Three-and-a-half-digit single-chip digital multimeter LSINJU9207/9208 and its applications

    Abstract: NJU9207/9208 are three-and-a-half-digit monolithic digital multimeter LSIs produced by New Japan Radio Corporation (NJRC). This article introduces the packaging form, pin function and internal structure of NJU9207/9208, and describes their main functions and principle, and finally the application circuit is given.

    Keywords: three-and-a-half-digit multimeter LSI NJU9207/9208

1 Overview

The NJU9207/9208 produced by New Japan Radio Corporation (NJRC) is a three-and-a-half-digit monolithic digital multimeter large-scale integrated circuit (LSI). Their operating voltage is only 3V and the operating current is no more than 1mA. It can directly drive the LCD display and maintain data and automatic range. It has a built-in voltage doubler and voltage regulator. At the same time, LSIs also contain dual slope A/D converters, piezoelectric buzzer drivers, battery life detectors, controllers and oscillators, etc., so they are very suitable for the use of pocket digital multimeters.

2 Pin functions and internal structure

NJU9207/9208 is packaged in QFP80. The pin arrangement of NJU9207 is shown in Figure 1. The pin position of NJU9208 is rotated 90° relative to NJU9207. Their internal structure block diagram is shown in Figure 2.

Table 1 lists the pin symbols and function descriptions of NJU9207/9208.

Table 1NJU9207/9208 pin functions

3 Functions and principles

3.1 Measurement function selection

Various measurements can be selected through the input pins of FC1~FC4. The specific operations are listed in Table 2. In the table, H is high level and L is low level. Measuring function selection can be achieved using mechanical locking (LOCK) or rotary switches.

Table 2 Measurement function selection

3.2 Operation switch

When the switch is operating, the device outputs a buzzer drive signal and a system reset signal.

The key input can be used to confirm the sound (Confirmation sound) signal (2kHz). On the rising edge or falling edge of the signal input to the FC1 ~ FC4 terminals, an output clock of 2kHz with a period of 31.25ms can be obtained at the BZ terminal, as shown in Figure 3.

The function of the system reset (RESET) signal is to synchronize with the rising edge or falling edge of the signal input to FC~FC4 when the range setting (SET), counter reset, data hold release and automatic reset are executed.

When the auto-range mode function changes, the range is set based on the level on the AFC terminal, as listed in Table 3.

When the function is changed during the A/D conversion cycle, the counter will output a reset signal to initialize the LSI. Later, the A/D conversion starts from the auto-zero cycle, as shown in Figure 4.

3.3 Continuous detection function

When the continuous detection function is selected, the 2kΩ range of the "kΩ" function is automatically set. By applying a voltage of 1.5V, the forward voltage drop of the diode can be measured. If the input is started, the displayed value corresponds to the battery voltage value.

3.5 All automatic range functions (A/M=high level)

Table 4 and Table 5 list the full-scale mode and range control mode respectively.

Table 4 Full-scale mode (AFC=low level)

Table 5 Full-scale mode (AFC=high level)

When the range selection terminal remains low for more than 1s or the function is changed through FC1~FC4 terminals, the range function can be changed from any fixed range to automatic range.

When the range hold terminal is low, the range is fixed at the operating range and has nothing to do with the signal input to the range selection terminal (RCI).

The 2kΩ clock output to the BZ terminal can directly drive the piezoelectric buzzer. The falling edge of the input signal at the RCI terminal. The period of the BZ output signal is approximately 31.25ms.

3.6 Manual range function (A/M=low level)

Manual range selection can be achieved through RC1~RC3. The specific selection methods are listed in Table 6.

3.7 LCD display function

Table 7 gives the specific method of LCD display control operation.

4 application circuit

The circuit of a 3-and-a-half-digit pocket digital multimeter designed using the NJU9207F chip is shown in Figure 5.

This digital multimeter is powered by two 1.5V batteries.

The voltage applied to the analog circuit is +3V on LSI pin 57, and the -3V LCD driver voltage is added from pin 52 (Vss2). The frequency of the quartz crystal oscillator connected in parallel on pins 49 and 50 is fosc=32.768kHz. The external 0.1μF capacitor and 500Ω resistor on pin 10 (C1) form an integrating circuit to provide a reset signal for the counter. The external components on pins 2 and 3 are used for AC-DC conversion, and pins 4 and 5 are used as the "+" and "-" input terminals of the AD converter (ADC) respectively. Overall, the number of components required for this pocket digital multimeter circuit is relatively small.

Table 6 RC1～RC3 manual range selection function

Table 7 Program Description