First, the design goal: 4 1/2 million use table (19999), the minimum resolution of 6 microvolts, automatically select the range.
Second, functional design requirements (range range):
DC voltage (DCV) DD 200 mV 2V 20V 200V 1000V
AC voltage (ACV) DD 200mV 2V 20V 200V 700V
DC current (DCA) DD 2mA 20mA 200mA 20A
AC current (ACA) DD2mA 20mA 200mA
Resistance (OHM) DDD 200 2K 20K 200K 2M20M
Third, the main chip: MSP430FE42X
Fourth, the operation mode: button DDDCV button, ACV button, DCA button, ACA button, OHM button
Fifth, the principle block diagram:
When performing AD measurements, the MSP430FE42X can select an external reference source or an internal reference source.
Here, when measuring voltage and current, select the internal reference source 1.25V, so that when the external voltage to be tested is 0.625V, the AD sample value is 65535, and when the voltage to be tested is -0.625, the AD sample value is 0. Since the minimum range of the design is 0.2V, it needs to be amplified to 0.625V to make it full scale, and then converted according to the displayed number of bits, that is, 0-20000 corresponds to 0-32767. The actual minimum resolution is 0.2/32767V = 6 microvolts.
When the voltage to be measured is greater than 0.2V, a voltage division process must be performed, generally using a voltage divider of 10 times, for example, 0.2V at 2V. The voltage divider is shown in Figure 1.
Figure 1 voltage divider
Similarly, when measuring current, it is also processed to make the current become a voltage before it can be measured. The principle diagram of current measurement is shown in Figure 2.
Figure 2 Current divider
Please note that the 20A input on the right in Figure 2 is directly connected, although a 20A fuse can be added.
The above is the measurement of DC voltage or DC current. When measuring AC voltage or AC current, it must be rectified. The rectifier circuit is shown in Figure 3.
Figure 3 AC rectifier circuit
The AC/DC converter circuit is composed of a non-inverting amplifier A1, rectifiers D2 and D3, DC blocking capacitors C18 and C19, smoothing filters R22 and C22, and R24 is a calibration resistor. This circuit can get the rms value of the input sine wave. D1 is used to reduce nonlinear distortion.
The measurement of the resistance is different from the measurement of voltage and current. The schematic diagram is shown in Figure 4.
The resistance measurement uses a proportional method, that is, when the current flowing through the resistance to be tested and the reference resistor are the same, Uin/Uref=Rx/Rref. According to the AD conversion characteristic of the FE42X, when the input voltage is half of the reference voltage, the full scale is That is, when the resistance to be tested is half of the reference resistance, the full scale. Therefore, the reference resistance of the 200 ohm file is 400 ohms. Assuming that the resistance to be tested is 100 ohms, since the voltage through the reference resistor and the resistance to be tested is 1.23V, the reference voltage is 1.23*(400/500)V, and the input is The voltage is 1.23*(100/500), and when the input voltage is 1.23*2/5 full scale, the current AD value is half of full scale -100 ohms. Of course, the AD at this time is to be converted by the range, that is, 0-20000 corresponds to 0-32767.
Sixth, a brief analysis of the actual implementation of the circuit:
1, DC voltage measurement:
The voltage to be tested passes through the voltage divider, and different voltage values ​​are generated on the respective voltage dividing resistors. At this time, the voltage of the input single-chip microcomputer is determined according to the magnitude of the voltage to be measured. Here, the voltage to be measured is divided by the HC4051. Since the voltage to be tested may be as high as 1000V, Panasonic's PHOTORELAY (its input is up to 1000V) is selected as the input of the voltage division. When the appropriate voltage divider voltage is selected, the voltage is about 3 times larger by the amplifier circuit composed of TLV2211 (so that the AD sample is full scale), and then the range conversion (0-20000 corresponds to 0-32767), then it can be obtained. Measure the voltage value.
2. AC voltage measurement:
The AC voltage measurement shares a voltage divider with the DC voltage measurement. After the voltage division, the voltage to be measured is rectified by the AC rectifier circuit composed of TLV2211 and then enters the amplifier circuit for measurement.
3. DC current measurement:
Since the current to be measured is as high as 200 mA, the current analog switch can pass a small current. Therefore, AQV201 (load current of 500 mA at 40 V) is used for current selection. The current to be measured is divided into voltages and then sent to the AD.
4, AC current measurement:
The AC current measurement shares a voltage divider with the DC current measurement. The difference is that after the partial pressure, it enters the AC rectifier circuit, then enters the amplifier circuit, and finally enters the AD.
5, resistance measurement:
The resistance measurement circuit selects the MAX4638 analog switch with small internal resistance to connect the reference resistors of different ranges to measure the resistance of the resistor to be tested. The AD uses an external reference voltage. The reference voltage is applied to the reference terminal through the subtraction circuit to obtain the voltage on the reference resistor, and the voltage on the resistor to be tested is directly sent to the measurement terminal.
6. Finally:
Since the input impedance of the MSP430FE42X is 500k, a follower is added to the AD input to increase its input impedance.
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