Key Technical Characteristics of Standard Digital Multimeters and Dual-Impedance Digital Multimeters
The basic structure of a regular digital multimeter is shown in the figure. The dual integration A/D converter is the "heart" of a digital multimeter, which enables the conversion of analog to digital signals. The peripheral circuits mainly include function converters, function and range selection switches, LCD or LED displays, as well as buzzer oscillation circuits, driving circuits, detection circuit on/off circuits, low voltage indication circuits, decimal point and symbol (polarity symbol, etc.) driving circuits.
The A/D converter is the core of a digital multimeter, using a single-chip large-scale integrated circuit 7106. 7106 adopts an internal XOR gate output, which can drive LCD displays and save electrode consumption. Its main features are: single power supply, wide voltage range, use of 9V stacked batteries to achieve miniaturization of the instrument, high input impedance, and use of internal analog switches to achieve automatic zeroing and polarity conversion. The disadvantage is that the A/D conversion speed is slow, but it can meet the needs of conventional electrical measurements.
Basic knowledge about impedance
Today, the majority of digital multimeters sold on the market for measuring industrial, electrical, and electronic systems have very high input circuit impedances, generally greater than 1 megohm. Simply put, when DMM is measuring a circuit, it has almost no impact on the performance of the circuit. And this is exactly what the vast majority of measurements require, especially for sensitive electronic or control circuits. Previously used troubleshooting tools, such as analog multimeters and solenoid valve testers, generally had low input circuit impedances, around 10 kiloohms or lower. Although these tools are not affected by stray voltages, they are only suitable for measuring power circuits or other situations where low input impedance does not adversely affect or alter circuit performance.
An exemplary combination of two input impedances
By using dual impedance instruments, technicians can troubleshoot sensitive electronic or control circuits, as well as faults that may include stray voltage circuits, and can more reliably determine whether there is voltage in the circuit.
For standard electrical measurements, it is generally better to use high impedance instruments unless there are stray voltages present.
In Fluke114, 116, and 117DMM, there is a significant impedance in the commonly used Vac and Vdc switch positions of the instrument, which can be used for troubleshooting tasks in most cases, especially for sensitive electronic loads. The low impedance function of Fluke is called Auto-V/LoZ. Among them, Auto-V represents automatic voltage, which can automatically determine whether the measured signal is an AC voltage or a DC voltage, and then select the correct function and range to display the correct information. LoZ represents low impedance (Z). This performance is a low impedance input for the tested circuit, which can reduce the possibility of reading errors caused by stray voltages and improve the accuracy of determining the presence or absence of voltage. When there is doubt about the reading (possibly due to stray voltage) or when measuring the presence of voltage, the Auto-V/LoZ switch position on the DMM can be used.
