Design of automatic range multimeter

1. Design goal: 4 1/2 multimeter (19999), minimum resolution 6 microvolts, automatic range selection.

2. Functional design requirements (range):

DC voltage (DCV) - 200 mV 2V 20V 200V 1000V

AC voltage (ACV) - 200mV 2V 20V 200V 700V

DC current (DCA) - 2mA 20mA 200mA 20A

AC current (ACA) - 2mA 20mA 200mA

Resistance (OHM) --- 200 2K 20K 200K 2M20M

3. Main chip: MSP430FE42X

4. Operation mode: Buttons - DCV button, ACV button, DCA button, ACA button, OHM button

5. 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, the internal reference source 1.25V is selected. In this way, when the external voltage to be measured is 0.625V, the AD sampling value is 65535, and when the voltage to be measured is -0.625, the AD sampling value is 0. Since the designed minimum range is 0.2V, it needs to be enlarged to 0.625V to make it full range, and then converted according to the displayed digits, 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, it must be divided, usually using a 10-fold voltage divider, for example, 2V is reduced to 0.2V, etc. The voltage divider is shown in Figure 1.

Figure 1 Voltage divider

Similarly, when measuring current, it must also be processed to convert the current into voltage before it can be measured. The current measurement principle diagram is shown in Figure 2.

Figure 2 Current divider

Please note that the 20A input on the right side of Figure 2 is connected directly, but of course a 20A fuse can also be added.

The above is the case of measuring DC voltage or DC current. When measuring AC voltage or AC current, rectification must be performed. The rectification circuit is shown in Figure 3.

Figure 3 AC rectifier circuit

The AC/DC conversion circuit consists of a common-mode amplifier A1, rectifiers D2 and D3, DC blocking capacitors C18 and C19, smoothing filters R22 and C22, etc. R24 is a calibration resistor. This circuit can obtain the effective value of the input sine wave. D1 is used to reduce nonlinear distortion.

The measurement of resistance is different from the measurement of voltage and current. The schematic diagram is shown in Figure 4.

The resistance measurement adopts the proportional method, that is, when the current flowing through the resistance to be measured and the reference resistance is the same, Uin/Uref=Rx/Rref. According to the AD conversion characteristics of FE42X, when the input voltage is half of the reference voltage, it is full scale, that is, when the resistance to be measured is half of the reference resistance, it is full scale. Therefore, the reference resistance of the 200 ohm range is 400 ohms. Assuming that the resistance to be measured is 100 ohms, since the voltage passing through the reference resistance and the resistance to be measured is 1.23V at this time, the reference voltage is 1.23*(400/500)V, and the input voltage is 1.23*(100/500). When the input voltage is 1.23*2/5, it is full scale, so the current AD value is half of the full scale - 100 ohms. Of course, the AD at this time needs to be converted to the range, that is, 0-20000 corresponds to 0-32767.

6. Brief analysis of the actual implementation circuit:

1. DC voltage measurement:

The voltage to be measured passes through the voltage divider, generating different voltage values ​​on each voltage divider resistor. At this time, the voltage input to the microcontroller must be determined according to the voltage to be measured. Here, HC4051 is used to divide the voltage to be measured. Since the voltage to be measured may be as high as 1000V, Panasonic's PHOTORELAY (whose input is up to 1000V) is selected as the input end of the voltage divider. After selecting the appropriate voltage divider, the voltage is amplified by the amplifier circuit composed of TLV2211 by about 3 times (to make the AD sample full scale), and then the range conversion is performed (0-20000 corresponds to 0-32767), and the voltage value to be measured can be obtained.

2. AC voltage measurement:

The AC voltage measurement and DC voltage measurement share a voltage divider. After 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 200mA, the current that can be passed by a general analog switch is relatively small, so AQV201 (load current 500mA at 40V) is selected for current selection. The current to be measured enters the amplifier circuit after voltage division and then is sent to the AD.

4. AC current measurement:

AC current measurement and DC current measurement share the same voltage divider. The difference is that after voltage division, it must enter the AC rectification circuit, then the amplification circuit, and finally the AD.

5. Resistance measurement:

The resistance measurement circuit uses the MAX4638 analog switch with very small internal resistance to connect reference resistors of different ranges, so as to measure the resistance value of the resistor to be measured. AD uses an external reference voltage, which is sent to the reference terminal after obtaining the voltage on the reference resistor through the subtraction circuit, while the voltage on the resistor to be measured is directly sent to the measurement terminal.

6. Finally:

Since the input impedance of MSP430FE42X is 500k, a follower is added to the AD input to increase its input impedance.