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A bandwidth of DC Amplifier

In Electronic Infomation Category: A | on April 15,2011

Abstract: The design of a circuit by the preamplifier can be pre-gain amplifier, low pass filter circuit, the latter stage amplifier circuit, the DC circuit and TPS73625DBVR datasheet and MCU control circuit bandwidth DC amplifier. One gain from the two-stage variable gain amplifier broadband amplifier AD603 component, the gain preset by the MCU, the filter second-order Butterworth filter, and TPS73625DBVR price and then stage amplifier circuit can amplify the output voltage RMS to 10 V. The whole design for maximum voltage gain AV 60 dB, and TPS73625DBVR suppliers and gain continuously adjustable, and its low production cost, power and high efficiency.

Frequency bandwidth of the amplifier is the ratio of upper and lower limits of the amplifier circuit is much larger than l. Such circuit is mainly used to amplify the video signal, pulse signal or RF signal. This paper presents a variable gain amplifier VGA AD603 to the core, combined with external analog and digital circuit design method of broadband amplifiers, the bandwidth of up to 10 MHz.

1 overall system design

The system consists of pre-amplification, can be pre-gain amplification, low-pass filter, after stage amplifier, the DC power module and the MCU control and display module six major sections. The specific circuit structure shown in Figure 1.

Figure 1 System Block Diagram

2 program demonstration and design

2.1 controllable gain amplifier scheme selection

Option One: DAC control gain. The program theory, as long as the D / As fast enough, the accuracy can be achieved over a wide enough range of another job, precision gain adjustment. However, digital control and final Gain (dB) but not a linear relationship between the exponential relationship, resulting in uneven gain adjustment, the accuracy decreased, and the limited dynamic range of the gain, it is not used; Option Two: Use the control voltage and gain linear programmable gain amplifier PGA, with a control voltage and gain (dB) linear variable gain amplifier to achieve gain control. With a voltage-controlled gain, easy MCU control, while reducing noise and interference.

Comprehensive comparison, the choice of options for using the AD603 variable gain amplifier for gain control amplifier.

2.2 input impedance matching scheme selection

Option One: The design of low-noise precision amplifier OP27 of radio-level before the class to follow, despite the low noise, high precision, but because of the bandwidth of only 8 MHz, up to the requirements of less than 10 MHz; Option Two: Using high-speed broadband transport amp OPA692 as a form of radio-level before the level follower. OPA692 op amp high-speed broadband, and its 5 V dual-supply operation, a gain of 2, bandwidth is 190 MHz, voltage conversion rate is 2 100 V / s.

By comparison, the use of Option Two. The AD603s input impedance is only 100 , using the OPA692 as an input before the class can meet the requirements, and can be well isolated from front-end circuit on the circuit after the level of interference, to achieve inter-stage impedance matching.

2.3 filter circuit options

Option One: The RC filter circuit, but a large RC filter attenuation; Option Two: Using high-speed broadband operational amplifier OPA690 design of second-order Butterworth filter, the pass band frequency response curve of the maximum flat, not rolling , and in the resistance band decreased gradually to zero. By comparison, the second option.

3 Theoretical analysis and parameter calculation

3.1 Principle of voltage gain control

Voltage gain control basic principle of AD603 gain: Gain = 40 VG +10, which, VG is the differential input voltage, the unit is V, Gain is the basic gain of AD603, the unit is dB. It can be seen from this, the number of units in dB for gain and linear relationship between voltage, so long as the microcontroller can calculate a simple linear logarithmic gain control, gain step can be accurately realized.

3.2 passband gain variation within the control analysis

To control the pass band gain variation, part of the second order Butterworth filter, the resistors and capacitors according to the formula (1), equation (2) calculation:

Where f0 is the passband cutoff frequency, Q is f = f0 when the voltage gain and pass-band value of the magnification ratio. Simulation data can be calculated.

3.3 suppression of DC Offset Drift

In the integrated operational amplifier inverting input and inverting input of the total resistance of the same external circumstances, can inhibit the zero drift, the other in the actual debugging, also joined the zero-side, which can effectively adjust the zero position.

3.4 amplifier stability analysis

At all levels of amplification circuit, the design of a voltage negative feedback are used to ensure the stable operation of the amplifier.

4 main function module

4.1 programmable gain amplifier

AD603 is a low-noise, precision-controlled variable gain amplifier, temperature stability, maximum gain error is 0.5 dB, the gain (dB) and control voltage (V) is linear, it can easily use of the D / A output voltage control amplifier gain, and its input current is very small, resulting in on-chip control circuit provides gain control of the external circuit voltage decreases, it is suitable form programmable gain amplifier. Programmable Gain Amplifier AD603 by the passive input attenuator, gain control interface and a fixed gain amplifier of three parts. When the gain bandwidth of 90 MHz range is -11 dB ~ +3 l dB; bandwidth of 9 MHz when 9 dB ~ 51 dB. Gain range can be divided into three modes of control: When the 5 feet and 7 feet off, the gain range is 9 dB ~ 51 dB, 5 feet and 7 feet when shorted, the gain variation range should be -11 dB ~ +3 l dB, when 5 feet and 7 feet of the indirect one resistor, can gain range to pan. To increase the control range, designed to take a two-stage cascade AD603 method shown in Figure 2.

Figure 2 Programmable Gain Amplifier

4.2 low-pass filter circuit

Design uses a special design filter software Filter Wiz Pro, to achieve high-speed broadband transport OPA690, as shown in Figure 3.

low-pass filter circuit in Figure 3

4.3 level after the amplifier

Stage power amplifier circuit after the op amp AD811 achieved. Analog Devices AD811 is the launch of a video with a current feedback operational amplifier, a gain G = 1,, -3 dB bandwidth of 140 MHz; a gain G = 2 Shi , -3 dB bandwidth of 140 MHz; a gain G = 10 Shi , -3 dB bandwidth of up to 100 MHz; voltage conversion rate of 2 500 V / s, fully meet the system requirements. Specific circuit after stage amplifier shown in Figure 4.

Figure 4 after amplification Circuit

4.4 preset and manual gain control

C8051F020 microcontroller is the core of the amplifier control, which mainly perform the following functions: receive the user to control the gain of key information, then the gain control voltage of the AD603 can be done to control display.

Manually preset gain the basic idea is: SCM keyboard settings, digital program-controlled by the D / A converter to produce a control output voltage, added to Figure 2, two AD603s 1 foot, and thus gain control and achieve easy and intuitive.

4.5 DC Power Supply

Power supply with full-wave bridge rectifier, filter and three-terminal regulator, output 15 V, 12 V, 9 V, 5 V supply voltage of the chip. Using 18 V AC input isolation transformer, rectifier diodes therefore choose IN5408, withstand such full compliance with its requirements. Rectifier filter is -type filter, filter works well, three-terminal regulator by the CM7815, CM7915, CM7812, CM7912, CM7809, CM7909, CM7805, CM7905 achieved.

4.6 immunity measures

Total system gain of 60 dB, so the use of effective measures to avoid interference from shock and reduce noise, this method uses the following:

(1) all signals across the capacitor and electrolytic coupling high-frequency ceramic capacitors connected in order to avoid high-frequency gain decreased.

(2) using coaxial cable. Input stage and output stage using BNC connectors, input stage and a power connection between the use of coaxial cable.

(3) the number of mode isolation. Digital part and analog part of the power isolation between the outside in addition, also joined the class after the DAC TLV5620 isolation optocouplers, thus successfully eliminating the digital signal to analog signal interference.

5 Software Design:

The system design software, using C8051F020, complete the pre-gain and display functions, the program flow shown in Figure 5.

software flow chart in Figure 5

The entire design, including analog and digital two parts, the use of integrated circuits and discrete components with the program, focusing on their strengths and played a simple design, excellent performance, ease of implementation results. The design for maximum voltage gain AV 60 dB, the input rms voltage Vi 10 mV, and the AV manually adjustable, 3 dB pass band is 0 ~ 10 MHz, bandwidth, good performance, low production costs, power efficiency, has a certain value.

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