This article refers to the address: http:// Key words: VGA; AGC; AD8367 1 Main features The AD8367 is a variable gain single-ended IF amplifier from AD that uses AD's advanced X-AMP architecture for superior gain control. Because it integrates the radiance root detector on the chip, it is also the world's first VGA chip that can realize single-chip closed-loop AGC. The chip features a high-performance 45dB variable gain amplifier that controls linear gain and operates reliably from any low frequency to 500MHz frequency range. The AD8367 has the following main features: â— single-ended input, single-ended output; â— Input impedance is 200Ω and output impedance is 50Ω; â— 3dB bandwidth is 500MHz; â— When the input terminal is at zero level, the output terminal level is half of the power supply voltage and is adjustable; â—With gain control feature selection and power shutdown control function; â— On-chip integrated square root detector, which can realize single-chip AGC application; â— The gain control characteristic is linear in dB; â— The operating frequency can be extended to any low frequency by an external capacitor. The functional block diagram of the AD8367 is shown in Figure 1. The chip consists mainly of a variable attenuator, a fixed gain amplifier, and a root detector. Its input stage is a variable attenuator with a total attenuation of 45dB, including a 200Ω single-ended ladder resistor network and a Gaussian interpolator. The resistor network consists of a 9-stage attenuation network with 5dB attenuation per stage, and the attenuation factor can be selected by a Gaussian interpolator. Each stage of the ladder network attenuates the input signal with a fixed number of decibels. When the attenuation is not an integer multiple of 5dB, under the control voltage, two adjacent attenuation nodes are turned on, and the weighted average of the discrete node attenuation is used to obtain the attenuation corresponding to the control voltage, and This method achieves smooth, monotonic attenuation characteristics. It provides a linearity error of better than ±0.5dB at an operating frequency of 200MHz and a linearity error of better than ±1dB at 400MHz over a gain control range of greater than 40dB. Followed by the attenuator is a fixed gain amplifier, which is mainly used to ensure that the AD8367 has a gain of 42.5dB and a bandwidth of 500MHz. It is actually an operational amplifier with a gain bandwidth product of 100 GHz, so when it works at high At the time of frequency, it still has good linearity. The AD8367 integrates a radian root detector at the output to detect the output signal level and compare it to an internally set 354mVrms level (corresponding to a 1Vp-p sine wave). When the output level exceeds the internal set level, a difference current is generated. Integrating this current with an external capacitor CAGC (including a 5pF built-in capacitor) connected between the DETO pin and ground produces an RSSI voltage proportional to the received signal strength, which can be used in AGC applications. AGC controls the voltage. The AD8367 is best suited for operation in a 200Ω impedance system and can be converted to other general-purpose impedance systems (from 50Ω for RF systems to 1kΩ for data converters) via resistor or reactive passive networks. In general, the design choice of the conversion network depends on specific system requirements such as bandwidth, return loss, noise figure, and absolute gain range. The AD8367 contains a passive variable attenuator and a fixed gain amplifier. The circuit noise and distortion performance are both a function of gain and control voltage, and the input folded noise increases proportionally with the amount of attenuation. The circuit has a noise figure of at least 7.5 dB at maximum gain, with a 1 dB reduction in gain and a 1 dB increase in noise figure. In the receiving system, if the received signal is weak, there will be a maximum gain and a minimum noise figure; and when the received signal level is high, the system will have a lower gain and a larger noise figure. Therefore, the variation of the circuit noise figure with the gain does not have a significant impact on the system. The distortion performance of the circuit is similar to the noise performance. When the AD8367 operates in a 200Ω source impedance system, its output stage is a low output impedance voltage buffer with a 50Ω damping resistor that reduces sensitivity to load reactance and parasitic parameters. 3.1 Universal VGA Amplifier The AD8367 is a general purpose VGA amplifier suitable for large control range voltage controlled gain applications. Because it has an operating bandwidth from any low frequency to 500 MHz, it can handle not only high frequency signals up to 500MHz, but also frequency extensions to adapt to the audio system. Figure 2 shows the basic connection circuit of the AD8367 when it is operating in VGA. In Figure 2, the circuit gain AV is proportional to the control voltage VGAIN. Since the gain control rate of the AD8367 is 50dB/V, the circuit gain AV can be calculated by the following equation when VGAIN is in V: AV=50VGAIN-5 When the linear gain control range of the circuit is -2.5dB to 42.5dB, it can be inferred from the above equation that the value range of VGAIN is 50mV to 950mV. The capacitor CHP is connected to an internal drift control loop that cancels the change in the dc balance of the signal path, setting the high-pass cutoff frequency of the signal path. When this capacitor is not used, a 500 kHz default high-pass cutoff frequency can be provided by the internal capacitor. The relationship between CHP and high-pass cutoff frequency is: fHP=10/(CHP+0.02) In the formula, the unit of fHP is kHz, and the unit of CHP is nF. In this way, the AD8367 extension can be applied to the audio field by increasing the value of CHP. 3.2 as an AGC amplifier Using an internally integrated precision-law root detector, the AD8367 can be easily configured as a single-chip AGC amplifier with the basic connections shown in Figure 3. When the AD8367 is used as an AGC amplifier, the reverse gain control mode needs to be selected. When the effective value of the output signal exceeds 354mV, the detector will output the RSSI voltage proportional to the input signal from the DETO terminal at a ratio of 20mV/dB. By adding the RSSI voltage as the AGC control voltage to the gain control terminal GAIN, a simple monolithic AGC amplifier with a control rate of 20 mV/dB can be constructed. When using a power supply lower than 5V, the output starting point and ratio of the detector will not change, that is, when the power supply voltage varies from 2.7V to 5.5V, the AGC characteristics of the circuit can be kept unchanged. According to the connection method of Figure 3, a control linearity better than 0.1 dB can be obtained in an input range greater than 35 dB. The time constant Ï„AGC of the circuit can be simply set by the AGC capacitor CAGC. In fact, Ï„AGC is the result of the interaction of the AGC capacitor CAGC and the 10kΩ on-chip equivalent resistance RAGC. Therefore, the time constant is as follows: Ï„AGC=RAGCCAGC An external detector should be used when the required AGC control point is different from the internal setpoint of the circuit. The DC level detected by the output terminal is amplified, divided, and added to the gain control terminal to obtain the required AGC control point. 3.3 Signal Power Detection Application Another benefit of using a radiant root detector is that its output acts as an RSSI voltage to reflect the signal power, enabling absolute power measurements for any given source impedance. Therefore, the AD8367 can also be used as a power detection chip to design a power meter or as an ac voltmeter that reads out in decibels. Its power detection range is 45dB. If the gain control in Figure 2 is not used, the RSSI voltage output from the DETO terminal can be used as the detection voltage of the input signal power. When used for input signal power detection, the voltage output is indicated only when the output signal level reaches 354mVrms. Water-cooled Capacitor is supercapacitor is a capacitor with a capacity of thousands of farads.According to the principle of capacitor, capacitance depends on the distance between the electrode and electrode surface area, in order to get such a large capacitance, as far as possible to narrow the distance between the super capacitor electrode, electrode surface area increased, therefore, through the theory of electric double layer and porous activated carbon electrode. 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2 Working principle
3 Typical applications
   It should be noted that the AGC loop using the error integration technique has a common weakness. When driven by a gradually increasing signal, the AGC control voltage increase reduces the gain. When the gain is reduced to its lowest value, an increase in the control voltage proportional to the input will have no effect on the gain and will therefore cause an input overload. In fact, the AGC amplifier configured with the AD8367 also has an input overload problem. Since its minimum gain is -2.5dB, an input with an input amplitude greater than 2.5dB above the control point will cause an overload, that is, an input signal power exceeding +6.5dBm will cause an input overload. Therefore, in actual use, it is best to control the maximum input level to 5 dB below the overload level to form a certain overload protection band. In the AGC application, it can also be applied to the audio field through band extension. When the CHP is as high as 1μF, the circuit can process audio signals with frequencies as low as 10Hz. By changing the values ​​of CHP, C4, and CAGC in Figure 2 to 1μF, a high-stability, low-distortion audio leveling circuit can be constructed.