GT6505 digital meter principle and multimeter design

GT6505 digital electric meter principle and multimeter design manual

The digital electric meter has been widely used in scientific research, industrial field and production life because of its intuitive display, high accuracy, strong resolution, perfect function, stable performance, small size and easy to carry. The digital electric meter works simply and can be used. Let the students understand and use this tool to design the measurement of physical quantities such as current, voltage, resistance, pressure, temperature, etc., so as to improve everyone's ability to do things and solve problems.

[Purpose]

1. Understand the basic principle of digital electricity meter and the selection principle of peripheral parameters of common double integral analog-to-digital conversion chip, the calibration principle of electric meter and the source of measurement error.

2. Understand the characteristics, composition and working principle of the multimeter.

3. Master the principle of voltage divider and shunt circuit and design multi-range measurement of voltage, current and resistance.

4. Understand the measurement of AC voltage, triode and diode related parameters.

5. Through the study of the principle of digital electricity meter, the digital electric meter can be flexibly applied in the sensor design.

[laboratory apparatus]

1, GT6505 digital meter principle and multimeter design experiment instrument.

2, four and a half universal digital multimeter. (Bring your own or my company)

3, oscilloscope. (Bring your own or my company)

4, ZX25a resistance box. (Bring your own or my company)

[Experimental principle]

First, the principle of digital electricity meter

The common physical quantities are the so-called analog quantities whose magnitude varies continuously. The pointer meter can directly display the analog voltage and current Venus. For the digital meter, the analog electric signal (usually the voltage signal) needs to be converted into a digital signal. Display and process again.

The digital signal is different from the analog signal, and its amplitude is discontinuous. That is to say, the size of the digital signal can only be some discrete value, so it needs to be quantized. If the minimum quantization unit is , the size of the digital signal is Integer multiple, the integer can be represented by a binary code. =0.1 mV, we put the measured voltage U with Compare, see U is How many times, and round the result to the integer N (binary). In general, N ≥ 1000 can meet the measurement accuracy requirements (quantization error ≤ 1 / 1000 = 0.1%). Therefore, the most common digital header The maximum number of readings is 1999, which is called the three-and-a-half (3 1/2) digital table. For example: U is 1061 times (0.1 mV), that is, N=1861, the result is 186.1 (mV). Such a digital header, together with the voltage polarity discrimination display circuit and the decimal point selection bit, can measure -199.9 to 199.9 mV. The voltage is displayed with an accuracy of 0.1 mV.

1. Basic working principle of double integral analog-to-digital converter (ICL7107)

The principle of the double-integral analog-to-digital conversion circuit is relatively simple. When the input voltage is Vx, the capacitor C with zero charge is charged in a constant time T1 (the current is proportional to the voltage Vx to be measured), so that the capacitor is bipolar. The amount of electricity will increase linearly with time. When the charging time T1 is reached, the accumulated electric quantity Q on the capacitor is proportional to the measured voltage Vx; then the capacitor is discharged at a constant current (the current is proportional to the reference voltage Vref), so that the capacitor pole The amount of electricity will decrease linearly until T2 is reduced to zero. Therefore, it can be concluded that T2 is also proportional to Vx. If the counter is used to count the clock at the start of T2, the end time stops counting and the count value is obtained. N2, then N2 is proportional to Vx.

The working principle of the double integral AD is based on the fact that the counter reading N2 is proportional to the input voltage Vx during the charging and discharging process of the above capacitor. Now we use the 3-bit half-analog converter ICL7107 used in the experiment as an example to describe its whole work. Process. The basic composition of the ICL7107 dual integral A/D converter is shown in Figure 1. It consists of an integrator, a zero-crossing comparator, a logic control circuit, a gate circuit, a counter, a clock source, a latch, and a decoder. And the display and other circuits are composed. The following mainly talk about its conversion circuit, which is roughly divided into three stages:

In the first stage, first, the voltage input pin is disconnected from the input voltage and connected to the ground terminal to discharge the accumulated electric energy on the capacitor C, and then the reference capacitor Cref is charged to the reference voltage value Vref, and the feedback loop is given to the auto-zeroing capacitor CAZ to compensate the buffer. The bias voltage of the amplifier, integrator and comparator. This phase is called the auto-zero phase.

The second phase is the signal integration phase (sampling phase), in which Vs is connected to Vx to be connected to the integrator, so that capacitor C will be charged at a constant current Vx/R, while the counter starts counting, when it is counted Logic control circuit for a certain value N1 (for a three-digit half-digital converter, N1=1000) The charging process is finished, so that the sampling time T1 is constant. If the clock pulse is TCP, then T1=N1*TCP. At this stage, the integrator output voltage Vo=-Qo/C (because Vo is opposite to Vx), Qo The amount of electricity obtained by charging the capacitor C for constant current (Vx/R) in the T1 time, so there is the following formula:

Qo= = (1)

Vo=- =- (2)

Figure 1 Double integral AD internal structure diagram

Figure 2 Integral and anti-integration phase graph

The third stage is the anti-integration phase (measurement phase), at which stage the logic control circuit has charged Reference capacitance Press and The opposite polarity is connected to the integration circuit via the buffer so that capacitor C will have a constant current Discharge, at the same time the counter starts counting, the electric quantity on the capacitor C decreases linearly. When the time T2 elapses, the capacitor voltage decreases to 0. The zero-value comparator outputs the gate control signal and then stops the counter counting and displays the counting result. The following relationship exists at this stage:

Vo+ =0 (3)

Substituting (2) into the above formula gives:

T2= Vx (4)

It can be seen from equation (4) that since both T1 and Vref are constant, T2 is proportional to Vx, as can be seen from Figure 2. If the minimum pulse unit of the clock is ,then , , substituted (4),

That is:

N2= Vx (5)

It can be concluded that the measured count value N2 is proportional to the measured voltage Vx.

For ICL7107, the signal integration phase time is fixed at 1000 TCP, that is, the value of N1 is unchanged at 1000. The count of N2 varies from 0 to 1999 with Vx, and the range of automatic zero calibration is 2999 to 1000, that is, The measurement period is always kept at 4,000 TCP constants. That is, N2max=2000=2*N1 at full scale, so Vxmax=2Vref, so if the reference voltage is 100mV, the maximum input voltage is 200mV; if the reference voltage is 1V, the maximum The input voltage is 2V.

For the working principle of ICL7107, we will not say more here. Below we mainly talk about its pin function and the selection of peripheral component parameters, so that students can learn to use the chip.

2, ICL7107 dual integral analog-to-digital converter pin function, selection of peripheral component parameters

Figure 3 ICL7107 chip pin diagram

Figure 4 ICL7107 and peripheral device connection diagram

The pin diagram of the ICL7107 chip is shown in Figure 3. The connection diagram of the ICL7107 chip is shown in Figure 4. The pin connected to the digital tube and the power supply pin are fixed in Figure 4, so the details are not detailed. 32 feet are analog common terminals, called COM terminals; 34th pins Vr+ and 35 pins Vr- are reference voltage positive and negative input terminals; 31st pin IN+ and 30 pins IN- are measuring voltage positive and negative input terminals; Cint and Rint respectively For the integral capacitor and the integral resistor, Caz is an auto-zero capacitor. They are connected to the chips 27, 28 and 29. The oscilloscope is connected to the 27th pin to observe the capacitor charging and discharging process described above. This pin corresponds to the experimenter. The oscilloscope interface Vint; the resistors R1 and C1 are combined with the internal circuit of the chip to provide the clock pulse oscillation source. The oscillating waveform can be measured by the oscilloscope from the 40th foot. The foot corresponds to the oscilloscope interface CLK on the experiment instrument. The speed of the clock frequency determines the conversion of the chip. Time (because the measurement cycle always keeps 4000 Tcp constant) and the accuracy of the measurement. Let's analyze the specific effects of these parameters:

Rint is the integral resistor, which is defined by the full-scale input voltage and the output current of the internal buffer amplifier used to charge the integrating capacitor. For the ICL7107, the normal value of the charging current is Iint=4uA, then Rint=full scale/4uA Therefore, when the full-scale is 200mV, that is, the reference voltage Vref=0.1V, Rint=50K, the actual selection of 47K resistance; when the full-scale is 2V, that is, the reference voltage Vref=1V, Rint=500K, the actual selection of 470K resistance.

Cint=T1*Iint/Vint, generally in order to reduce the power frequency 50Hz interference during measurement, the T1 time is usually chosen to be 0.1S, and then the following analysis, so that because the maximum value of the integrated voltage Vint=2V, so: Cint=0.2uF In practice, 0.22uF is selected.

For the ICL7107, the oscillation frequency of the 38-pin input is: f0=1/(2.2*R1*C1), and the counting pulse frequency of the analog-to-digital conversion is 4 times that of f0, that is, Tcp=1/(4*f0), so measurement Period T=4000*Tcp=1000/f0, integration time (sampling time) T1=1000*Tcp=250/fo. ​​So the size of fo directly affects the speed of conversion time. If the frequency is too fast or too slow, it will affect the measurement accuracy and linearity. Degree, students can analyze the value of R1 while observing the waveform of the 40th foot of the chip and the value displayed on the digital tube during the experiment. In general, in order to improve the ability to resist 50Hz power frequency interference during the measurement process, The integration time of A/D conversion should be selected as an integral multiple of 50Hz power frequency period, that is, T1=n*20ms. Considering the linearity and test effect, we take T1=0.1m (n=5), so T=0.4 S, f0=40kHz, A/D conversion speed is 2.5 times/second. From T1=0.1=250/f0, if C1=100pF, then R1≈112.5KΩ. In order to let students better understand the clock frequency in the experiment. For the effect of A / D conversion, we let R1 can be adjusted, the adjustment potentiometer is the potentiometer RWC in the experiment instrument.

3. Measurement of common physical parameters with ICL7107A/D converter

Figure 5

Image 6

Figure 7

(1) Implementation of DC voltage measurement (DC voltmeter)

I: When the reference voltage Vref=100mV, Rint=47KΩ. At this time, the DC voltage of 0～2V is measured by the voltage division method. The circuit diagram is shown in Figure 5.

II: Directly make the reference voltage Vref=1V, Rint=470KΩ to measure the DC voltage of 0~2V, the circuit diagram is shown in Figure 6.

(2) Implementation of DC current measurement (DC ammeter)

There are usually two methods for measuring DC current. The first one is the ohmic voltage drop method. As shown in Figure 7, the measured current flows through a certain value of resistance Ri, and then the 200mV voltmeter is used to measure the fixed value resistor. The voltage drop Ri*Is (if Vref=100mV, it is guaranteed that Ri*Is≤200mV), because the resistance is connected to the circuit under test, this measurement method will have an influence on the original circuit, and the measurement current becomes Is'=R0. *Is/(R0+Ri), so the larger the internal resistance of the circuit under test, the smaller the error. The second method is to measure the current by an IV conversion circuit composed of an operational amplifier. No effect, but due to the limitations of the op amp's own parameters, it can only be used in the measurement circuit for small currents, so it will not be detailed here.

(3) Implementation of resistance value measurement (ohmmeter)

I: When the reference voltage is selected at 100mV, select Rint=47KΩ at this time. The wiring diagram of the test is shown in Figure 8. In the figure, Dw is the test reference voltage, and Rt is the positive temperature coefficient (PTC) thermistor. The reference voltage is lower than 100mV, and it is also possible to prevent damage to the conversion chip when the high voltage is detected by mistake. Therefore, it is necessary to satisfy Vr ≤ 100mV when Rx=0. The operation principle of 7107 described above exists:

Vr=(Vr+)–(Vr-)=Vd*Rs/(Rs+Rx+Rt) (6)

IN=(IN+)–(IN-)=Vd*Rx/(Rs+Rx+Rt) (7)

From the aforementioned theory N2/N1=IN/Vr there are:

Rx=(N2/N1)*Rs (8)

Therefore, it can be concluded from the above equation that the measurement range of the resistance is always 0 to 2 RsΩ.

II: When the reference voltage is selected at 1V, select Rint=470KΩ at this time. The test circuit can be implemented with Figure 9. This circuit is only for reference by interested students. Because it does not have a protection circuit, it must be guaranteed that Vr≤1V. For multi-range experiments (multimeter design experiments), for the convenience of design, our reference voltage will be chosen to be 100mV, except for the proportional method to measure the resistance we make Rint=470KΩ and also make Rint= when performing diode forward voltage drop measurement. 470KΩ plus a reference voltage of 1V.

Second, the digital multimeter design

Commonly used multimeters need AC/DC voltage, AC and DC current, resistance, and triode And the measurement of the forward voltage drop of the diode, etc., Figure 10 is the basic principle diagram of the multimeter measurement. Below we mainly talk about the measurement of several parameters mentioned:

The GT6505 digital electric meter principle and multimeter design experiment instrument used in this experiment, its core is composed of double integral analog-to-digital A/D conversion decoding driver integrated chip ICL7107 and peripheral components, LED digital tube. For students better To understand its working principle, we have reserved 9 inputs in the instrument, including 2 measuring voltage inputs (IN+, IN-), 2 reference voltage inputs (Vr+, Vr-), and 3 decimal point drive inputs. Terminals (dp1, dp2, and dp3) and analog common (COM) and ground (GND).

1. DC voltage range extension measurement

2. DC current range extension measurement (reference voltage 100mV)

3. AC voltage and AC current measurement (reference voltage 100mV)

4, resistance measurement circuit (reference voltage 0 ~ 1V)

5. Measurement of triode parameter hFE (reference voltage 100mV)

6, diode forward voltage drop measurement (reference voltage 1V)

[Experimental content and steps]

First, the composition of the experimental instrument

The function of this module.

1. ICL7107 analog-to-digital conversion and its display module, as shown in Figure 23, "1".

2. The range switch module, as shown in Figure 23, "2".

3. AC voltage and current module, providing AC voltage and current, and adjusting by potentiometer in the module.

4, DC voltage and current module, providing DC voltage and current, through the potentiometer in the module to adjust.

5. The component module to be tested provides one diode, one resistor, one NPN transistor and one PNP transistor.

6. The AD reference voltage module provides the reference voltage of the analog-to-digital converter and is adjusted by the potentiometer in the module.

7. The reference resistor module provides one adjustable reference resistor and one adjustable resistance to be tested.

8, AC and DC voltage conversion module, the AC voltage is converted into DC voltage, the potentiometer in the module to adjust.

9, resistance file protection module to prevent overvoltage damage to the instrument.

10, current file protection module to prevent overcurrent.

11, NPN triode measurement module, PNP triode measurement module, diode measurement module.

12. Range extended voltage dividers a, b, shunts a, b, and step resistance modules.

Second, the principle of digital electricity meter experiment

â—† Measurement of DC voltage

â—† Measurement of DC current

â—† Measurement of resistance

â—† Calibration of 200mV AC voltage

â—† 20mA AC current measurement

â—† Calibration and measurement of diode forward voltage drop

â—† Measurement of triode hFE parameters

Third, the multimeter design experiment

1. Design and manufacture multi-range DC digital voltmeter

2, design and manufacture multi-range DC digital ammeter

3, design and manufacture multi-range resistance meter

4, design and manufacture multi-range AC voltmeter

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