The accuracy of a digital multimeter (DMM) is the core indicator for measuring the degree to which its measurement results are close to the true values, and is influenced by various factors. The following analysis is conducted from four dimensions: hardware design, external environment, operating standards, and maintenance calibration:

1、 The Influence of Hardware Design and Manufacturing Process

1. Performance of core components

-Characteristics of Analog to Digital Converter (ADC)

The number of bits in an ADC directly determines the resolution. For example, the theoretical resolution of a 24 bit ADC can reach 1/16777216, but in practical applications, a balance needs to be struck between conversion speed and noise suppression capability. High precision ADCs typically use the Δ - ∑ architecture, combined with oversampling techniques to reduce quantization noise.

-Stability of reference source

The temperature coefficient (TC) of the internal reference voltage is crucial, and high-quality products use temperature compensated Zener diodes or bandgap reference sources to control the temperature drift within ± (1 × 10 ⁻⁶/℃). For example, the dedicated benchmark chip used in Fluke 8846A has an annual stability of ± 0.0002%.

-Front end circuit optimization

The selection of input stage operational amplifiers affects the input impedance and common mode rejection ratio (CMRR). The precision FET input operational amplifier can reduce the input bias current to the pA level, and with the help of shielded driving technology, eliminate cable capacitance effects.

2. PCB layout and grounding design

-Partition isolation strategy

The digital and analog areas are strictly separated, and the power layer and ground plane are arranged in layers. Key signal lines are routed using differential pairs.

-Optimization of multi-layer board stacking structure

The high-frequency filtering capacitor is placed tightly against the IC pin, and a π - type LC filtering network is installed at the power inlet. For the measurement of weak signals at the μ V level, even ceramic substrates are used to reduce dielectric absorption effects.

2、 The constraints of external environmental conditions

1. Temperature fluctuation effect

-Temperature drift compensation mechanism

When the working temperature exceeds the range of (23 ± 5) ℃, for every 1 ℃ increase, a typical multimeter will produce an additional 0.0005% reading error. Some models have built-in PTC thermistor arrays, which are used in conjunction with software algorithms for nonlinear compensation. The experiment shows that the error amplification of the compensated equipment can be controlled within 0.002% within the range of -10~50 ℃.

-Local hotspot prevention and control

Long term full load operation can cause transformers and rectifier bridges to heat up, and it is recommended to work continuously for no more than 8 hours. If necessary, install a cooling fan to maintain a temperature difference of less than 3 ℃ inside the machine.

2. Electromagnetic Compatibility Challenge

-Conducted interference suppression

The power port is equipped with a magnetic ring to filter out high-frequency noise, and the USB/RS232 interface is configured with ferrite beads. Actual test data shows that unprotected mobile phone calls nearby can cause ± 2 count jumps in AC measurements.

-Radiation coupling prevention

When measuring high impedance circuits, human proximity may introduce hundreds of Hz interference. The solutions include using shielding fixtures, shortening the length of test leads, and enabling averaging mode (taking the average of N samples).

3、 Human variable control of operational norms

1. Principle of Range Selection

-Best Matching Rule

When the measured voltage is within the range of 1/3 to 2/3 of the current measurement range, the relative error is minimized. For example, when measuring a 1.2V battery, selecting a 2V range can result in lower significant bit loss compared to a 20V range. Although automatic range switching is convenient, there may be repeated gear jumps near the critical point, and manual locking is more reliable at this time.

-Overload protection mechanism

Incorrect connection of high voltage may cause the fuse to melt or even burn out the voltage divider resistor. Regular products are equipped with dual protection: transient suppression diode clamp+PTC self recovery insurance. Daily use should cultivate the habit of estimating before connecting.

2. Test attachment management

-Probe contact quality

A banana plug with severe oxidation will increase the contact resistance to the Ω level, causing severe attenuation of millivolt level signals. Recommend wiping gold-plated contacts with alcohol swabs every month and replacing aged spring plates annually.

-Test line loss correction

The distribution parameters of the standard test line are approximately 0.05 μ H/m+100pF/m, forming a resonant cavity in the MHz frequency band. Low loss coaxial cables should be selected for precision measurement, and the actual line length should be compensated through the menu input.

4、 Periodic calibration and traceability system

1. Measurement chain transmission requirements

-Three level calibration system

① Factory initial calibration: using Fluke 5720A multifunctional calibrator, tracing back to national standards;

② Periodic verification: sent to the provincial metrology institute every year, in accordance with JJG (Military Industry) 117-201X regulations;

③ On site verification: Conduct rapid verification using standard resistors/voltage sources every week, and save the original data for future reference.

-Uncertainty evaluation model

Composite standard uncertainty, where u ₁ comes from the extended uncertainty of the superior standard, and u ₂ is the standard deviation introduced by repeatability. The typical UC value of an industrial grade multimeter is about 0.005% to 0.01%.

2. Long term stability monitoring

-Historical data trend analysis

Establish a database to record the key parameters of each calibration and draw an annual drift curve. If a certain indicator is found to exceed the standard for three consecutive times, immediately initiate preventive maintenance procedures.

-Life prediction of key components

The service life of electrolytic capacitors is about 5-8 years, and there will be an increase in ESR before expiration. Capacity changes can be detected through LCR tables to prepare spare parts in advance.