**1. Introduction**
In the design of an analog-digital hybrid system, the sampling of analog signals is typically done using a dedicated A/D converter along with a special decoding and latching circuit to convert the signal into a suitable digital format. However, this approach often leads to a more complex circuit design and requires multiple integrated chips, making the overall system less efficient and harder to implement. To address these limitations, the high-integration chip ICL7107 is utilized as the sampling module in this system. This choice simplifies the circuit design significantly and enhances the reliability of the entire setup. The ICL7107 integrates essential components such as buffers, integrators, voltage comparators, reference sources, and digital logic circuits, reducing the need for external components and improving performance.
**2. Overall System Design**
This paper presents a digital voltmeter capable of measuring DC voltages with a 3.5-digit display. It supports five measurement ranges: 0–199 mV, 0–1.99 V, 0–19.99 V, 0–199.9 V, and 0–1999 V. The device can measure both positive and negative voltages, automatically switch between ranges, and provide stable, easy-to-read results. The system block diagram is shown in Figure 1, which outlines key modules including voltage testing, analog channel processing, A/D conversion, decoding and latching, display control, range detection, and decimal point drive.
**3. Implementation of System Modules**
**3.1 A/D Conversion and Latching/Decoding Module**
The ICL7107 is used as the core component for A/D conversion. This dual-function A/D converter includes internal analog circuits such as buffers, integrators, voltage comparators, and reference sources, as well as digital components like oscillators, counters, latches, and decoders. With minimal external components—such as resistors, capacitors, and a display—it provides fast and accurate conversion. The ICL7107 operates in three main stages: automatic zero (AZ), signal integration (INT), and reverse integration (DE). Each cycle is controlled by internal logic, ensuring accurate output corresponding to the input signal. The relationship between the input voltage and the count value is given by:
VIN = T
Where 1000 represents the integration time (in pulse periods), and T is the reverse integration time at full scale (2000 counts). The external component configuration is illustrated in Figure 2.
**3.2 Voltage Division Module**
To meet the input requirements of the next stage, the tested voltage must be scaled down to ensure it stays within the A/D converter’s range of 200 mV. This is achieved through a voltage divider circuit, as shown in Figure 3. The circuit ensures that the input to the A/D remains below 200 mV, allowing accurate measurement across all ranges.
**3.3 Analog Voltage Channel Module**
This module is responsible for delivering the divided voltage to the A/D converter. It uses an analog switch to select the appropriate voltage divider ratio based on the measured voltage. This allows for precise and flexible operation, with multiple switching channels and stable performance.
**3.4 Over/Underrange Identification Module**
The ICL7107 includes built-in over- and underrange detection capabilities. When overrange occurs, the highest digit displays '1', while others remain blank. For underrange, the highest digits show '0'. By analyzing specific segments (Abk, eH, gH), the system determines the range status. The over/underrange signals are combined into a single output, Y, which triggers a range switch if needed. The formula for Y is:
Y = Y1 + Y2 = bH × gH + Abk × gH × eH
The circuit, shown in Figure 4, uses a comparator to convert the signal into standard levels, with U2C reversing the eH signal for proper logic handling. If an overrange condition exists, the output Y becomes '0', signaling the need for range adjustment.
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