How to achieve better accuracy?

Everything about accuracy starts with the voltage reference used in the DMM.

The voltage references are used by Analog to Digital Converter (ADC) to compare unknown measured voltage with the known referenced one.

The most important characteristics of voltage reference are:

• Absolute voltage value.
• Long-Term stability measured in uV/√kHour or ppm from Vref/√kHour.
• Noise for frequency from DC to 10Hz/10Khz measured in [ppm] or [peak to peak uV] or [RMS uV]
• Voltage reference drift due temperature changes measured in ppm/uV per °C.
• Hysteresis : voltage reference shift due to temperature cycling.

Optional characteristics can include:

• Voltage reference drift due humidity changes.
• Voltage reference drift due mechanical stress of the PCB

One of the best voltage reference which is used in the high end DMMs is the LTZ1000 from a company called Linear technology. DMMs which are using LTZ1000 are HP/Agilent/Keysight 3458A, Keithley 2002, Fluke 8508A, Datron/Wavetek 1271/1281, Prema 6048, Advantest R6581.

The second most popular voltage reference is the LM399 again from the Linear technology company. Used in Keithley 2000. 2001 and 2110

Below can be found a comparison between these two famous voltage references and they typical characteristics values:

VRef Type	LTZ1000	LM399
Reference voltage (typ)	7.2 V	6.95 V
Long-term stability after 1000 hours typical in ppm  (uV peak to peak)	~0.28 ppm (2 uV)	8 ppm (55.6 uV)
Noise typical value in ppm  (uV peak to peak)	~0.16 ppm (1.2 uV)	~2.85 ppm (19.8 uV)
Temperature drift typical value in ppm / °C for 5°C difference (uV peak to peak) [ppm for 1°C]	0.25 ppm (1.8 uV) [0.05 ppm/°C]	1.5 ppm (10.425 uV) [0.3 ppm/°C]
Total error (typical) in ppm after 1000h and 5°C difference  (uV peak to peak)	~0.69 ppm (~5 uV)	~12.35 ppm (~85 uV)
Price, 1 pcs	Min 45 USD	~10 USD

Note, that overall accuracy of DMM includes also accuracy of the used ADC, operational amplifiers, resistors etc..