How High Are the Requirements for Achieving a 7.5-Digit DMM?
Many instrumentation applications require high accuracy, such as digital multimeters (DMMs), three-phase standard meters, field instrument calibrators, high-precision DAQ systems, electronic scales/laboratory balances, seismic geophysical instruments, and source meters (SMUs)/power measurement units (PMUs) in automatic test equipment (ATE). These applications require very high accuracy in measuring DC or low-frequency AC signals, and in most cases, the relevant components used to implement application selection need to have low INL, high resolution, good stability, and repeatability. Among all these applications, DMM is the most representative one.
In order to build DMMs with seven and a half bits or higher accuracy, the industry typically uses multi slope integral ADCs based on discrete components. Although this type of ADC can ensure reasonable measurement accuracy, its design and debugging are quite complex, so many engineers use commercial ADC ICs to complete the design. In the past decade, 24 bit ∑ - Δ ADCs on the market have been widely used in the design of six and a half bit DMMs. To achieve seven and a half bit accuracy and linearity, higher performance ADCs must be used. Another challenge comes from the reference voltage source, which requires complex external signal conditioning circuits to achieve ultra-low temperature drift for deep buried Zener diode reference voltage sources.
This article will introduce a high-precision signal chain solution constructed by a low INL SAR ADC, a fully integrated ultra-low temperature drift precision reference voltage source, a four channel matching resistor network, and a zero drift low-noise amplifier. The theoretical analysis and calculation of accuracy in the article can serve as a reference and guidance for practical circuit design and testing.
