AN-934 60 Db Wide Dynamic Range, Low Frequency

AN-934

APPLICATION NOTE One Technology Way • P. O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com

60 dB Wide Dynamic Range, Low Frequency AGC Circuit Using a Single VGA by James Staley

INTRODUCTION Controllable Gain Element Low frequency automatic gain control (AGC) circuits are used A VGA is a special type of amplifier, which controls its gain by in audio and power equipment for applications such as sensitive electronic means instead of by a set of fixed resistors, as is the microphone preamplifiers (preamps) and regulators. An AGC case with the classic op amp circuit. VGAs are the familiar and circuit, a closed-loop feedback system, is shown in Figure 1. The preferred solution for automatic gain control circuits in a loop consists of a controllable gain element, a detector, a stable variety of communications applications. reference and a comparison circuit. VGAs operate at frequencies from hundreds of kilohertz up to CONTROLLABLE GAIN hundreds of megahertz. An ideal VGA performs as a linear ELEMENT amplifier, without introducing distortion or otherwise corrupting the desired signal. INPUT OUTPUT GAIN CONTROL When a VGA is used, the gain element is an amplifier combined VOLTAGE with electronic volume control. In this example, the controllable – DETECTOR gain element is further reduced to an electronic potentiometer + and a fixed gain amplifier and it adjusts the loop gain by REFERENCE attenuating the input signal, without contributing significant

06975-001 Figure 1. AGC Circuit Using a Variable Gain Amplifier distortion. The other fundamental elements of the loop are the detector, a stable reference, and a summing circuit that senses This application note describes a low frequency AGC circuit the state of the loop, compares it to the stable reference, and using a wide dynamic range AD8336 variable gain amplifier adjusts the output accordingly. (VGA) as the gain control element, an AD736 rms-to-dc converter as the detector, a low cost rail-to-rail AD8551 op amp, A functional block diagram of the AD8336 is shown in Figure 2. and an ADP3339 LDO as the reference. Because of its wide controllable gain range and circuit flexibility, the AD8336 is featured in this application note.

PRAO VGAI 8 9

INPP 4 ATTENUATOR PrA 34dB 1 VOUT –60dB TO 0dB INPN 5 AD8336

GAIN CONTROL PWRA 2 BIAS INTERFACE

10 13 3 11 12

VNEG VPOS VCOM GPOS GNEG 6975-002 0 Figure 2. AD8336 Functional Block Diagram

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TABLE OF CONTENTS Introduction ...... 1 AGC Circuit Design Example...... 3 Circuit Design ...... 3 AGC Circuit Operation...... 4

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70 CIRCUIT DESIGN GAIN CHARACTERISTICS 60 COMPOSITE GAIN An audio AGC requires the following features: VGA STAGE GAIN 50 • Wide dynamic range, which is the ability to amplify very FOR PREAMP GAIN = 26dB 40 USEABLE GAIN RANGE low level signals and very large signals. OF AD8336 30 • Amplification with low distortion over the entire operat- 20 ing range. GAIN (dB) GAIN 10 • A means of adjusting the minimum and maximum 0 gain limits. –10 FOR PREAMP GAIN = 12dB The AD8336 described in this application note uses the Analog FOR PREAMP GAIN = 6dB –20 Devices, Inc. exclusive X-AMP® architecture, consisting of a –30 06975-004 ladder network with multiple taps spaced in equi-resistive –0.7 –0.5 –0.3 –0.1 0.1 0.3 0.5 0.7 increments, and accessed by an array of differential amplifiers. VG (V) See Figure 3. Figure 4. Gain Characteristics of the AD8336 for Various Operating Conditions OUTPUT GAIN INTERPOLATOR FIXED GAIN AMPLIFIER AGC CIRCUIT DESIGN EXAMPLE Gm Signal Voltage levels The range of signal voltages to be controlled, the supply voltages –6dB –6dB VGA 60dB IN and input and output voltage levels, all highly interactive factors, influence the topology of an AGC circuit. In this example, the goal is to fully exploit the full 60 dB gain control range of the

10 SECTION LADDER NETWORK 06975-003 AD8336. First, assume power supply voltages of ±5 V. Figure 3. AD8336 Ladder Network With a known power supply voltage, the stabilized output This circuit architecture offers several important advantages: voltage is established. Because saturation in either the preamplifier or the 34 dB fixed gain stage limits the available • A passive resistor ladder network performs the gain control output swing to about 7 V p-p, a nominal maximum swing of function, introducing no distortion. 5 V p-p is easily attainable. With a preamp output-voltage swing • The gain element is a fixed gain op amp. Because the gain of 5 V p-p and the X-AMP attenuator set for −26 dB (0.05×), of the op amp remains unchanged, the application benefits the output voltage is 250 mV p-p. If the preamp gain is set at from constant bandwidth, distortion, and overload per- −1× (unity inverting gain (equivalent to a noise gain of 2×), formance optimized over a wide range of operating the maximum input voltage is 5 V p-p. Finally, with a gain conditions. range of 60 dB, the minimum input voltage is 5 mV p-p. The The AD8336 features a wide gain range (60 dB) and extended AGC circuit operates with an input voltage range of 60 dB supply voltage, capable of operating with power supplies up to (5 mV p-p to 5 V p-p), with a fixed output voltage of 250 mV p-p. ±15 V. It features an uncommitted preamplifier and permits Control Voltage Levels inverting, noninverting, or differential input configurations. The differential gain control input of the AD8336 performs any The preamplifier and VGA sections are completely independ- level shifting required for the available control voltages consid- ent, and the VGA can be used as a standalone element if no erably simplifying the gain control drive circuitry. In this preamp is needed. The gain control inputs are fully differential. example, the GNEG input (Pin 12) is biased at 0.75 V and the Figure 4 shows the gain characteristic for the VGA, for two gain range voltage at GPOS is 1.5 V. values of preamplifier gain. Detector The detector is an AD736 rms-to-dc converter and provides an accurate dc control voltage directly proportional to the rms value of the output signal. The output of the AD736 drives the inverting input of an op amp connected for very high dc gain for accurate loop control.

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Comparison Circuit Table 1. The AD8551 is a single-supply rail-to-rail op amp with a very EIN EOUT (mV p-p) low offset voltage. The voltage applied to the noninverting input (V rms) 20 Hz 100 Hz 1 kHz 5 kHz 10 kHz 20 kHz is the reference voltage, and establishes the rms value of the 0.001 125 130 136 135 140 140 output. The voltage to be compared is the detector voltage from 0.002 245 255 253 253 260 265 the rms–to-dc converter. When the comparison input falls 0.003 251 250 251 253 257 258 below the reference the comparison output voltages increases to 0.005 250 250 250 251 256 258 restore the output to its nominal level. 0.01 250 250 250 251 255 255 0.1 250 250 250 251 254 254 AGC CIRCUIT OPERATION 1 250 250 250 251 254 254 Table 1 lists the data for the AGC control for six frequencies 1.5 250 250 250 251 254 254 from 1 mV to 2 V rms input. Refer to Figure 6 for a plot 1.8 250 249 250 250 254 254 showing the flat output level vs. input for the typical audio 2 250 256 261 266 266 266 frequency range of 20 Hz to 20 kHz. The output level is flat over the 2 mV rms to 2 V rms range.

1kΩ GAIN RANGE: –26 TO +34dB 8 9 (0.05× TO 50×)

4 100Ω 10µF NP PrA ATTENUATOR INPUT 1kΩ 34dB OUTPUT 5 0dB –60dB TO 0dB 1 AD8336 4.7nF 0dB

0.1µF 2 GAIN CONTROL 1 8 BIAS C COM INTERFACE C 2 7 VIN +VS +5V 10 133 12 11 AD736 1MΩ 3 6 2.2µF 10kΩ CF OUTPUT +5V –5V +5V 4 5 ADP3339AKC-1.5 1.5V –5V –VS CAV 0.022µF 0.1µF 0.1µF 1kΩ 0.75V 33µF 3 2 1 +5V 1kΩ IN OUT GND 7 2 10kΩ 6 9.09kΩ 3 AD8551 4 OUT 88mV 1kΩ TAB

06975-005 Figure 5. AGC Circuit Using the AD8336

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240 OUTPUT (mV p-p) (mV OUTPUT 230

220 20Hz 5kHz 210 100Hz 10kHz 1kHz 20kHz

200 06975-006 0.0010.01 0.1 1 10 INPUT (V rms) Figure 6. AGC Performance at Various Frequencies ©2007 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. AN06975-0-11/07(0)

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