Usually A/D conversion are required to use the A/D conversion chip, it has 8 kB RAM and 128 kB of on-chip flash memory (Flash EEPROM), 2 kB electrically erasable programmable read-only memory (EEPROM) and the multiple functions of the interface, A/D module built-in MC9S12XS128 analog-to-digital converter is 16 channel, 12 bit precision, multi input multiplexing, successive approximation, so it can save hardware circuit using A/D conversion chip and design, can reduce the cost, improve the stability and reliability of the system. But the single chip analog input can only accept the unipolar forward analog signal, which can not directly carry on the analog digital conversion of bipolar analog signal. In general design, often to form a bipolar -ui-+ui analog signal through voltage shift circuit into 0 ~ 5 V unipolar signal, which makes the translation circuit A/D conversion accuracy doubled, and the stability is decreased. In this paper, using the symmetrical circuit design, the microcontroller can receive the A/D signal from 0 to 5 V to expand to +5 ~ V -5, A/D conversion range expanded 1 times, stability also greatly improved.

1, circuit design

　　Figure 1 Schematic diagram of the overall circuit

1.1 design principles
When the input signal is amplified by the amplifying circuit, if the signal is positive, the diode 1 cut-off signal, lossless input from the AD0 port, while the positive signal by the inverter for inverting into a negative signal, 2 diode conduction, so the signal received by the AD1 port is 2 of the pressure drop through the diode guide negative as long as the negative voltage is less than the allowable value of A/D input port, the port acquisition A/D value is 0, so in this case the A/D value is AD0 value;
On the contrary, when the input signal is negative, 1 diode conduction, AD0 port to receive the data for the 0, and the signal is reversed after inverter, diode 2 cutoff, AD1 port to receive data.
If AD=AD0-AD1, when the signal is positive, AD=AD0-0, is positive; when the signal is negative, AD=0-AD1, is negative. At this point, the AD data can be received from the original 0 to 5 V extended to +5 ~ V -5.
1.2 diode selection
If the linearity is considered, the positive voltage drop of the diode should be selected, for example, 1 N4007. But the 1N4007 forward voltage is about 0.7 V, when the diode conduction when the signal corresponding to the received A/D port for -0.7 V, it will burn microcontroller, so from the security considerations should be conducted to choose a relatively low pressure drop diode. The pressure drop of PMEG2010 is about 0.1 V, the pressure drop of 1N60 is about 0.2 ~ 0.3 V, can protect the chip from being burnt. On the premise of safety, the linearity of the data is analyzed.

2, experiment and conclusion
2.1 PMEG2010
In this paper, we take a kind of signal as the input signal to measure whether the output signal and the input signal have a linear relationship.
Table 1 is a diode for the PMEG2010 is the measured data, the data can be obtained after fitting the diode for PMEG2010 when the resulting curve, as shown in figure 2.

As shown in Figure 2, the nonlinear relationship between the input and the output near the zero point is obvious. The reason is that the reverse leakage current of PMEG2010 is too large, and the leakage current and the size of the input signal is nonlinear, so the measured data and the input value is nonlinear.
2.2 1N60
When the PMEG2010 is replaced by 1N60, a set of data is measured again, and the linear correlation is analyzed.

　　Table 2 the input and output values of diode for 1N60

Figure 3 is the fitting curves, it is shown that the input value and the output value has a good linear relationship, fitting curve equation was y=0.049 421335942257 8+2.261 80317823524 * x, the correlation coefficient is R=0.999 72, can meet the general needs of measurement.

3, the end of the language
After the experimental data verification and reference to relevant literature, we can draw the following conclusions: (1) the circuit design can effectively solve the relationship between unipolar and bipolar signal MCU receives the signal, the MCU can be received and processed by the bipolar signal. (2) through the design of the symmetrical circuit, the A/D can receive the signal from the original 0 to 5 V to +5 ~ V -5, effectively expand the range of A/D. (3) when the diode is selected properly, the linear relationship of the data can be guaranteed. (4) the design uses the monolithic integrated circuit A/D module, has saved the A/D chip to use, causes the design cost to reduce.


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