Circuit Note CN-0050 Devices Connected/Referenced in this Circuit Note Circuit Designs Using Analog Devices Products Apply these product pairings quickly and with confidence. For more information and/or support call 1-800-AnalogD (1-800-262-5643) or visit www.analog.com/circuits. ADL5330 Variable Gain Amplifier (VGA) AD8318 70 dB Logarithmic Detector/Controller Stable, Closed-Loop Automatic Power Control for RF Applications CIRCUIT FUNCTION AND BENEFITS with an automatic gain control (AGC) loop. Figure 1 shows the ADL5330 operating in an AGC loop. The addition of the AD8318 log amp allows the AGC to have improved temperature stability over a wide output power control range. The circuit described in this document provides closed-loop, automatic power control using a VGA (ADL5330) and a log detector (AD8318). Due to the high temperature stability of the AD8318, this circuit provides stability over temperature because the AD8318 RF detector ensures the same level of temperature stability at the output of the ADL5330 VGA. The addition of the log amp detector converts the ADL5330 from an open-loop variable gain amplifier to a closed-loop output power control circuit. Because the AD8318, like the ADL5330, has a linear-in-dB transfer function, the POUT vs. setpoint transfer function also follows a linear-in-dB characteristic. To operate the ADL5330 VGA in an AGC loop, a sample of the output RF must be fed back to the detector (typically using a directional coupler and additional attenuation). A setpoint voltage is applied by a DAC to the VSET input of the detector while VOUT is connected to the GAIN pin of the ADL5330. Based on the detector’s defined linear-in-dB relationship between VOUT and the RF input signal, the detector adjusts the voltage on the GAIN pin (the detector’s VOUT pin is an error amplifier output) until the level at the RF input corresponds to the applied setpoint voltage. GAIN settles to a value that results in the correct balance between the input signal level at the detector and the setpoint voltage. CIRCUIT DESCRIPTION Although the ADL5330 variable gain amplifier provides accurate gain control, precise regulation of output power can be achieved +5V +5V RF INPUT SIGNAL RF OUTPUT SIGNAL 120nH VPSx 120nH COMx 100pF 100pF OPHI INHI ADL5330 100pF DIRECTIONAL COUPLER OPLO INLO 100pF ETC1-1-13 (M/A-COM) GAIN 412Ω ETC1-1-13 (M/A-COM) +5V ATTENUATOR 1kΩ DAC* VOUT VPSx VSET AD8318 LOG AMP CLPF 220pF 1nF INHI 1nF INLO CMxP *SEE COMMON VARIATIONS SECTION 08515-001 SETPOINT VOLTAGE Figure 1. ADL5330 Operating in an Automatic Gain Control Loop in Combination with the AD8318 (Simplified Schematic: Decoupling and All Connections Not Shown) Rev. B “Circuits from the Lab” from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any“Circuit from the Lab”. (Continued on last page) 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 ©2008–2010 Analog Devices, Inc. All rights reserved. CN-0050 Circuit Note The basic connections for operating the ADL5330 in an AGC loop with the AD8318 are shown in Figure 1. The AD8318 is a 1 MHz to 8 GHz precision demodulating logarithmic amplifier. It offers a large detection range of 60 dB with ±0.5 dB temperature stability. The gain control pin of the ADL5330 is controlled by the output pin of the AD8318. This voltage, VOUT, has a range of 0 V to near VPSx. To avoid overdrive recovery issues, the AD8318 output voltage can be scaled down using a resistive divider to interface with the 0 V to 1.4 V gain control range of the ADL5330. T AM MODULATED INPUT 1 AD8318 OUTPUT 2 A coupler/attenuation of 23 dB is used to match the desired maximum output power from the VGA to the top end of the linear operating range of the AD8318 (at approximately −5 dBm at 900 MHz). 4 20 3 10 2 0 1 –10 0 –20 –1 –30 –2 –40 –3 –50 0.4 0.6 0.8 1.0 1.2 1.4 1.6 SETPOINT VOLTAGE (V) 1.8 2.0 –4 2.2 ERROR (dB) 30 3 08515-003 ADL5330 OUTPUT CH1 250mV Ω CH2 200mV CH3 250mV Ω M2.00ms T 0.00000s Figure 3. Oscilloscope Showing an AM Modulated Input Signal For the AGC loop to remain in equilibrium, the AD8318 must track the envelope of the ADL5330 output signal and provide the necessary voltage levels to the ADL5330 gain control input. Figure 3 shows an oscilloscope screen shot of the AGC loop depicted in Figure 1. A 100 MHz sine wave with 50% AM modulation is applied to the ADL5330. The output signal from the ADL5330 is a constant envelope sine wave with amplitude corresponding to a setpoint voltage at the AD8318 of 1.5 V. Also shown is the gain control response of the AD8318 to the changing input envelope. T AD8318 WITH PULSED VSET 08515-002 OUTPUT POWER (dBm) The detector’s error amplifier uses CLPF, a ground-referenced capacitor pin, to integrate the error signal (in the form of a current). A capacitor must be connected to CLPF to set the loop bandwidth and to ensure loop stability. T T 1 Figure 2. ADL5330 Output Power vs. AD8318 Setpoint Voltage, PIN = −1.5 dBm Figure 2 shows the transfer function of the output power vs. the VSET voltage over temperature for a 900 MHz sine wave with an input power of −1.5 dBm. Note that the power control of the AD8318 has a negative sense. Decreasing VSET, which corresponds to demanding a higher signal from the ADL5330, tends to increase GAIN. The AGC loop is capable of controlling signals just under the full 60 dB gain control range of the ADL5330. The performance over temperature is most accurate over the highest power range, where it is generally most critical. Across the top 40 dB range of output power, the linear conformance error is well within ±0.5 dB over temperature. The broadband noise added by the logarithmic amplifier is negligible. ADL5330 OUTPUT CH1 2.00V CH2 50.0mVΩ M10.0µs A CH1 T 20.2000µs 2.60V 08515-004 2 Figure 4. Oscilloscope Showing the ADL5330 Output Figure 4 shows the response of the AGC RF output to a pulse on VSET. As VSET decreases to 1 V, the AGC loop responds with an RF burst. Response time and the amount of signal integration are controlled by the capacitance at the AD8318 CLPF pin—a function analogous to the feedback capacitor around an integrating amplifier. An increase in the capacitance results in slower response time. The circuit must be constructed on a multilayer printed circuit board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see the MT-031 Tutorial and the MT-101 Tutorial and the ADL5330 and ADL8318 evaluation board layouts). Rev. B | Page 2 of 3 Circuit Note CN-0050 On the underside of the ADL5330 and AD8318 chip scale packages, there is an exposed compressed paddle. This paddle is internally connected to the chip’s ground. Solder the paddle to the low impedance ground plane on the printed circuit board to ensure specified electrical performance and to provide thermal relief. It is also recommended that the ground planes on all layers under the paddle be stitched together with vias to reduce thermal impedance. COMMON VARIATIONS LEARN MORE Dana Whitlow, Design and Operation of Automatic Gain Control Loops for Receivers in Modern Communications Systems, Analog Devices Wireless Seminar, Chapter 7, 2006. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of “AGND” and “DGND,” Analog Devices. MT-073 Tutorial, High Speed Variable Gain Amplifiers, Analog Devices. This circuit can be used to implement a constant power out function (fixed setpoint with variable input power) or a variable power out function (variable setpoint with fixed or variable input power). If a lower output power control range is desired, the AD8318 log amp (60 dB power detection range) can be replaced with either the AD8317 (50 dB power detection range) or the AD8319 (45 dB power detection range). For a constant output power function, the lowest dynamic range detector (AD8319) is adequate because the loop always servos the detector input power to a constant level. MT-077 Tutorial, Log Amp Basics, Analog Devices. The ADL5330 VGA, which is optimized for transmit applications, can be replaced by the AD8368 VGA. The AD8368 is optimized for receive application low frequencies of up to 800 MHz and provides 34 dB of linear-in-dB voltage-controlled variable gain. ADL5330 Evaluation Board There are a number of DACs suitable for this application. All of the following DACs have internal references: Single: AD5660/AD5640/AD5620 (16-bit/14-bit/12-bit), 9/10—Rev. A to Rev. B Changes to Figure 1 .......................................................................... 1 Changes to Circuit Description Section......................................... 1 Changes to Common Variations Section ....................................... 3 Dual: AD5663R/AD5643R/AD5623R (16-bit/14-bit/12-bit) 11/09—Rev. 0 to Rev. A Quad: AD5664R/AD5644R/AD5624R (16-bit/14-bit/12-bit) Updated Format ................................................................. Universal Changes to Circuit Note Title .......................................................... 1 MT-078 Tutorial, High Speed Log Amps, Analog Devices. MT-101 Tutorial, Decoupling Techniques, Analog Devices. Data Sheets ADL5330 AD8318 AD8317 AD8319 AD8318 Evaluation Board REVISION HISTORY 10/08—Revision 0: Initial Release (Continued from first page) "Circuits from the Lab" are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the "Circuits from the Lab" in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the "Circuits from the Lab". Information furnished by Analog Devices is believed to be accurate and reliable. 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