2 channel bandwidth: 2Mhz;
3 slew rate: 45V / us;
4 crosstalk: -120dB (1kHz);
5-pin programmable, closed-loop gain: ±1 and ±2;
6 closed loop gain accuracy and matching: 0.05%;
7 channel offset voltage: 100μV (AD630BD);
8350kHz full power bandwidth.
Ad630 pin diagram and function
AD630 realizes precision rectifier circuit
The working principle of this circuit can be illustrated by the following figure. The two op amps inside the AD630 form a non-inverting and inverting amplifier with a gain of two, and then use an analog switch to switch the two paths. When the input signal is positive, the analog switch is turned to the end of the same direction amplifier, and when the input signal is negative, the analog switch is turned to the end of the inverting amplifier.
The gain of the above circuit is 2, and other multiple gains can be achieved with the AD630. Not much is introduced here, you can refer to the AD630 chip manual if you need it. This circuit can operate in the range of input signal frequencies from DC to hundreds of kHz. The best operating frequency range is from DC to a few kHz. In this band, the effect of this circuit should be the best of these precision rectification circuits. The input impedance of the above circuit varies with the polarity of the input voltage. When the input voltage is positive, the input impedance is high, and when the input voltage is negative, the input impedance is low. Therefore, there is a certain requirement on the output impedance of the signal source. If the output impedance of the input signal is high, it is necessary to increase the level of the first buffer.
Phase-locked amplifier circuit of AD630
The schematic diagram of the phase-locked amplifying circuit of the AD630 is shown in Figure 2.
The waveform at point A is the bilaterally modulated waveform after the detected signal and the carrier are modulated, and the point B is the waveform after the bilateral modulated waveform and the noise are superimposed. The 9th pin of the AD630 is connected to the carrier signal, and the square wave and sine wave are feasible. With a reference phase. The output of the AD630 is connected to an integrating circuit and a low-pass filter to achieve perfect recovery of the signal.
AD630 realizes online measurement of battery internal resistance
1, the principle of measurement
The basic principle of realizing online measurement of battery internal resistance is shown in Figure 1.
Figure 1 block diagram of the internal measurement of battery internal resistance
When the signal source injects an AC current signal into the battery, the AC voltage signal and the input current generated at both ends of the battery are measured, and the internal resistance of the battery can be calculated:
Where: Vrms is the effective value of the AC voltage signal at both ends of the battery; Irms is the effective value of the AC current signal in the input battery.
The internal resistance of the battery is measured by the alternating current method, and it is not necessary to discharge the battery. In theory, the battery can be measured in any state.
In the actual measurement, since the internal resistance of the battery is in the micro-ohm or milli-ohm level, after a certain current is injected, the voltage signal generated at both ends of the battery is very weak, and is often submerged by noise. After amplification, the voltage is measured. It is difficult to distinguish useful signals, and the principle of correlation detection is needed to measure the AC voltage signals at both ends of the battery.
The principle of using the correlator to detect weak signals is shown in the relevant detection part of Figure 1. It consists of a switch multiplier and an integrator. The weak signal detected at both ends of the battery is preamplified and input to the multiplier signal input. The sinusoidal signal injected into the battery is converted into a square wave signal by a circuit, and then input to the multiplier reference signal terminal. If the useful signal at both ends of the battery is Vs(t) and the noise mixed is n1(t), the mixed signal at the input is f1(t)=Vs(t)+n1(t); the useful signal at the reference end is Vr(t). -Ï„); When the mixed noise is n2(t-Ï„), the mixed signal at the reference end is f2(t-Ï„)=Vr(t-Ï„)+n2(t-Ï„).
According to the principle of correlation detection, the multiplication by the multiplier, the signal and noise, noise and noise are independent of each other, their correlation function is zero, only the signal is related to the signal, and can be detected from the noise. Specifically, it can be expressed as:
When the sinusoidal signal detected at both ends of the battery is Vs(t), the square wave reference signal is Vr(t-Ï„):
Since the signal frequency at both ends of the battery is the same as the fundamental frequency of the reference signal, ie ωr=ωs, the output of the integrator is:
Where: K is only related to the transmission coefficient of the integrator; φ is the phase difference between the detection signal and the reference signal.
If φ = 0 is adjusted, the output DC signal reaches the maximum value, which fully demonstrates that noise is suppressed after passing through the multiplier and the integrator. When the input signal and the circuit transmission coefficient are constant, the output signal is only proportional to the internal resistance of the battery. As long as the AC voltage across the battery and the AC current through the battery are measured, the internal resistance of the battery can be calculated. , to achieve online measurement.
2, the hardware circuit design of the measurement system
The system block diagram designed according to the above principle is shown in Figure 2. The path selection switch circuit, the preamplifier bandpass filter, the AD630 multiplier circuit, the integrator circuit, the AC constant current signal generating circuit, the square wave conversion circuit, Sampling circuit, single-chip control system and external display communication. Since the internal resistance of the battery is small, it is necessary to reduce the influence of the wire impedance on the internal resistance of the battery, so the four-lead connection method is employed. The AC constant current signal output by the system is connected to both ends of the battery, and then the voltage signal generated by the internal resistance of the battery is connected to the input transfer switch circuit. After power-on, the phase difference between the detection signal and the reference signal is first adjusted by the single-chip microcomputer! Make it 0. After the measurement is started, the current measurement path is gated by the analog switch CD4052. The path sets a standard sampling resistor in the circuit for injecting an alternating current signal into the battery to measure the current value of the alternating current signal; and then strobes the voltage measuring path to measure the voltage value. . The collected signal is processed by amplification and filtering, and then sent to the single-chip microcomputer, and the internal resistance of the battery is calculated by the formula (1).
Figure 2 block diagram of the battery internal resistance online measurement system
2.1 amplification filter circuit
Since the collected signal is very weak, it must be preamplified and then input into the correlator. As shown in Figure 3, the low noise preamplifier consists of an instrumentation amplifier AD620 and a bandpass filter.
Figure 3 schematic diagram of the preamplifier circuit
The AD620 is a high performance instrumentation amplifier with stable performance and adjustable gain. The amplification factor is determined by the resistance RG between pin 1 and pin 8, G = 1 + (49.4 kΩ / RG). After the signal is amplified, a bandpass signal of 0.4~3 kHz is detected by a band pass filter and sent to the signal end of the multiplier. The DC amplifying circuit adopts the high-precision op amp OP27 to realize the program-controlled gain amplification. The feedback resistance of the amplifier is selected by the analog switch CD4052, and the amplification factor is selected by the single-chip microcomputer to make the signal within the optimal A/D acquisition voltage range.
2.2 related arithmetic circuit
In the design, the correlator adopts AD630 produced by AD Company. This is a high-precision balanced modulator. The internal resistance is high stability SiCr film resistor, which ensures the accuracy and stability of its work.
Its signal processing applications include balanced modulation and demodulation, synchronous detection, phase detection, quadrature detection, phase sensitive detection, lock amplification, and square wave multiplication.
The AD630 logic diagram is shown in Figure 4. It can be thought of as integrating two preamplifiers, a precision comparator for strobing the preamplifier, and a multiplexer and output stage integrating op amp. With a high switching speed and fast and stable linear amplifier, the switching distortion is minimized due to the fast response time of the comparator. In addition, there is extremely low crosstalk between channels. The AD630 is typically used for high precision signal processing and instrumentation with a wide dynamic range. In the phase-locked amplifying circuit, when it is used as a synchronous demodulator, it is possible to recover a weak signal in a 100 dB noise background. The optimal operating frequency of the AD630 is at 1 kHz, so the signal and reference signal injected into the battery are chosen to be 1 kHz, and 1 kHz is also in the appropriate battery internal resistance frequency response range, but it still works normally at a few megahertz.
The schematic diagram of the relevant detection circuit realized by using AD630 as a multiplier is shown in Fig. 5. Among them, AMPA and AMPB are configured as a positive phase amplifier and an inverting amplifier, respectively. The input signal is a signal to be detected and a reference signal. The signal to be detected is sent through pin 1, and the reference signal is input to the comparison amplifier through pin 9. The signal to be detected is flipped inside the device according to the positive and negative of the carrier signal, realizing the switch multiplication function.
Figure 4AD630 device logic diagram
Figure 5AD630 implementation of the relevant detection circuit schematic
3. Experimental results and analysis
3.1 Preamplification and filtering results analysis
The preamplification requirement in the design is 100 times. According to the RG calculation formula RG=49.4kΩ/(G-1) in the AD620, the RG is 499Ω.
In this case, the amplifier circuit with capacitance error of #5% and resistance error of ±1% is simulated by MulTIsim software. As shown in Fig. 6, channel A is the input signal, and channel B is the output signal amplified by AD620. The effective value of the signal is 13.621mV, and the output is 1.36*8V, which enables accurate and stable amplification.
Figure 6AD620 achieves accurate and stable amplification waveforms
3.2 Bandpass Filtering Results Analysis
Bandpass filtering is achieved by a first-order low-pass filter and a first-order high-pass filter. The low-pass filter is a multi-feedback LPF, as shown in the U3 level in Figure 3, which can be solved as:
Let R1=R2=R3=R, C1=C2=C, you can get:
Since the pass band is cut off at the time, the cutoff frequency f=037/(2RC) can be solved. According to the design requirements, R=20kΩ and C=1nF are selected, and the frequency characteristics of the simulation are shown in Fig. 7.
Figure 7 Frequency characteristics of the low-pass filter
It can be seen from Fig. 7 that the corresponding frequency is 3 kHz when the gain is -3 dB, and the high-pass filter frequency characteristic of the same design is shown in Fig. 8.
Figure 8 Frequency characteristics of the high-pass filter
3.3AD630 results analysis
Connect the circuit according to the AD630 design requirements, and realize the multiplication effect as shown in Figure 9. Channel 3 is the input signal, channel 2 is the reference signal, channel 1 is the output signal, the signal terminal and the reference terminal input the 1 kHz sinusoidal signal, and the output is two. The result of multiplying the signals. After the AD630 is multiplied, the multiplied signal is sent to the integrator to filter the noise from the signal and become a DC signal. The signal is mixed with 30dB of noise, and the correlation detector with AD630 as the core is shown in Figure 10. As shown in Figure 10, channel 3 is the original signal, and channels 4 and 1 are the mixed signal and the signal waveform after passing through AD630; channel 2 is the integral. The subsequent DC signal has a value equal to the value of the original signal after passing the correlation detection. This design is very good at suppressing noise, and the desired signal is well detected in the internal resistance measurement system.
Figure 9AD630 Multiplier Input/Output Waveform
Figure 10 correlator detection performance
3.4 System test results analysis
According to the idea in the paper, a set of battery internal resistance online measurement system was designed and compared with the results measured by stanfordSR830. The test battery is a 12V, 15A·h lead-acid battery that is used for about one year. The test results are shown in Table 1. It can be seen from the measurement data of Table 1 that the system is basically consistent with the measurement results of stanford SR830.
Figure 11 is a graph showing the internal resistance of a 6V, 4.5A·h battery during on-line measurement. Figure 11 is a graph showing the internal resistance of a 6V, 4.5A·h battery during discharge. After the electricity was discharged, the discharge current was selected to be 650 mA. During the discharge process, the internal resistance value gradually increases, and the internal resistance change rate is small at the initial stage of discharge, and there is a significant change at the beginning. When the remaining capacity of the battery is 50% or more, the internal resistance value changes little. When the capacity drops below 40%, the internal resistance value changes significantly, especially when the capacity is less than 20%, the internal resistance value decreases with the capacity. Sharply increase, at this time should pay attention to the battery in time to avoid damage to the battery.
Table 1 internal resistance test comparison results
Figure 11 discharge characteristics of the internal resistance of the battery
Figure 12 is a graph of internal resistance during battery charging. After discharging the battery to the cut-off voltage, it is charged with a current of 200 mA, and the internal resistance is measured online during the charging process. It can be seen from the test results that the charging process is exactly opposite to the change of the discharging process. At the beginning, the internal resistance first decreases sharply, then slowly changes, and finally it does not change. The same change in internal resistance accounts for the change in capacity.
Figure 12 Charging characteristics of battery internal resistance
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