ
Max
AD600A/AD602A
Min
Typ
Max
Pins 2 to 3; Pins 6 to 7
98
95
RS = 50 Ω, Maximum Gain
RS = 200 Ω, Maximum Gain
f = 100 kHz
VOUT = 100 mV rms
RL ≥ 500 Ω
f ≤ 10 MHz
± 2.5
f = 3 MHz; Full Gain Range
VG = 0 V, f = 1 MHz to 10 MHz
RL= 200 Ω, VOUT = ± 1 V Peak, Rpd = 1 kΩ
100
2
1.4
5.3
2
30
102
35
275
±3
2
50
±2
±2
–60
± 2.5
100
2
1.4
5.3
2
30
105
35
275
±3
2
50
±2
±2
–60
Unit
Ω
pF
nV/√Hz
dB
dB
dB
MHz
V/µs
V
Ω
mA
ns
ns
dBc
0 dB to 3 dB Gain
3 dB to 37 dB Gain
37 dB to 40 dB Gain
VG = –625 mV to +625 mV
VG = –625 mV to +625 mV
0
–0.5
–1
+0.5
± 0.2
–0.5
10
10
+1
+0.5
0
50
50
–0.5
–1.0
–1.5
+0.5
± 0.2
–0.5
10
10
+1.5
+1.0
+0.5
65
65
dB
dB
dB
mV
mV
–10 dB to –7 dB Gain
–7 dB to +27 dB Gain
27 dB to 30 dB Gain
VG = –625 mV to +625 mV
VG = –625 mV to +625 mV
0
–0.5
–1
+0.5
± 0.2
–0.5
5
5
+1
+0.5
0
30
30
–0.5
–1.0
–1.5
+0.5
± 0.2
–0.5
10
10
+1.5
+1.0
+0.5
45
45
dB
dB
dB
mV
mV
32
32.3
+2.5
1
50
30.5
–0.75
32
33.5
+2.5
1
50
dB/V
V
µA
nA
MΩ
dB/µs
0.8
V
V
µs
kΩ
400
mV
nV/√Hz
3 dB to 37 dB (AD600); –7 dB to +27 dB (AD602) 31.7
–0.75
0.35
10
15
40
Pins 1 to 16; Pins 8 to 9
Full 40 dB Gain Change
0.35
10
15
40
0.8
2.4
ON to OFF, OFF to ON
Pins 4 to 3; Pins 5 to 6
2.4
0.3
30
± 10
65
0.3
30
100
± 10
65
–80
–70
POWER SUPPLY
Specified Operating Range
Quiescent Current
± 4.75
11
–80
–70
± 5.25
12.5
± 4.75
22
dB
dB
± 5.25
14
V
mA
NOTES
1
Typical open or short-circuited input; noise is lower when system is set to maximum gain and input is short-circuited. This figure includes the effects of both voltage
and current noise sources.
2
Using resistive loads of 500 Ω or greater, or with the addition of a 1 kΩ pull-down resistor when driving lower loads.
3
The dc gain of the main amplifier in the AD600 is X113; thus an input offset of only 100 µV becomes an 11.3 mV output offset. In the AD602, the amplifier’s gain is
X35.7; thus, an input offset of 100 µV becomes a 3.57 mV output offset.
Specifications shown 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 guaranteed, although only those shown in boldface are tested on all production units.
Specifications subject to change without notice.
–2–
REV. C
AD600/AD602
ABSOLUTE MAXIMUM RATINGS 1
Supply Voltage ± VS . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 7.5 V
Input Voltages
Pins 1, 8, 9, 16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± VS
Pins 2, 3, 6, 7 . . . . . . . . . . . . . . . . . . . . . . ± 2 V Continuous
. . . . . . . . . . . . . . . . . . . . . . . . . ± VS for 10 ms
Pins 4, 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± VS
Internal Power Dissipation2 . . . . . . . . . . . . . . . . . . . . 600 mW
Operating Temperature Range (J) . . . . . . . . . . . . 0°C to 70°C
Operating Temperature Range (A) . . . . . . . . –40°C to +85°C
Operating Temperature Range (S) . . . . . . . –55°C to +125°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Lead Temperature Range (Soldering 60 sec) . . . . . . . . . 300°C
NOTES
1
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.
2
Thermal Characteristics:
16-Lead Plastic Package: θJA = 85°C/W
16-Lead SOIC Package: θJA = 100°C/W
16-Lead Cerdip Package: θJA = 120°C/W
ORDERING GUIDE
Model
Gain
Range
Temperature
Range
Package
Option1
AD600AQ
AD600AR
AD600AR-REEL
AD600AR-REEL7
AD600JN
AD600JR
AD600JR-REEL
AD600JR-REEL7
AD600SQ/883B2
0 dB to 40 dB
0 dB to 40 dB
0 dB to 40 dB
0 dB to 40 dB
0 dB to 40 dB
0 dB to 40 dB
0 dB to 40 dB
0 dB to 40 dB
0 dB to 40 dB
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
0°C to 70°C
0°C to 70°C
0°C to 70°C
0°C to 70°C
–55°C to +125°C
Q-16
R-16
13" Reel
7" Reel
N-16
R-16
13" Reel
7" Reel
Q-16
AD602AQ
AD602AR
AD602AR-REEL
AD602AR-REEL7
AD602JN
AD602JR
AD602JR-REEL
AD602JR-REEL7
AD602SQ/883B3
–10 dB to +30 dB
–10 dB to +30 dB
–10 dB to +30 dB
–10 dB to +30 dB
–10 dB to +30 dB
–10 dB to +30 dB
–10 dB to +30 dB
–10 dB to +30 dB
–10 dB to +30 dB
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
0°C to 70°C
0°C to 70°C
0°C to 70°C
0°C to 70°C
–55°C to +150°C
Q-16
R-16
13" Reel
7" Reel
N-16
R-16
13" Reel
7" Reel
PIN FUNCTION DESCRIPTIONS
Pin Mnemonic
Description
1
C1LO
2
A1HI
3
A1LO
4
GAT1
5
GAT2
6
A2LO
7
A2HI
8
C2LO
9
C2HI
10
A2CM
11
12
13
14
15
A2OP
VNEG
VPOS
A1OP
A1CM
16
C1HI
CH1 Gain-Control Input “LO” (Positive
Voltage Reduces CH1 Gain)
CH1 Signal Input “HI” (Positive Voltage
Increases CH1 Output)
CH1 Signal Input “LO” (Usually Taken to
CH1 Input Ground)
CH1 Gating Input (A Logic “HI” Shuts Off
CH1 Signal Path)
CH2 Gating Input (A Logic “HI” Shuts Off
CH2 Signal Path)
CH2 Signal Input “LO” (Usually Taken to
CH2 Input Ground)
CH2 Signal Input “HI” (Positive Voltage
Increases CH2 Output)
CH2 Gain-Control Input “LO” (Positive
Voltage Reduces CH2 Gain)
CH2 Gain-Control Input “HI” (Positive
Voltage Increases CH2 Gain)
CH2 Common (Usually Taken to CH2
Output Ground)
CH2 Output
Negative Supply for Both Amplifiers
Positive Supply for Both Amplifiers
CH1 Output
CH1 Common (Usually Taken to CH1
Output Ground)
CH1 Gain-Control Input “HI” (Positive
Voltage Increases CH1 Gain)
CONNECTION DIAGRAM
16-Lead Plastic DIP (N) Package
16-Lead Plastic SOIC (R) Package
16-Lead Cerdip (Q) Package
Q-16
NOTES
1
N = Plastic DIP; Q = Cerdip; R = Small Outline IC (SOIC).
2
Refer to AD600/AD602 Military data sheet. Also available as 5962-9457201MEA.
3
Refer to AD600/AD602 Military data sheet. Also available as 5962-9457202MEA.
C1LO
1
A1HI
2
A1LO
3
GAT1
4
16 C1HI
+
15 A1CM
A1
14 A1OP
–
13 VPOS
REF
GAT2
5
A2LO
6
12 VNEG
–
11 A2OP
A2
A2HI
7
C2LO
8
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 AD600/AD602 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.
REV. C
–3–
10 A2CM
+
9
AD600 / AD602
C2HI
WARNING!
ESD SENSITIVE DEVICE
AD600/AD602–Typical Performance Characteristics
0.45
0.35
GAIN ERROR – dB
0.25
0.15
20dB
10dB
17dB
7dB
0.05
0
0
–45
–45
–90
–90
–0.05
–0.15
–0.25
–0.35
–0.45
–0.7
–0.5 –0.3 –0.1 0.1
0.3
0.5
GAIN CONTROL VOLTAGE – Volts
100k
0.7
TPC 1. Gain Error vs. Gain Control
Voltage
1M
10M
FREQUENCY – Hz
100M
TPC 2. AD600 Frequency and Phase
Response vs. Gain
VG = 0V
10dB/DIV
CENTER
FREQ 1MHz
10kHz/DIV
9.8
GROUP DELAY – ns
9.6
9.4
9.2
9.0
8.8
8.6
8.4
8.2
–0.5 –0.3 –0.1 0.1
0.3
0.5
GAIN CONTROL VOLTAGE – Volts
TPC 4. AD600 and AD602 Typical
Group Delay vs. VC
TPC 5. Third Order Intermodulation Distortion, VOUT = 2 V p-p,
RL = 500 Ω
INPUT IMPEDANCE – 99
97
GAIN = 20dB
GAIN = 0dB
96
95
94
–1.8
–2.0
–2.2
–2.4
–2.6
–2.8
–3.0
–3.2
–3.4
0
50
100 200 500 1000 2000
LOAD RESISTANCE – 5
1V VOUT
4
AD600
1µs
100
3
2
AD602
90
1
0
–1
–2
10
0%
1V VC
–3
93
92
100k
–1.6
TPC 6. Typical Output Voltage vs.
Load Resistance (Negative Output
Swing Limits First)
OUTPUT
OUTPUT OFFSET VOLTAGE – mV
GAIN = 40dB
100
98
–1.2
–1.4
6
102
101
100M
–1.0
0.7
INPUT
8.0
–0.7
1M
10M
FREQUENCY – Hz
TPC 3. AD602 Frequency and Phase
Response vs. Gain
NEGATIVE OUTPUT VOLTAGE LIMIT – Volts
10.0
100k
1M
10M
FREQUENCY – Hz
TPC 7. Input Impedance vs.
Frequency
100M
–4
–0.7
–0.5 –0.3 –0.1 0.1
0.3
0.5
GAIN CONTROL VOLTAGE – Volts
0.7
TPC 8. Output Offset vs. Gain
Control Voltage (Control Channel
Feedthrough)
–4–
TPC 9. Gain Control Channel
Response Time. Top: Output Voltage, 2 V max, Bottom: Gain Control Voltage VC = ± 625 mV
REV. C
AD600/AD602
50mV
50mV
10
0%
5V
90
90
OUTPUT
100
10
10
0%
100ns
5V
TPC 10. Gating Feedthrough to
Output, Gating Off to On
100ns
AD600
OUTPUT
0
–10
AD600
–20
–30
AD602
–40
–30
–60
AD600: G = 40dB
AD602: G = 30dB
BOTH: RL = 500
VIN = 0V
RS = 50
AD602
–70
–35
10M
100M
TPC 16. CMRR vs. Frequency
REV. C
–80
100k
1M
10M
FREQUENCY – Hz
–20
–30
AD600: CH1 G = 40dB, VIN = 0
CH2 G = 20dB, VIN = 100mV
AD602: CH1 G = 30dB, VIN = 0
CH2 G = 0dB, VIN = 316mV
BOTH: VOUT = 1V RMS1, RS = 50,
RL = 500
CH1 VOUT
CROSSTALK = 20log
CH2 VIN
{
}
–40
–50
AD600
–60
–70
AD602
–80
100M
TPC 17. PSRR vs. Frequency
–5–
500ns
TPC 15. Transient Response
Minimum Gain
10
–50
100k
1M
FREQUENCY – Hz
1V
10
–25
10k
500ns
0
–15
–40
1k
10
0%
20
–10
PSRR – dB
CMRR – dB
10
0%
90
TPC 14. Output Stage Overload
Recovery Time
AD600: G = 20dB
AD602: G = 10dB
BOTH: V CM = 100mV RMS
VS = 5V
RL = 500
TA = 25C
–20
90
200mV
10
–10
100
200ns
TPC 13. Input Stage Overload
Recovery Time
500mV
100
CROSSTALK – dB
1V
500ns
TPC 12. Transient Response,
Medium and High Gain
INPUT
OUTPUT
INPUT
OUTPUT
INPUT
10
0%
0
100mV
1V
90
–5
0%
TPC 11. Gating Feedthrough to
Output, Gating On to Off
500mV
100
5
1V
100
INPUT
OUTPUT
90
INPUT
INPUT
OUTPUT
100
–90
100k
1M
10M
FREQUENCY – Hz
100M
TPC 18. Crosstalk Between A1
and A2 vs. Frequency
AD600/AD602
THEORY OF OPERATION
To understand the AD600, it will help to think in terms of a
mechanical means for moving this slider from left to right; in
fact, it is voltage controlled. The details of the control interface
are discussed later. Note that the gain is exactly determined
at all times, and a linear decibel relationship is automatically
guaranteed between the gain and the control parameter that
determines the position of the slider. In practice, the gain
deviates from the ideal law, by about ±0.2 dB peak (see Figure 6).
The AD600 and AD602 have the same general design and
features. They comprise two fixed gain amplifiers, each preceded by a voltage-controlled attenuator of 0 dB to 42.14 dB
with independent control interfaces, each having a scaling factor
of 32 dB per volt. The gain of each amplifier in the AD600 is
laser trimmed to 41.07 dB (X113), providing a control range
of –1.07 dB to 41.07 dB (0 dB to 40 dB with overlap), while
the AD602 amplifiers have a gain of 31.07 dB (X35.8) and
provide an overall gain of –11.07 dB to 31.07 dB (–10 dB to
+30 dB with overlap).
The advantage of this topology is that the amplifier can use
negative feedback to increase the accuracy of its gain. Also, since
the amplifier never has to handle large signals at its input, the
distortion can be very low. Another feature of this approach is
that the small-signal gain and phase response, and thus the
pulse response, are essentially independent of gain.
The following discussion describes the AD600. Figure 1 is a
simplified schematic of one channel. The input attenuator is a
seven-section R-2R ladder network, using untrimmed resistors
of nominally R = 62.5 Ω, which results in a characteristic resistance of 125 Ω ± 20%. A shunt resistor is included at the input
and laser trimmed to establish a more exact input resistance of
100 Ω ± 2%, which ensures accurate operation (gain and HP
corner frequency) when used in conjunction with external resistors or capacitors.
Noise Performance
An important reason for using this approach is the superior
noise performance that can be achieved. The nominal resistance
seen at the inner tap points of the attenuator is 41.7 Ω (one third
of 125 Ω), which exhibits a Johnson noise spectral density (NSD)
of 0.84 nV/√Hz (that is, √4kTR) at 27°C, which is a large fraction
of the total input noise. The first stage of the amplifier contributes another 1.12 nV/√Hz, for a total input noise of 1.4 nV/√Hz.
The noise at the 0 dB tap depends on whether the input is
short-circuited or open-circuited. When shorted, the minimum
NSD of 1.12 nV/√Hz is achieved. When open, the resistance
of 100 Ω at the first tap generates 1.29 nV/√Hz, so the noise
increases to a total of 1.71 nV/√Hz. This last calculation would
be important if the AD600 were preceded, for example, by a
900 Ω resistor to allow operation from inputs up to ± 10 V rms.
However, in most cases the low impedance of the source will
limit the maximum noise resistance.
GAT1
SCALING
REFERENCE
PRECISION PASSIVE
INPUT ATTENUATOR
GATING
INTERFACE
C1HI
VG
A1OP
C1LO
A1CM
GAIN CONTROL
INTERFACE
0dB
–12.04dB
–6.02dB
–18.06dB
–22.08dB
–36.12dB
–30.1dB
–42.14dB
A1HI
A1LO
500
R – 2R LADDER NETWORK
62.5
Note that the signal inputs are not fully differential. A1LO,
A1CM (for CH1), A2LO, and A2CM (for CH2) provide
separate access to the input and output grounds. This recognizes that even when using a ground plane, small differences
will arise in the voltages at these nodes. It is important that
A1LO and A2LO be connected directly to the input ground(s).
Significant impedance in these connections will reduce the gain
accuracy. A1CM and A2CM should be connected to the load
ground(s).
RF2
2.24k(AD600)
694(AD602)
It will be apparent from the foregoing that it is essential to use a
low resistance in the design of the ladder network to achieve low
noise. In some applications this may be inconvenient, requiring
the use of an external buffer or preamplifier. However, very few
amplifiers combine the needed low noise with low distortion at
maximum input levels, and the power consumption required to
achieve this performance is quite high (due to the need to maintain
very low resistance values while also coping with large inputs).
On the other hand, there is little value in providing a buffer
with high input impedance, since the usual reason for this—
the minimization of loading of a high resistance source—is not
compatible with low noise.
RF1
20
FIXED-GAIN
AMPLIFIER
41.07dB(AD600)
31.07(AD602)
Figure 1. Simplified Block Diagram of Single Channel of
the AD600 and AD602
The nominal maximum signal at input A1HI is 1 V rms (± 1.4 V
peak) when using the recommended ± 5 V supplies, although
operation to ± 2 V peak is permissible with some increase in HF
distortion and feedthrough. Each attenuator is provided with a
separate signal “LO” connection for use in rejecting commonmode, the voltage between input and output grounds. Circuitry
is included to provide rejection of up to ± 100 mV.
Apart from the small variations just mentioned, the signal-tonoise (S/N) ratio at the output is essentially independent of the
attenuator setting, since the maximum undistorted output is 1 V
rms and the NSD at the output of the AD600 is fixed at 113
times 1.4 nV/√Hz, or 158 nV/√Hz. Thus, in a 1 MHz bandwidth,
the output S/N ratio would be 76 dB. The input NSD of the
AD600 and AD602 are the same, but because of the 10 dB
lower gain in the AD602’s fixed amplifier, its output S/N ratio is
10 dB better, or 86 dB in a 1 MHz bandwidth.
The signal applied at the input of the ladder network is attenuated by 6.02 dB by each section; thus, the attenuation to each of
the taps is progressively 0, 6.02, 12.04, 18.06, 24.08, 30.1, 36.12,
and 42.14 dB. A unique circuit technique is employed to interpolate between these tap points, indicated by the “slider” in Figure
1, providing continuous attenuation from 0 dB to 42.14 dB.
–6–
REV. C
AD600/AD602
The Gain-Control Interface
Common-Mode Rejection
The attenuation is controlled through a differential, high impedance (15 MΩ) input, with a scaling factor that is laser trimmed to
32 dB per volt, that is, 31.25 mV/dB. Each of the two amplifiers
has its own control interface. An internal bandgap reference
ensures stability of the scaling with respect to supply and temperature variations, and is the only circuitry common to both channels.
A special circuit technique provides rejection of voltages appearing between input grounds (A1LO and A2LO) and output
grounds (A1CM and A2CM). This is necessary because of the
“op amp” form of the amplifier, as shown in Figure 1. The
feedback voltage is developed across the resistor RF1 (which, to
achieve low noise, has a value of only 20 Ω). The voltage
developed across this resistor is referenced to the input common,
so the output voltage is also referred to that node.
When the differential input voltage VG = 0 V, the attenuator
“slider” is centered, providing an attenuation of 21.07 dB,
resulting in an overall gain of 20 dB (= –21.07 dB + 41.07 dB).
When the control input is –625 mV, the gain is lowered by
20 dB (= 0.625 × 32), to 0 dB; when set to 625 mV, the gain is
increased by 20 dB, to 40 dB. When this interface is overdriven in either direction, the gain approaches either –1.07 dB
(= –42.14 dB + 41.07 dB) or 41.07 dB (= 0 + 41.07 dB),
respectively.
ACHIEVING 80 dB GAIN RANGE
The gain of the AD600 can be calculated using the following
simple expression:
Gain (dB) = 32 VG + 20
(1)
where VG is in volts. For the AD602, the expression is:
Gain (dB) = 32 VG + 10
For zero differential signal input between A1HI and A1LO,
the output A1OP simply follows the voltage at A1CM. Note that
the range of voltage differences which can exist between A1LO
and A1CM (or A2LO and A2CM) is limited to about ±100 mV.
TPC 16 shows typical common-mode rejection ratio versus
frequency.
(2)
The two amplifier sections of the X-AMP can be connected
in series to achieve higher gain. In this mode, the output of
A1 (A1OP and A1CM) drives the input of A2 via a high-pass
network (usually just a capacitor) that rejects the dc offset. The
nominal gain range is now –2 dB to +82 dB for the AD600 or
–22 dB to +62 dB for the AD602.
Operation is specified for VG in the range from –625 mV dc
to +625 mV dc. The high impedance gain-control input ensures
minimal loading when driving many amplifiers in multiple-channel
applications. The differential input configuration provides flexibility in choosing the appropriate signal levels and polarities for
various control schemes.
There are several options in connecting the gain-control inputs.
The choice depends on the desired signal-to-noise ratio (SNR)
and gain error (output ripple). The following examples feature
the AD600; the arguments generally apply to the AD602, with
appropriate changes to the gain values.
For example, the gain-control input can be fed differentially to
the inputs, or single-ended by simply grounding the unused
input. In another example, if the gain is to be controlled by a
DAC providing a positive only ground referenced output, the
“Gain Control LO” pin (either C1LO or C2LO) should be
biased to a fixed offset of 625 mV, to set the gain to 0 dB
when “Gain Control HI” (C1HI or C2HI) is at zero, and to
40 dB when at 1.25 V.
In the sequential mode of operation, the SNR is maintained at
its highest level for as much of the gain control range possible,
as shown in Figure 2. Note here that the gain range is 0 dB to
80 dB. Figure 3 shows the general connections to accomplish
this. Both gain-control inputs, C1HI and C2HI, are driven in
parallel by a positive only, ground referenced source with a
range of 0 V to 2.5 V.
Sequential Mode (Maximum S/N Ratio)
85
It is a simple matter to include a voltage divider to achieve other
scaling factors. When using an 8-bit DAC having a FS output of
2.55 V (10 mV/bit), a divider ratio of 1.6 (generating 6.25 mV/
bit) would result in a gain setting resolution of 0.2 dB/ bit. Later,
we will discuss how the two sections of an AD600 or AD602
may be cascaded, when various options exist for gain control.
80
75
S/N RATIO – dB
70
Signal-Gating Inputs
Each amplifier section of the AD600 and AD602 is equipped
with a signal gating function, controlled by a TTL or CMOS
logic input (GAT1 or GAT2). The ground references for these
inputs are the signal input grounds A1LO and A2LO, respectively. Operation of the channel is unaffected when this input is
LO or left open-circuited. Signal transmission is blocked when
this input is HI. The dc output level of the channel is set to
within a few millivolts of the output ground (A1CM or A2CM),
and simultaneously the noise level drops significantly. The
reduction in noise and spurious signal feedthrough is useful
in ultrasound beam-forming applications, where many amplifier
outputs are summed.
REV. C
65
60
55
50
45
40
35
30
–0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
VG
Figure 2. S/N Ratio vs. Control Voltage Sequential Control
(1 MHz Bandwidth)
An auxiliary amplifier that senses the voltage difference
between input and output commons is provided to reject
this common voltage.
–7–
AD600/AD602
A2
A1
–40.00dB
INPUT
0dB
–41.07dB
–40.00dB
C1HI
1.07dB
41.07dB
C1LO
–42.14dB
C1HI
41.07dB
OUTPUT
40dB
41.07dB
OUTPUT
80dB
VO2 = 1.908V
VO1 = 0.592V
VC = 0V
(a)
–0.51dB
–1.07dB
–0.51dB
C1HI
41.07dB
40.56dB
–41.63dB
C1HI
C1LO
C1LO
VG2
VG1
VO2 = 1.908V
VO1 = 0.592V
VC = 1.25V
(b)
0dB
38.93dB
0dB
INPUT
0dB
OUTPUT
0dB
VG2
VG1
INPUT
0dB
41.07dB
C1LO
C1HI
41.07dB
41.07dB
C1LO
C1LO
VG2
VG1
VO2 = 1.908V
VO1 = 0.592V
VC = 2.5V
–2.14dB
C1HI
(c)
Figure 3. AD600 Gain Control Input Calculations for Sequential Control Operation
The gains are offset (Figure 4) such that A2’s gain is increased
only after A1’s gain has reached its maximum value. Note that
for a differential input of –700 mV or less, the gain of a single
amplifier (A1 or A2) will be at its minimum value of –1.07 dB;
for a differential input of +700 mV or more, the gain will be at
its maximum value of 41.07 dB. Control inputs beyond these
limits will not affect the gain and can be tolerated without damage or foldover in the response. See the Specifications section of
this data sheet for more details on the allowable voltage range.
The gain is now
Gain (dB) = 32 VC
When VC is set to zero, VG1 = –0.592 V and the gain of A1 is
1.07 dB (recall that the gain of each amplifier section is 0 dB for
VG = 625 mV); meanwhile, VG2 = –1.908 V so the gain of A2
is –1.07 dB. The overall gain is thus 0 dB (see Figure 3a).
When VC = 1.25 V, VG1 = 1.25 V – 0.592 V = 0.658 V, which
sets the gain of A1 to 40.56 dB, while VG2 = 1.25 V – 1.908 V =
–0.658 V, which sets A2’s gain at –0.56 dB. The overall gain is
now 40 dB (see Figure 3b). When VC = 2.5 V, the gain of A1 is
41.07 dB and that of A2 is 38.93 dB, resulting in an overall gain
of 80 dB (see Figure 3c). This mode of operation is further
clarified by Figure 5, which is a plot of the separate gains of A1
and A2 and the overall gain versus the control voltage. Figure 6
is a plot of the gain error of the cascaded amplifiers versus the
control voltage.
(3)
where VC is the applied control voltage.
+41.07dB
Parallel Mode (Simplest Gain-Control Interface)
40.56dB
A1
In this mode, the gain-control voltage is applied to both inputs
in parallel—C1HI and C2HI are connected to the control voltage, and C1LO and C2LO are optionally connected to an offset
voltage of 0.625 V. The gain scaling is then doubled to 64 dB/V,
requiring only 1.25 V for an 80 dB change of gain. The amplitude of the gain ripple in this case is also doubled, as shown in
Figure 7, and the instantaneous signal-to-noise ratio at the
output of A2 decreases linearly as the gain is increased (Figure 8).
+38.93dB
A2
*
20dB
*
+1.07dB
–0.56dB
–1.07dB
0.592
GAIN
(dB)
–2.14
0
0
0.625
20
1.908
1.25
40
1.875
60
2.5
80
VC (V)
82.14
Low Ripple Mode (Minimum Gain Error)
As can be seen in Figures 6 and 7, the output ripple is periodic.
By offsetting the gains of A1 and A2 by half the period of the
ripple, or 3 dB, the residual gain errors of the two amplifiers
can be made to cancel. Figure 9 shows the much lower gain
ripple when configured in this manner. Figure 10 plots the
S/N ratio as a function of gain; it is very similar to that in the
“Parallel Mode.”
*GAIN OFFSET OF 1.07dB, OR 33.44mV
Figure 4. Explanation of Offset Calibration for
Sequential Control
–8–
REV. C
90
75
80
70
70
65
60
60
S/N RATIO – dB
OVERALL GAIN – dB
AD600/AD602
50
40
A1
30
COMBINED
55
50
45
A2
20
40
10
35
0
–10
–0.5
0.0
0.5
1.0
1.5
2.0
2.5
30
0.0
3.0
0.2
0.4
0.6
5
1.2
4
1.0
3
0.8
2
0.6
1
0
–1
–2
–3
–4
0.0
–0.2
–0.4
–0.6
–0.8
–7
–1.0
1.0
1.4
0.2
–6
0.5
1.2
0.4
–5
0.0
1.0
Figure 8. SNR for Cascaded Stages — Parallel
Control
GAIN ERROR – dB
GAIN ERROR – dB
Figure 5. Plot of Separate and Overall Gains in
Sequential Control
–8
–0.5
0.8
VC
VC
1.5
2.0
2.5
–1.2
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3
3.0
VC
VC
Figure 6. Gain Error for Cascaded Stages —
Sequential Control
Figure 9. Gain Error for Cascaded Stages —
Low Ripple Mode
5
80
4
75
3
70
S/N RATIO – dB
GAIN ERROR – dB
2
1
0
–1
–2
65
60
55
50
–3
45
–4
40
–5
–6
–0.1
0
0.2
0.4
0.6
0.8
1.0
35
0.0
1.2
VC
0.4
0.6
0.8
1.0
1.2
VC
Figure 7. Gain Error for Cascaded Stages —
Parallel Control
REV. C
0.2
Figure 10. ISNR vs. Control Voltage —
Low Ripple Mode
–9–
1.4
AD600/AD602
APPLICATIONS
Increasing Output Drive
The full potential of any high performance amplifier can only be
realized by careful attention to details in its applications. The
following pages describe fully tested circuits in which many such
details have already been considered. However, as is always true
of high accuracy, high speed analog circuits, the schematic is
only part of the story; this is no less true for the AD600 and
AD602. Appropriate choices in the overall board layout and the
type and placement of power supply decoupling components are
very important. As explained previously, the input grounds
A1LO and A2LO must use the shortest possible connections.
The AD600/AD602’s output stage has limited capability for
negative-load driving capability. For driving loads less than
500 Ω, the load drive may be increased by approximately 5 mA
by connecting a 1 kΩ pull-down resistor from the output to the
negative supply (Figure 12).
Driving Capacitive Loads
For driving capacitive loads of greater than 5 pF, insert a 10 Ω
resistor between the output and the load. This lowers the possibility of oscillation.
The following circuits show examples of time-gain control for
ultrasound and for sonar, methods for increasing the output
drive, and AGC amplifiers for audio and RF/IF signal processing using both peak and rms detectors. These circuits also
illustrate methods of cascading X-AMPs for either maintaining
the optimal S/N ratio or maximizing the accuracy of the gaincontrol voltage for use in signal measurement. These AGC
circuits may be modified for use as voltage-controlled amplifiers
for use in sonar and ultrasound applications by removing the
detector and substituting a DAC or other voltage source for
supplying the control voltage.
GAIN-CONTROL
VOLTAGE
C1LO
VIN
A1LO
GAT1
CONTROL VOLTAGE,
VG
3
15
+
A1
4
A1OP
13
5
A2LO
6
–
A2HI
7
+
C2LO
8
A1CM
14
–
GAT2
C1HI
12
VPOS
+5V
1k
VNEG
–5V
11 A2OP
ADDED
PULL-DOWN
RESISTOR
10 A2CM
AD600/
AD602
9
C2HI
Figure 12. Adding a 1 kΩ Pull-Down Resistor Increases the
X-AMP’s Output Drive by About 5 mA. Only the A1 Connections Are Shown for Simplicity.
Realizing Other Gain Ranges
Larger gain ranges can be accommodated by cascading amplifiers. Combinations built by cascading two amplifiers include
–20 dB to +60 dB (using one AD602), –10 dB to +70 dB (1/2
of an AD602 followed by 1/2 of an AD600), and 0 dB to 80 dB
(one AD600). In multiple-channel applications, extra protection
against oscillation can be provided by using amplifier sections
from different packages.
An Ultralow Noise VCA
VOLTAGE-OUTPUT
DAC
–625mV
2
16
A2
Ultrasound and sonar systems share a similar requirement: both
need to provide an exponential increase in gain in response to a
linear control voltage, that is, a gain control that is “linear in
dB.” Figure 11 shows the AD600/AD602 configured for a control voltage ramp starting at –625 mV and ending at +625 mV
for a gain-control range of 40 dB. The polarity of the gain-control
voltage may be reversed and the control voltage inputs C1HI and
C1LO reversed to achieve the same effect. The gain-control
voltage can be supplied by a voltage-output DAC such as the
AD7242, which contains two complete DACs, operates from ±5 V
supplies, has an internal reference of +3 V, and provides ± 3 V of
output swing. As such, it is well-suited for use with the AD600/
AD602, needing only a few resistors to scale the output voltage of the DACs to the levels needed by the AD600/AD602.
40dB
1
REF
Time-Gain Control (TGC) and Time-Variable Gain (TVG)
+625mV
0dB
A1HI
The two channels of the AD600 or AD602 can operate in
parallel to achieve a 3 dB improvement in noise level, providing
1 nV/√Hz without any loss of gain accuracy or bandwidth.
VG
A1
GAIN
C1LO
A1HI
A1LO
GAT1
1
16
2
+
3
–
15
A1
A1OP
13
REF
5
A2LO
6
A1CM
14
4
GAT2
C1HI
12
–
VPOS
VNEG
+5V
–5V
11 A2OP
A2
A2HI
7
C2LO
8
+
10 A2CM
9
C2HI
AD600 or AD602
Figure 11. The Simplest Application of the X-AMP Is as a
TGC or TVG Amplifier in Ultrasound or Sonar. Only the A1
Connections Are Shown for Simplicity.
In the simplest case, as shown in Figure 13, the signal inputs
A1HI and A2HI are tied directly together. The outputs A1OP
and A2OP are summed via R1 and R2 (100 Ω each), and the
control inputs C1HI/C2HI and C1LO/C2LO operate in parallel.
Using these connections, both the input and output resistances
are 50 Ω. Thus, when driven from a 50 Ω source and terminated in a 50 Ω load, the gain is reduced by 12 dB, so the gain
range becomes –12 dB to +28 dB for the AD600 and –22 dB to
+18 dB for the AD602. The peak input capability remains unaffected (1 V rms at the IC pins, or 2 V rms from an unloaded
50 Ω source). The loading on each output, with a 50 Ω load, is
effectively 200 Ω, because the load current is shared between
the two channels, so the overall amplifier still meets its specified
maximum output and distortion levels for a 200 Ω load. This amplifier can deliver a maximum sine wave power of 10 dBm to the load.
–10–
REV. C
AD600/AD602
GAIN-CONTROL
VOLTAGE
VG
– +
C1LO
A1HI
A1LO
GAT1
VIN
GAT2
A2LO
A2HI
C2LO
1
16
C1HI
A1CM
2
15
+
3
A1
4
13
REF
5
12
6
–
7
+
100
A1OP
14
–
11
A2
10
8
9
VPOS
VNEG
A2OP
VOUT
+5V
–5V
50
100
A2CM
C2HI
AD600 or AD602
Figure 13. An Ultralow Noise VCA Using the AD600 or
AD602
A Low Noise, 6 dB Preamplifier
In some ultrasound applications, a high input impedance preamplifier is needed to avoid the signal attenuation that results
from loading the transducer by the 100 Ω input resistance of the
X-AMP. High gain cannot be tolerated, because the peak transducer signal is typically ± 0.5 V, while the peak input capability
of the AD600 or AD602 is only slightly more than ±1 V. A
gain of two is a suitable choice. It can be shown that if the
preamplifier’s overall referred-to-input (RTI) noise is to be
the same as that due to the X-AMP alone (1.4 nV/√Hz), then
the input noise of a nX2 preamplifier must be √(3/4) times as
large, that is, 1.2 nV/√Hz.
+5V
R1
49.9
An inexpensive circuit using complementary transistor types
chosen for their low rbb, is shown in Figure 14. The gain is
determined by the ratio of the net collector load resistance to
the net emitter resistance. It is an open-loop amplifier. The gain
will be X2 (6 dB) only into a 100 Ω load, assumed to be provided by the input resistance of the X-AMP; R2 and R7 are in
shunt with this load, and their value is important in defining the
gain. For small-signal inputs, both transistors contribute an
equal transconductance that is rendered less sensitive to signal
level by the emitter resistors R4 and R5. They also play a dominant role in setting the gain.
This is a Class AB amplifier. As VIN increases in a positive direction, Q1 conducts more heavily and its re becomes lower while
Q2 increases. Conversely, increasingly negative values of VIN
result in the re of Q2 decreasing, while the re of Q1 increases.
The design is chosen such that the net emitter resistance is
essentially independent of the instantaneous value of VIN, resulting in moderately low distortion. Low values of resistance and
moderately high bias currents are important in achieving the low
noise, wide bandwidth, and low distortion of this preamplifier.
Heavy decoupling prevents noise on the power supply lines from
being conveyed to the input of the X-AMP.
Table I. Measured Preamplifier Performance
Measurement
Value
Unit
Gain (f = 30 MHz)
Bandwidth (–3 dB)
Input Signal for
1 dB Compression
Distortion
VIN = 200 mV p-p
6
250
dB
MHz
1
V p-p
0.27
0.14
0.44
0.58
1.03
%
%
%
%
nV/√Hz
1.4
15
± 150
±5
15
kΩ
pF
µA
V
mA
1F
VIN = 500 mV p-p
R2
174
Q1
MRF904
R3
562
1F
R4
42.2
0.1F
–5V
VIN
INPUT
GROUND
R5
42.2
100
RIN OF X AMP
+5V
0.1F
R6
562
1F
A Low Noise AGC Amplifier with 80 dB Gain Range
Q2
MM4049
R7
174
OUTPUT
GROUND
1F
R8
49.9
–5V
Figure 14. A Low Noise Preamplifier for the AD600 and
AD602
REV. C
System Input Noise
Spectral Density (NSD)
(Preamp plus X-AMP)
Input Resistance
Input Capacitance
Input Bias Current
Power Supply Voltage
Quiescent Current
HD2
HD3
HD2
HD3
Figure 15 provides an example of the ease with which the AD600
can be connected as an AGC amplifier. A1 and A2 are cascaded,
with 6 dB of attenuation introduced by the 100 Ω resistor R1,
while a time constant of 5 ns is formed by C1 and the 50 Ω of
net resistance at the input of A2. This has the dual effect of (a)
lowering the overall gain range from 0 dB–80 dB to 6 dB–74 dB
and (b) introducing a single-pole low-pass filter with a
–3 dB frequency of about 32 MHz. This ensures stability at the
maximum gain for a slight reduction in the overall bandwidth.
The capacitor C4 blocks the small dc offset voltage at the output of A1 (which might otherwise saturate A2 at its maximum
gain) and introduces a high pass corner at about 8 kHz, useful
in eliminating low frequency noise and spurious signals which
may be present at the input.
–11–
AD600/AD602
5V
R3
46.4k
5V
R4
3.74k
C1LO
RF
INPUT
1
A1HI
2
A1LO
3
GAT1
GAT2
A2LO
A2HI
C2LO
VG
16
+
15
A1
14
–
4
13
REF
5
6
12
–
11
A2
7
8
10
+
AD600
9
AD590
300A
(at 300K)
+5V
C1HI
A1CM
A1OP
VPOS
VNEG
C4
0.1F
+5V DEC
C2
1F
R1
100
FB
0.1F
+5V DEC
Q1
2N3904
0.1F
C1
100pF
–5V DEC
C3
15pF
–5V DEC
A2OP
R2 +
806 VPTAT
1% –
FB
RF
OUTPUT
A2CM
–5V
POWER SUPPLY
DECOUPLING NETWORK
C2HI
Figure 15. This Accurate HF AGC Amplifier Uses Just Three Active Components
A simple half-wave detector is used, based on Q1 and R2. The
average current into capacitor C2 is just the difference between
the current provided by the AD590 (300 µA at 300 K, 27°C)
and the collector current of Q1. In turn, the control voltage VG
is the time integral of this error current. When VG (thus the gain)
is stable, the rectified current in Q1 must, on average, exactly
balance the current in the AD590. If the output of A2 is too
small to do this, VG will ramp up, causing the gain to increase,
until Q1 conducts sufficiently. The operation of this control
system will now be described in detail.
First, consider the particular case where R2 is zero and the output voltage VOUT is a square wave at, say, 100 kHz, well above
the corner frequency of the control loop. During the time VOUT
is negative, Q1 conducts. When VOUT is positive, it is cut off.
Since the average collector current is forced to be 300 µA and the
square wave has a 50% duty-cycle, the current when conducting
must be 600 µA. With R2 omitted, the peak value of VOUT
would be just the VBE of Q1 at 600 µA (typically about 700 mV)
or 2 VBE peak-to-peak. This voltage, hence the amplitude at which
the output stabilizes, has a strong negative temperature coefficient
(TC), typically –1.7 mV/°C. While this may not be troublesome
in some applications, the correct value of R2 will render the
output stable with temperature.
To understand this, first note that the current in the AD590 is
closely proportional to absolute temperature (PTAT). In fact,
this IC is intended for use as a thermometer. For the moment,
assume that the signal is a square wave. When Q1 is conducting,
VOUT is the now the sum of VBE and a voltage that is PTAT and
that can be chosen to have an equal but opposite TC to that of
the base-to-emitter voltage. This is actually nothing more than
the “bandgap voltage reference” principle in thinly veiled disguise!
When we choose R2 so that the sum of the voltage across it
and the VBE of Q1 is close to the bandgap voltage of about
1.2 V, VOUT will be stable over a wide range of temperatures,
provided, that Q1 and the AD590 share the same thermal
environment.
Since the average emitter current is 600 µA during each half-cycle
of the square wave, a resistor of 833 Ω would add a PTAT voltage of 500 mV at 300 K, increasing by 1.66 mV/°C. In practice,
the optimum value of R2 will depend on the transistor used and,
to a lesser extent, on the waveform for which the temperature
stability is to be optimized; for the devices shown and sine wave
signals, the recommended value is 806 Ω. This resistor also
serves to lower the peak current in Q1 and the 200 Hz LP
filter it forms with C2 helps to minimize distortion due to ripple
in VG. Note that the output amplitude under sine wave conditions will be higher than for a square wave, since the average
value of the current for an ideal rectifier would be 0.637 times as
large, causing the output amplitude to be 1.88 (= 1.2/0.637) V, or
1.33 V rms. In practice, the somewhat nonideal rectifier results
in the sine wave output being regulated to about 1.275 V rms.
An offset of 375 mV is applied to the inverting gain-control
inputs C1LO and C2LO. Thus the nominal –625 mV to +625 mV
range for VG is translated upwards (at VG´) to –0.25 V for minimum gain to 1 V for maximum gain. This prevents Q1 from
going into heavy saturation at low gains and leaves sufficient
“headroom” of 4 V for the AD590 to operate correctly at high
gains when using a 5 V supply.
In fact, the 6 dB interstage attenuator means that the overall
gain of this AGC system actually runs from –6 dB to +74 dB.
Thus, an input of 2 V rms would be required to produce a 1 V
rms output at the minimum gain, which exceeds the 1 V rms
maximum input specification of the AD600. The available gain
range is therefore 0 dB to 74 dB (or, X1 to X5000). Since the
gain scaling is 15.625 mV/dB (because of the cascaded stages)
the minimum value of VG´ is actually increased by 6 × 15.625 mV,
or about 94 mV, to –156 mV, so the risk of saturation in Q1
is reduced.
The emitter circuit of Q1 is somewhat inductive (due its finite ft
and base resistance). Consequently, the effective value of R2
increases with frequency. This would result in an increase in the
stabilized output amplitude at high frequencies, but for the addition of C3, determined experimentally to be 15 pF for the 2N3904
for maximum response flatness. Alternatively, a faster transistor
can be used here to reduce HF peaking. Figure 16 shows the ac
response at the stabilized output level of about 1.3 V rms. Figure 17 demonstrates the output stabilization for sine wave
inputs of 1 mV to 1 V rms at frequencies of 100 kHz, 1 MHz,
and 10 MHz.
–12–
REV. C
AD600/AD602
AGC OUTPUT CHANGE – dB
A Wide Range, RMS-Linear dB Measurement System
(2 MHz AGC Amplifier with RMS Detector)
3dB
0.1
100
1
10
FREQUENCY – MHz
RELATIVE OUTPUT – dB
Figure 16. AC Response at the Stabilized Output Level
of 1.3 V RMS
+0.2
100kHz
0
1MHz
–0.2
Monolithic rms-dc converters provide an inexpensive means to
measure the rms value of a signal of arbitrary waveform, and
they also may provide a low accuracy logarithmic (“decibelscaled”) output. However, they have certain shortcomings. The
first of these is their restricted dynamic range, typically only
50 dB. More troublesome is that the bandwidth is roughly proportional to the signal level; for example, the AD636 provides a
3 dB bandwidth of 900 kHz for an input of 100 mV rms, but
has a bandwidth of only 100 kHz for a 10 mV rms input. Its
logarithmic output is unbuffered, uncalibrated and not stable
over temperature. Considerable support circuitry, including at
least two adjustments and a special high TC resistor, is required
to provide a useful output.
All of these problems can be eliminated using an AD636 as
the detector element in an AGC loop, in which the difference
between the rms output of the amplifier and a fixed dc reference are nulled in a loop integrator. The dynamic range and
the accuracy with which the signal can be determined are now
entirely dependent on the amplifier used in the AGC system. Since
the input to the rms-dc converter is forced to a constant amplitude, close to its maximum input capability, the bandwidth is no
longer signal dependent. If the amplifier has an exactly exponential (“linear-dB”) gain-control law, its control voltage VG is
forced by the AGC loop to be have the general form:
10MHz
VOUT = VSCALE log 10
–0.4
0.001
0.01
0.1
INPUT AMPLITUDE – V RMS
1
Figure 17. Output Stabilization vs. RMS Input for
Sine Wave Inputs at 100 kHz, 1 MHz, and 10 MHz
While the “bandgap” principle used here sets the output amplitude to 1.2 V (for the square wave case), the stabilization point
can be set to any higher amplitude, up to the maximum output
of ± (VS – 2) V that the AD600 can support. It is only necessary
to split R2 into two components of appropriate ratio whose
parallel sum remains close to the zero-TC value of 806 Ω. This
is illustrated in Figure 18, which shows how the output can be
raised, without altering the temperature stability.
5V
AD590
300A
(at 300K)
TO AD600 PIN 16
C2
1F
Q1
2N3904
R2B
C3
15pF
TO AD600 PIN 11
R2A
+
VPTAT
–
R2 = R2A R2B ≈ 806
RF
OUTPUT
Figure 18. Modification in Detector to Raise Output to
2 V RMS
REV. C
VIN ( RMS )
VREF
(4)
Figure 19 shows a practical wide dynamic range rms-responding
measurement system using the AD600. Note that the signal
output of this system is available at A2OP, and the circuit can
be used as a wideband AGC amplifier with an rms-responding
detector. This circuit can handle inputs from 100 µV to 1 V rms
with a constant measurement bandwidth of 20 Hz to 2 MHz,
limited primarily by the AD636 rms converter. Its logarithmic
output is a loadable voltage accurately calibrated to 100 mV/dB,
or 2 V per decade, which simplifies the interpretation of the
reading when using a DVM and is arranged to be –4 V for an
input of 100 µV rms input, zero for 10 mV, and +4 V for a
1 V rms input. In terms of Equation 4, VREF is 10 mV and
VSCALE is 2 V.
Note that the peak “log output” of ± 4 V requires the use of
± 6 V supplies for the dual op amp U3 (AD712) although lower
supplies would suffice for the AD600 and AD636. If only ± 5 V
supplies are available, it will be either necessary to use a reduced
value for VSCALE (say 1 V, in which case the peak output would
be only ± 2 V) or restrict the dynamic range of the signal to
about 60 dB.
As in the previous case, the two amplifiers of the AD600 are
used in cascade. However, the 6 dB attenuator and low-pass
filter found in Figure 1 are replaced by a unity gain buffer
amplifier U3A, whose 4 MHz bandwidth eliminates the risk of
instability at the highest gains. The buffer also allows the use of
a high impedance coupling network (C1/R3) that introduces a
high-pass corner at about 12 Hz. An input attenuator of 10 dB
(X0.316) is now provided by R1 + R2 operating in conjunction
with the AD600’s input resistance of 100 Ω. The adjustment
provides exact calibration of the logarithmic intercept VREF in
critical applications, but R1 and R2 may be replaced by a fixed
–13–
AD600/AD602
VRMS
AF/RF
OUTPUT
C1
0.1F
C4
4.7F
+6V DEC
CAL
0dB
INPUT
1V RMS
MAX
(SINE WAVE)
R1
115
C1LO
A1HI
R2 200
A1LO
GAT1
R3
133k
U3A
1/2
AD712
VG
15.625mV/dB
GAT2
A2LO
A2HI
C2LO
R4
3.01k
+6V
1
2
3
16
15
+
A1
14
–
4
13
REF
5
6
12
–
11
A2
7
8
C1HI
1 VINP
A1CM
A1OP
–6V
DEC
VPOS
+6V
DEC
VNEG
–6V
DEC
A2OP
2
NC
C2
2F
U1 AD600
FB
13 NC
0.1F
3 VNEG
12 NC
4 CAVG
11 NC
COMM 10
5 VLOG
NC
6 BFOP
LDLO
9
7 BFIN
VRMS
8
A2CM
9
U2
AD636
NC
10
+
VPOS 14
R7
56.2k
0.1F
–6V
DEC
R6
3.16k
FB
+316.2mV
C2HI
1/2
AD712
+6V
DEC
U3B
C3
1F
–6V
POWER SUPPLY
DECOUPLING
NETWORK
VOUT
+100mV/dB
0V = 0dB(AT 10mV RMS)
R5
16.2k
NC = NO CONNECT
Figure 19. The Output of This Three-IC Circuit Is Proportional to the Decibel Value of the RMS Input
resistor of 215 Ω if very close calibration is not needed, since the
input resistance of the AD600 (and all other key parameters of it
and the AD636) are already laser trimmed for accurate operation.
This attenuator allows inputs as large as ± 4 V to be accepted,
that is, signals with an rms value of 1 V combined with a crest
factor of up to 4.
the input required at the AD636 to balance the loop. Finally,
note that unlike most AGC circuits, needing strong temperature
compensation for the internal “kT/q” scaling, these voltages,
and thus the output of this measurement system, are temperature stable, arising directly from the fundamental and exact
exponential attenuation of the ladder networks in the AD600.
The output of A2 is ac coupled via another 12 Hz high-pass
filter formed by C2 and the 6.7 kΩ input resistance of the
AD636. The averaging time constant for the rms-dc converter is
determined by C4. The unbuffered output of the AD636 (at Pin
8) is compared with a fixed voltage of 316 mV set by the positive supply voltage of 6 V and resistors R6 and R7. VREF is
proportional to this voltage, and systems requiring greater calibration accuracy should replace the supply dependent reference
with a more stable source.
Typical results are presented for a sine wave input at 100 kHz.
Figure 20 shows that the output is held very close to the setpoint of 316 mV rms over an input range in excess of 80 dB.
To check the operation, assume an input of 10 mV rms is
applied to the input, which results in a voltage of 3.16 mV rms
at the input to A1, due to the 10 dB loss in the attenuator. If the
system operates as claimed, VOUT (and hence VG) should be
zero. This being the case, the gain of both A1 and A2 will be
20 dB and the output of the AD600 will therefore be 100 times
(40 dB) greater than its input, which evaluates to 316 mV rms,
425
400
375
350
VOUT – mV
Any difference in these voltages is integrated by the op amp
U3B, with a time constant of 3 ms formed by the parallel sum of
R6/R7 and C3. Now, if the output of the AD600 is too high, V
rms will be greater than the “setpoint” of 316 mV, causing the
output of U3B—that is, VOUT—to ramp up (note that the integrator is noninverting). A fraction of VOUT is connected to the
inverting gain-control inputs of the AD600, so causing the gain
to be reduced, as required, until V rms is exactly equal to 316 mV,
at which time the ac voltage at the output of A2 is forced to be
exactly 316 mV rms. This fraction is set by R4 and R5 such
that a 15.625 mV change in the control voltages of A1 and
A2—which would change the gain of the cascaded amplifiers by
1 dB—requires a change of 100 mV at VOUT. Notice here that
since A2 is forced to operate at an output level well below its
capacity, waveforms of high crest factor can be tolerated throughout the amplifier.
450
325
300
275
250
225
200
175
150
10V
100V
1mV
10mV
100mV
INPUT SIGNAL – V RMS
1V
10V
Figure 20. The RMS Output of A2 Is Held Close to the
“Setpoint” 316 mV for an Input Range of Over 80 dB
This system can, of course, be used as an AGC amplifier, in
which the rms value of the input is leveled. Figure 21 shows
the “decibel” output voltage. More revealing is Figure 22, which
shows that the deviation from the ideal output predicted by
Equation 1 over the input range 80 µV to 500 mV rms is within
± 0.5 dB, and within ± 1 dB for the 80 dB range from 80 µV to
800 mV. By suitable choice of the input attenuator R1 + R2,
this could be centered to cover any range from 25 mV to 250 mV
to, say, 1 mV to 10 V, with appropriate correction to the value
of VREF. Note that VSCALE is not affected by the changes in the
–14–
REV. C
AD600/AD602
range. The gain ripple of ±0.2 dB seen in this curve is the result of
the finite interpolation error of the X-AMP. Note that it occurs
with a periodicity of 12 dB—twice the separation between the
tap points (because of the two cascaded stages).
16
15
14
13
U1
AD600
5
12
4
11
3
10
9
VOUT – Volts
2
C1HI
1
NC 2
A1OP
VPOS
VNEG
–6V DEC
+6V DEC
–6V DEC
A2OP
3
VNEG
4
CAVG
U2
AD636
NC 5 VLOG
NC 6 BFOP
7
BFIN
C2HI
–46.875mV
–6V
DEC
–1
C2
2F
A2CM
1
0
VINP
A1CM
10k 78.7 78.7
NC = NO CONNECT
+46.875mV
+6V
DEC
10k
3dB OFFSET
MODIFICATION
–2
–3
Figure 23. Reducing the Gain Error Ripple
–4
The error curve shown in Figure 24 demonstrates that over the
central portion of the range the output voltage can be maintained
very close to the ideal value. The penalty for this modification is
the higher errors at the extremities of the range. The next two
applications show how three amplifier sections can be cascaded
to extend the nominal conversion range to 120 dB, with the
inclusion of simple LP filters of the type shown in Figure 15.
Very low errors can then be maintained over a 100 dB range.
–5
10V
100V
1mV
10mV
100mV
INPUT SIGNAL – V RMS
1V
10V
Figure 21. The dB Output of Figure 19’s Circuit Is Linear
Over an 80 dB Range
2.5
2.0
2.5
2.0
1.0
1.5
0.5
OUTPUT ERROR – dB
OUTPUT ERROR – dB
1.5
0
–0.5
–1.0
–1.5
–2.0
–2.5
10V
0.5
0
–0.5
–1.0
–1.5
100V
1mV
10mV
100mV
INPUT SIGNAL – V RMS
1V
–2.0
10V
–2.5
10V
Figure 22. Data from Figure 20 Presented as the Deviation
from the Ideal Output Given in Equation 4
This ripple can be canceled whenever the X-AMP stages are
cascaded by introducing a 3 dB offset between the two pairs
of control voltages. A simple means to achieve this is shown
in Figure 23: the voltages at C1HI and C2HI are “split” by
± 46.875 mV, or ± 1.5 dB. Alternatively, either one of these pins
can be individually offset by 3 dB and a 1.5 dB gain adjustment
made at the input attenuator (R1 + R2).
REV. C
1.0
100V
1mV
10mV
100mV
INPUT SIGNAL – V RMS
1V
10V
Figure 24. Using the 3 dB Offset Network, the Ripple
Is Reduced
100 dB to 120 dB RMS Responding Constant Bandwidth AGC
Systems with High Accuracy dB Outputs
The next two applications double as both AGC amplifiers and
measurement systems. In both, precise gain offsets are used to
achieve either (1) a very high gain linearity of ± 0.1 dB over
the full 100 dB range or (2) the optimal signal-to-noise ratio
at any gain.
–15–
AD600/AD602
C1LO
INPUT
1V RMS
MAX
(SINE WAVE)
A1HI
A1LO
GAT1
GAT2
A2LO
A2HI
C2LO
1
2
3
16
15
+
A1
14
–
4
13
REF
5
6
12
–
11
A2
7
10
+
8
U1 AD600
9
C1LO
C1HI
A1CM
VPOS
VNEG
+5V
DEC
–5V
DEC
A2OP
C2
0.1F
R4
133k
A1HI
U3A
A1LO
R3
200
1/4
AD713
R1
133k
A2CM
GAT1
GAT2
R5
1.58k
A2LO
U3B
C3
220pF
–2dB
–62.5mV
A2HI
1/4
AD713
C2HI
+5V
0dB
+2dB
+62.5mV
R8
127
R9
10k
C2LO
16
2
15
+
3
A1
14
–
4
13
REF
5
12
–
6
10
+
8
A1CM
A1OP
VPOS
VNEG
C4
2F
VOUT
+5V
DEC
–5V
DEC
11 A2OP
A2
7
C1HI
C2HI
9
U2 AD600
A2CM
+5V
–5V
FB
R2
487
C1
0.1F
A1OP
1
R6
10k
0.1F
R7
127
+5V
DEC
C5
22F
0.1F
–5V
DEC
+5V DEC
FB
1 VINP
–5V
POWER SUPPLY
DECOUPLING
NETWORK
NC
–5V
DEC
+5V DEC
R15
19.6k
R14
301k
Q1
2N3906
R16
6.65k
2
VPOS 14
U4
AD636
13 NC
3 VNEG
12 NC
4 CAVG
11 NC
R11
46.4k
C6
4.7F
R10
3.16k
COMM 10
NC
5 VLOG
NC
6 BFOP
LDLO
9
7 BFIN
VRMS
8
U3C
R12
11.3k
+316.2mV
VLOG
1/4
AD713
NC = NO CONNECT
R13
3.01k
Figure 25. RMS Responding AGC Circuit with 100 dB Dynamic Range
A 100 dB RMS/AGC System with Minimal Gain Error
(Parallel Gain with Offset)
5
4
–16–
3
LOGARITHMIC OUTPUT – V
Figure 25 shows an rms-responding AGC circuit that can
equally well be used as an accurate measurement system. It
accepts inputs of 10 µV to 1 V rms (–100 dBV to 0 dBV) with
generous overrange. Figure 26 shows the logarithmic output, VLOG,
which is accurately scaled 1 V per decade, that is 50 mV/dB, with
an intercept (VLOG = 0) at 3.16 mV rms (–50 dBV). Gain offsets
of ± 2 dB have been introduced between the amplifiers, provided
by the ± 62.5 mV introduced by R6–R9. These offsets cancel a
small gain ripple which arises in the X-AMP from its finite interpolation error, which has a period of 18 dB in the individual VCA
sections. The gain ripple of all three amplifier sections without
this offset (in which case the gain errors simply add) is shown in
Figure 27; it is still a remarkably low ± 0.25 dB over the 108 dB
range from 6 µV to 1.5 V rms. However, with the gain offsets
connected, the gain linearity remains under ± 0.1 dB over the
specified 100 dB range (Figure 28).
2
1
0
–1
–2
–3
–4
–5
1V
10V
100V
1mV
10mV
100mV
1V
10V
INPUT SIGNAL – V RMS
Figure 26. VLOG Plotted vs. VIN for Figure 25‘s Circuit
Showing 120 dB AGC Range
REV. C
AD600/AD602
2.0
1.5
GAIN ERROR – dB
1.0
0.5
0.1
0
–0.1
–0.5
–1.0
–1.5
–2.0
1V
10V
100V
1mV
10mV 100mV
INPUT SIGNAL – V RMS
1V
10V
Figure 27. Gain Error for Figure 19 Without the 2 dB
Offset Modification
2.0
1.5
GAIN ERROR – dB
1.0
0.5
0.1
0
–0.1
–0.5
–1.0
–1.5
–2.0
1V
10V
100V
1mV
10mV 100mV
INPUT SIGNAL – V RMS
1V
10V
Figure 28. Adding the 2 dB Offsets Improves the
Linearization
The maximum gain of this circuit is 120 dB. If no filtering were
used, the noise spectral density of the AD600 (1.4 nV/√Hz)
would amount to an input noise of 8.28 µV rms in the full bandwidth (35 MHz). At a gain of one million, the output noise
would dominate. Consequently, some reduction of bandwidth is
mandatory, and in the circuit of Figure 25 it is due mostly to
a single-pole low-pass filter R5/C3, which provides a –3 dB
frequency of 458 kHz, which reduces the worst-case output
noise (at VAGC) to about 100 mV rms at a gain of 100 dB. Of
course, the bandwidth (and hence output noise) could be easily
reduced further, for example, in audio applications, merely by
increasing C3. The value chosen for this application is optimal
in minimizing the error in the VLOG output for small input signals.
The AD600 is dc-coupled, but even miniscule offset voltages at
the input would overload the output at high gains, so high-pass
filtering is also needed. To provide operation at low frequencies,
two simple zeros at about 12 Hz are provided by R1/C1 and
R4/C2; op amp sections U3A and U3B (AD713) are used to
provide impedance buffering, since the input resistance of the
AD600 is only 100 Ω. A further zero at 12 Hz is provided by C4
and the 6.7 kΩ input resistance of the AD636 rms converter.
The rms value of VLOG is generated at Pin 8 of the AD636; the
averaging time for this process is determined by C5, and the
REV. C
value shown results in less than 1% rms error at 20 Hz. The
slowly varying V rms is compared with a fixed reference of
316 mV, derived from the positive supply by R10/R11. Any
difference between these two voltages is integrated in C6, in
conjunction with op amp U3C, the output of which is VLOG. A
fraction of this voltage, determined by R12 and R13, is returned
to the gain control inputs of all AD600 sections. An increase in
VLOG lowers the gain, because this voltage is connected to the
inverting polarity control inputs.
Now, in this case, the gains of all three VCA sections are being
varied simultaneously, so the scaling is not 32 dB/V but 96 dB/V,
or 10.42 mV/dB. The fraction of VLOG required to set its scaling
to 50 mV/dB is therefore 10.42/50, or 0.208. The resulting fullscale range of VLOG is nominally ±2.5 V. This scaling allows the
circuit to operate from ± 5 V supplies. Optionally, the scaling
can be altered to 100 mV/dB, which would be more easily
interpreted when VLOG is displayed on a DVM, by increasing
R12 to 25.5 kΩ. The full-scale output of ± 5 V then requires the
use of supply voltages of at least ± 7.5 V.
A simple attenuator of 16.6 ± 1.25 dB is formed by R2/R3 and
the 100 Ω input resistance of the AD600. This allows the reference level of the decibel output to be precisely set to zero for an
input of 3.16 mV rms, and thus center the 100 dB range between
10 µV and 1 V. In many applications R2/R3 may be replaced
by a fixed resistor of 590 Ω. For example, in AGC applications, neither the slope nor the intercept of the logarithmic output
is important.
A few additional components (R14–R16 and Q1) improve the
accuracy of VLOG at the top end of the signal range (that is, for
small gains). The gain starts rolling off when the input to the
first amplifier, U1A, reaches 0 dB. To compensate for this nonlinearity, Q1 turns on at VLOG ~ 1.5 V and increases the feedback
to the control inputs of the AD600s, thereby needing a smaller
voltage at VLOG to maintain the input to the AD636 to the setpoint of 316 mV rms.
A 120 dB RMS/AGC System with Optimal S/N Ratio
(Sequential Gain)
In the last case, all gains were adjusted simultaneously, resulting
in an output signal-to-noise ratio (S/N ratio) which is always less
than optimal. The use of sequential gain control results in a
major improvement in S/N ratio, with only a slight penalty in
the accuracy of VLOG, and no penalty in the stabilization accuracy
of VAGC. The idea is to increase the gain of the earlier stages
first (as the signal level decreases) and maintain the highest S/N
ratio throughout the amplifier chain. This can be easily
achieved with the AD600 because its gain is accurate even when
the control input is overdriven. That is, each gain control
“window” of 1.25 V is used fully before moving to the next
amplifier to the right.
Figure 29 shows the circuit for the sequential control scheme.
R6 to R9 with R16 provide offsets of 42.14 dB between the
individual amplifiers to ensure smooth transitions between the
gain of each successive X-AMP, with the sequence of gain
increase being U1A first, then U1B, and lastly U2A. The adjustable attenuator provided by R3 + R17 and the 100 Ω input
resistance of U1A as well as the fixed 6 dB attenuation provided
by R2 and the input resistance of U1B are included both to set
VLOG to read 0 dB when VIN is 3.16 mV rms and to center the
100 dB range between 10 µV rms and 1 V rms input. R5 and
C3 provide a 3 dB noise bandwidth of 30 kHz. R12 to R15
–17–
AD600/AD602
0dB
ADJUST
INPUT
C1LO
R3
R17 200
115
A1HI
A1LO
GAT1
GAT2
A2LO
A2HI
C2LO
1
2
3
16
15
+
A1
14
–
4
13
REF
5
12
6
–
7
+
11
A2
8
10
U1 AD600
9
C1LO
C1HI
A1CM
R2
100
C1
0.1F
A1OP
U3A
R1
+6V 133k
DEC
–6V
C2
R5
DEC 0.1F
5.36k
VPOS
VNEG
A2OP
R4
133k
A2CM
1/4
AD713
3
GAT1
15
13
12
–
7
+
8
A1OP
VPOS
VNEG
C4
2F
VOUT
+5V
DEC
–5V
DEC
11 A2OP
A2
C2LO
A1CM
14
–
6
C1HI
A1
5
A2HI
1/4
AD713
+
REF
A2LO
U3B
16
4
GAT2
C3
0.001F
R7
R8
1k 294
2
A1LO
C2HI
R6
3.4k
1
A1HI
10
9
U2 AD600
A2CM
C2HI
R9
R16
1k 287
+5V
+6V
C5
22F
FB
+6V DEC
0.1F
+6V
DEC
1 VINP
0.1F
–6V
DEC
NC
–6V
DEC
FB
–6V
POWER SUPPLY
DECOUPLING
NETWORK
VPOS 14
U4
AD636
13 NC
3 VNEG
12 NC
4 CAVG
11 NC
R11
56.2k
R10
3.16k
C6
4.7F
COMM 10
NC
5 VLOG
NC
6 BFOP
LDLO
9
7 BFIN
VRMS
8
U3C
+6V DEC
R15
5.11k
2
R13
866
R12
1k
+316.2mV
R14
7.32k
VLOG
1/4
AD713
NC = NO CONNECT
Figure 29. 120 dB Dynamic Range RMS Responding Circuit Optimized for S/N Ratio
change the scaling from 625 mV/decade at the control inputs to
1 V/decade at the output and at the same time center the dynamic
range at 60 dB, which occurs if the VG of U1B is equal to zero.
These arrangements ensure that the VLOG will still fit within the
± 6 V supplies.
Figure 30 shows VLOG to be linear over a full 120 dB range.
Figure 31 shows the error ripple due to the individual gain functions which is bounded by ±0.2 dB (dotted lines) from 6 µV to 2 V.
The small perturbations at about 200 µV and 20 mV, caused
by the impracticality of matching the gain functions perfectly,
are the only sign that the gains are now sequential. Figure 32 is
a plot of VAGC that remains very close to its set value of 316 mV
rms over the full 120 dB range.
5
4
To compare the signal-to-noise ratios in the “simultaneous”
and “sequential” modes of operation more directly, all interstage attenuation was eliminated (R2 and R3 in Figure 25, R2
in Figure 29), the input of U1A was shorted, R5 was selected to
provide a 20 kHz bandwidth (R5 = 7.87 kΩ), and only the gain
control was varied, using an external source. The rms value of
the noise was then measured at VOUT and expressed as an S/N
ratio relative to 0 dBV, this being almost the maximum output
capability of the AD600. Results for the simultaneous mode can
be seen in Figure 33. The S/ N ratio degrades uniformly as the
gain is increased. Note that since the inverting gain control was
used, the gain in this curve and in Figure 34 decreases for more
positive values of the gain-control voltage.
LOGARITHMIC OUTPUT – V
3
2
1
0
–1
–2
–3
–4
–5
1V
10V
100V
1mV
10mV
100mV
1V
10V
INPUT SIGNAL – V RMS
Figure 30. VLOG Is Essentially Linear Over the Full 120 dB
Range
–18–
REV. C
AD600/AD602
In contrast, the S/N ratio for the sequential mode is shown in
Figure 34. U1A always acts as a fixed noise source; varying its
gain has no influence on the output noise. This is a feature of
the X-AMP technique. Thus, for the first 40 dB of control
range (actually slightly more, as explained below), when only
this VCA section has its gain varied, the S/N ratio remains constant. During this time, the gains of U1B and U2A are at their
minimum value of –1.07 dB.
2.0
1.5
GAIN ERROR – dB
1.0
0.5
0.2
0
–0.2
–0.5
90
–1.0
80
–1.5
10V
100V
1mV
10mV 100mV
INPUT SIGNAL – V RMS
1V
S/N RATIO – dB
–2.0
1V
70
10V
Figure 31. The Error Ripple Due to the Individual Gain
Functions
400
60
50
40
30
20
10
0
–1.183 –0.558
GAIN ERROR – mV
350
0.692
1.317
1.942
2.567
3.192
3.817
Figure 34. S/N Ratio vs. Control Voltage for Sequential
Gain Control (Figure 29)
300
250
200
1V
10V
100V
1mV
10mV 100mV
INPUT SIGNAL – V RMS
1V
10V
Figure 32. VAGC Remains Close to Its Setpoint of
316 mV RMS Over the Full 120 dB Range
90
80
S/N RATIO – dB
70
60
40
30
10
0
0
208.3 416.6 625.0
–833.2 –625.0 –416.6 –208.3
CONTROL VOLTAGE, VC (10.417mV/dB) – mV
For the next 40 dB of control range, the gain of U1A remains
fixed at its maximum value of 41.07 dB and only the gain of
U1B is varied, while that of U2A remains at its minimum value
of –1.07 dB. In this interval, the fixed output noise of U1A is
amplified by the increasing gain of U1B and the S/N ratio progressively decreases.
Once U1B reaches its maximum gain of 41.07 dB, its output
also becomes a gain independent noise source; this noise is
presented to U2A. As the control voltage is further increased,
the gains of both U1A and U1B remain fixed at their maximum
value of 41.07 dB, and the S/N ratio continues to decrease.
Figure 34 clearly shows this, because the maximum S/N ratio
of 90 dB is extended for the first 40 dB of input signal before it
starts to roll off.
This arrangement of staggered gains can be easily implemented
because, when the control inputs of the AD600 are overdriven,
the gain limits to its maximum or minimum values without side
effects. This eliminates the need for awkward nonlinear shaping
circuits that have previously been used to break up the gain
range of multistage AGC amplifiers. It is the precise values of the
AD600’s maximum and minimum gain (not 0 dB and 40 dB
but –1.07 dB and +41.07 dB) that explain the rather odd values
of the offset values that are used.
50
20
833.2
Figure 33. S/N Ratio vs. Control Voltage for Parallel Gain
Control (Figure 25)
REV. C
0.067
CONTROL VOLTAGE, VC (31.25mV/dB) – V
The optimization of the output S/N ratio is of obvious value in
AGC systems. However, in applications where these circuits are
considered for their wide range logarithmic measurements capabilities, the inevitable degradation of the S/N ratio at high gains
need not seriously impair their utility. In fact, the bandwidth of
the circuit shown in Figure 25 was specifically chosen to improve
measurement accuracy by altering the shape of the log error curve
(Figure 31) at low signal levels.
–19–
AD600/AD602
Dimensions shown in inches and (mm)
Dimensions shown in inches and (mm)
16-Lead Plastic DIP Package
(N-16)
16-Lead Cerdip Package
(Q-16)
0.87 (22.1) MAX
0.31
(7.87)
0.840 (21.34) MAX
16
9
1
8
0.005 (0.13)
MIN
0.25
(6.35)
0.080 (2.03) MAX
16
9
0.310 (7.87)
0.220 (5.59)
PIN 1
1
0.035
(0.89)
0.18 (4.57)
MAX
0.011
(0.28)
0.125
(3.18)
MIN
0.18
(4.57)
0.018
(0.46)
0.100
(2.54)
C00538–0–5/02(C)
OUTLINE DIMENSIONS
0.033
(0.84)
8
0.060 (1.52)
0.015 (0.38)
0.200 (5.08)
MAX
0.320 (8.13)
0.290 (7.37)
0.150
0.200 (5.08)
(3.81) MIN
0.125 (3.18)
0.023 (0.58) 0.100 0.070 (1.78) SEATING
15
0.014 (0.36) (2.54) 0.030 (0.76) PLANE
0
BSC
7.62 (0.3)
0.015 (0.38)
0.008 (0.20)
Dimensions shown in millimeters and (inches)
16-Lead SOIC Package
(R-16)
10.50 (0.413)
9
16
7.60 10.65
(0.299) (0.419)
8
1
0.75
(0.030)
1.27 (0.05)
REF
2.65
(0.104)
0.30
(0.012)
0.49
(0.019)
0.32
(0.013)
1.07
(0.042)
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
Location
Page
5/02–Data Sheet changed from REV. B to REV. C.
Changes to SPECIFICATIONS .................................................................................................................................................... 2
Renumber Tables and TPCs .................................................................................................................................................. Global
8/01–Data Sheet changed from REV. A to REV. B.
Changes to Accuracy Section of AD600A/AD602A column ............................................................................................................ 2
Changes to Ordering Guide ............................................................................................................................................................ 3
Changes to Figure 3 ....................................................................................................................................................................... 8
–20–
REV. C
PRINTED IN U.S.A.
Revision History