Design and Implementation of Range-switched Digital Voltmeter Based on ICL7107 Device

In the design of an analog-digital hybrid system, the sampling of the analog signal is generally performed using a dedicated A/D converter, plus a special decoding and latching circuit to convert the analog signal into a suitable digital signal. However, this makes the design circuit of the system more complicated, and more integrated chips are used, which brings inconvenience to the design. In order to overcome the shortcomings in the above design, the high integration chip ICL7107 is used as the sampling module for the analog signal in the design of the system, which makes the circuit design simpler and the reliability is greatly improved.

The voltmeter designed in this paper is a digital voltmeter with 3/2 digit DC voltage measurement. The measurement range is DC 0~199mV, 0~1.99V, 0~19.99V, 0~199.9V, 0~1999V, a total of 5 Range. The voltage value is stable, the reading is convenient, the positive and negative voltages can be measured, and the range can be automatically switched, which is convenient to use. The system block diagram is shown in Figure 1. The system can be divided into test voltage conversion, analog voltage channel, A/D conversion and decoding latch, display, over/under range identification and range switching and decimal point drive.

Use ICL7107. ICL7107 is a dual-product type A/D converter.

It also integrates analog part circuits of A/D converters such as buffers, integrators, voltage comparators, positive and negative voltage reference sources and analog switches, as well as digital circuit parts such as oscillator sources, counters, latches, and decoders. , drivers and control logic circuits, etc., only need to externally connect a small number of resistors, capacitors and display devices, you can complete the analog to digital conversion to meet the design requirements. The display is stable and readable and the measurement response is fast, which is the key to this design. One cycle of ICL7107 is to use 4000 count pulse time as one cycle time of A/D conversion, and each cycle is divided into three stages: automatic zero (AZ), signal integral (INT) and inverse integral (DE). The internal logic control circuit continuously repeats the control signals of the AZ, INT, and DE stages, and timely coordinates the coordination of the counter, the latch, and the decoder so that the output corresponds to the value of the input signal. The value of the input analog quantity is equal to the count value T in its internal value, ie:

VIN value = T value

Where: 1000 is the integration time (1000 pulse periods);

T is the inverse integration time (2000 at full scale).

The external component parameter setting and circuit of the chip are shown in Figure 2.

Figure 2 ICL7107 circuit connection diagram

As the measurement object of the voltmeter, the voltage to be tested must meet the input requirements of the next-level circuit. The design requirement is 0 Ì„1999V, and the full-scale value of the A/D converter is 200mV, so this module is quite important. The circuit is shown in Figure 3. The voltage dividing circuit ensures that the value of A/D is VIN<200mV. 3.3 Analog voltage channel module.

The main function of this module is to deliver the voltage to be tested after voltage division to A/D. It can select the most suitable voltage divider value according to different voltages to be tested, and adopt analog switch as the strobe channel. Its small size and stable operation. There are more switching channels.

The ICL7107 is designed with over- and under-range functions. When over-range, the A/D converter is in an overflow state. At this time, the highest bit is displayed as 1, and the other low bits are not displayed. If the second highest bit b and H are both high, The level indicates the overrange; when the range is underway, the highest and the next highest digits display the number 0. When the highest bit Abk is 1, this bit is not displayed; when the hundred digit is zero, 0 is displayed. Analysis of the state of the pen segment can be determined as long as the Abk and the e and g segments of the hundred digits are taken out. Combine the overrange and underrange signals into one signal. When there is one of the two conditions, the range is selected to be larger than the first range, so cycle until the appropriate range is switched. If Y1 indicates overrange, then Y1=bH&TImes;gH, Y2 indicates underrange, then Y2=Abk&TImes;gH&TImes;eH, so the over-range output is expressed as:

Y=Y1+Y2=bH&TImes;gH+Abk×gH×eH

The circuit is shown in Figure 4. The voltage comparator is used to convert it to a standard high and low level, and the comparator U2C is used to reverse the eH:

In summary, when there is an over-balance condition, the Y output is "0".

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