I. Overall Scheme Design
Program-controlled amplifier scheme of 1. 1
The topic requires that the input signal amplitude of the amplifier is 10mV, that is, the peak-to-peak value is 20mV, the voltage gain is 60dB, the gain step is 10dB, the voltage gain error is less than 5%, and the passband is 100hz-40khz.
Scheme 1: amplify six channels with different amplitudes with low noise operational amplifier OP37 as required, and then switch channels with relays or analog switches. The scheme is simple in hardware implementation and accurate in gain control. However, the control is relatively complex and involves many components, which is not conducive to the rapid realization of the system.
Scheme 2: Voltage-controlled gain connection using the variable gain amplifier AD603. AD603 is a low noise, broadband (maximum 90M gain bandwidth product) variable gain operational amplifier. According to the gain formula given in the literature, the continuous control of gain can be directly obtained, and the result is logarithmic value. The gain range from 0db to 60dB can be conveniently realized.
Based on the above analysis, we think that scheme 2 has the best operability, and it can easily meet the peak-to-peak requirements after operational amplifier adjustment, so scheme 2 is selected.
1.2 program-controlled filtering scheme
The topic requires that the filters can be set as low-pass and high-pass filters, the cutoff frequency of -3dB is 1k ~ 20kHz, the frequency step is 1K~20KHz, and the total gain of the two systems is not more than 30dB. We are considering the following options:
Scheme 1: Low-pass, Qualcomm and band-pass filters can be realized by off-the-shelf filter chips, such as Maxim's switched capacitor filter chip MAX262, and the center frequency range can be from DC to 140KHz, which can well meet the requirements of the topic.
Scheme 2: A state adjustable filter is adopted, which consists of three operational amplifiers and resistors and capacitors. The circuit is relatively simple and can realize three outputs: LPF, BPF and HPF. You can switch the center frequency with different resistors by using the resistor that determines the Q value and the capacitor that determines the center frequency, and realize the cutoff frequency of 1k ~ 20kHz and the frequency step of 1K~20KHz.
Scheme 3: It is realized by using a state adjustable filter. Using the R-2R resistor network in DA, the resistance is changed by controlling the digital quantity, so as to change the center frequency of the filter to meet the requirements of the topic.
After comparison, I think the first scheme is better, but because I didn't buy the chip of switched capacitor filter, I gave up this scheme and adopted the third scheme, which uses digital quantity to control the internal resistance network of DAC0832 to realize the change of center frequency. The scheme has strong operability and can easily meet the requirements.
1.3 fourth-order elliptic filter
The scheme of the fourth-order elliptic filter is as follows:
Scheme 1: The second-order elliptic low-pass filter can be obtained by using the LP, BP and HP outputs of the state adjustable variable filter, and the fourth-order elliptic low-pass filter can be realized by connecting two second-order sections in series.
Scheme 2: Using operational amplifier, with the help of the existing filter solving software and the circuit directly given by the software, and making appropriate adjustments, the fourth-order low-pass elliptic filter can be realized.
Comparing the two schemes, the first scheme can meet the requirements of the topic well, but it is more troublesome to debug and has higher technical requirements. The second scheme has simple circuit form and fast manufacturing speed, so the second scheme is adopted.
1.4 amplitude-frequency characteristic tester
The subject requires us to make an amplitude-frequency characteristic tester. The frequency sweep range is 100 Hz ~ 200 kHz, and the frequency step is 10KHz. The frequency sweep scheme adopts DDS chip, which can realize linear frequency sweep well. There are two main detection schemes:
Scheme 1: the scheme of first detecting and then amplifying. The detection and amplification scheme generally adopts the peak detection method. When the output is detected, a step voltage divider is used, and then a DC amplifier is used to amplify the signal for measurement. However, it should be noted that the parameters of the detector diode will affect the resolution of the minimum signal.
Scheme 2: the scheme of amplifying first and then detecting. The amplification detection scheme generally adopts the average detection method. The input signal first enters the step voltage divider, and then is amplified and sent to the average detector. However, it should be noted that the bandwidth of the amplifier will affect the frequency range of the measured signal.
The frequency range we want to test is 100 Hz ~ 200 kHz, and the requirements for amplifier gain and bandwidth product are not very high, so the scheme of amplification and detection is adopted.
Second, the system design:
The whole system is divided into two parts, program-controlled filtering module and amplitude-frequency characteristic curve testing module. The block diagram of the system module is as follows: (graphic province)
2. Implementation of1programmable amplifier
According to the requirements of the topic, when the input peak-to-peak value is 20mV, the dynamic range of the program-controlled amplifier should be 60dB, and the output peak-to-peak value should be 2mV-20V. This design adopts continuous mode (optimal signal-to-noise ratio) of AD603. This scheme combines the gain curves of two AD603 to extend the dynamic range. As shown in the figure below, the combined gain range is about -20dB to 60dB, which can meet the requirements of the topic dynamic range.
Because the power supply voltage of AD603 is low, the requirement of output voltage range can be adjusted and realized by using an operational amplifier with higher power supply voltage based on AD603 amplification.
2.4 Realization of amplitude-frequency characteristic curve tester
When testing the amplitude-frequency characteristic curve, the frequency sweep is realized by MSP 430F161single chip microcomputer controlled DDS, and the detection adopts AD637 to detect the effective value, and ADS8505 is used as the peripheral of single chip microcomputer to sample the output of AD637. The minimum system of single chip microcomputer adopts infrared remote control keyboard and 320x640 LCD screen.
Third, the system software flow chart:
3. 1 The flow chart of program-controlled filter is as follows: (graphic province)
3.2 Flow chart of amplitude-frequency tester: (save the chart)
Fourth, system testing.
4. 1 test instrument:
4.2 Main test results:
4.3 amplifier test data
The average gain error of 60dB test is 1.85%, and the average voltage gain error is 3.37%.
4.4 Test data of filter parameters
The average error of cutoff frequency is 0.027%.
4.5 Elliptic filter parameter test
The passband error of -3dB is (50-49.6)/50 = 0.8%. Because of the large noise, even the DC output is more than 70 mV, it can basically be considered that the gain of 200K is inaccurate under the existing conditions. The parameters of this design are measured under the condition of oscilloscope isolation.
4.6 Error analysis
By modifying the circuit parameters obtained by simulation, the system obtains good index parameters. However, there is still a big error in theoretical analysis, which is basically as follows:
When 1. is calibrated, F40 signal generator is used to generate 20mV signal. The signal-to-noise ratio of the signal is very poor, and the interference of noise power will cause the introduction of errors, as well as the signal source of the amplitude-frequency characteristic tester.
2. The transistor millivolt meter used for calibration is a mechanical meter itself, which will cause a relatively large error, and at the same time, the human eye will also introduce a relatively large error when reading.
There are many analog circuits in this topic. When debugging, all aspects will be integrated to compensate the circuit, and the balance will be made according to the requirements of the topic, which will also cause some errors.
4. The use of power supply is often the key to introduce circuit noise, especially when measuring small signals.
Circuit correction: Through error analysis, this design has done some operations to reduce the error.
1. Using the self-made PCB of AD9954 to improve the signal-to-noise ratio of the output signal.
2. When using the instrument, take multiple averages to reduce the error between the analog instrument and the human eye reading.
3. PCB the key analog circuits, and specially handle the grounding of signals, so as to reduce the debugging difficulty of analog circuits.
4. The power supply system adopts R-type transformer with good isolation to reduce the introduction of noise from the source.
Summary of verb (abbreviation of verb) design
After several days of hard work, we finally achieved all the requirements given by the topic. At the same time, due to the tight time, the system is not perfect. For example, the graphic display of the amplitude-frequency characteristic tester is not very humanized. Although we know that the elliptical filter is the result of superposition of several filters, there is not enough time to deduce and make it, but deepening the understanding of electronic design knowledge and teamwork is the biggest gain of the competition.