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..  

Why accuracy matters?

The accuracy shows what is the real measured value within confidence interval.
Every DMM has accuracy specification from which can be calculated the interval in which the real value of the parameter lies within it.

The accuracy given in the DMM specification is relative, given in last digits, percents (%) or part per millions (ppm) and have to be calculated to absolute value.

The relative accuracy is given for a time period, temperature range and warm-up period. For example :

• 90 days, 1 year, 2 years from the date of the last calibration
• ±5°C difference between the real temperature and the calibrated tempearture
• 2 hours warm-up period

Here an example with 3 real DMM will be given : 

• Low end : Uni-T UT61A which cost ~ 50 USD
• Middle : Keithley 2110 which cost ~ 600 USD
• High end : Keysight 3458A which cost ~ 10000 USD

Lets assume that we measured 5V on the USB power supply with these 3 DMM. 

Tip : how to convert ppm to an absolute value -> ppm/1,000,000 * value

DMM Model	Range	Relative accuracy ±(readings + range)	Absolute accuracy
UT61A	40V	±(0.5% + 1 digit) Resolution for 40V is 0.01V	0.5% * 5V = 0.025V 1 digit * 0.01V = 0.01V Total : ± 0.035V (35mV)
Keithley 2110	10V	±(0.012% + 0.002%) 1 year, 23° ±5°C	0.012%*5V= 0.0006V 0.002%*10V= 0.0002V Total : ± 0.0008V (0.8mV)
Keysight 3458A	10V	±(8 ppm + 0.05 ppm) 1 year, 23° ±5°C	8ppm*5V = 0.00004V 0.05ppm*10V=0.0000005V Total : ± 0.0000405V (0.0405mV)

You can see the difference of absolute accuracy and the price which is paid for.

For ~50 USD, your real measurement value is between 4.965V and 5.035V.

For ~600 USD, your real measurement value is between 4.9992V and 5.0008V.

For ~10000 USD, your real measurement value is between 4.9999595V and 5.0000405V.

Because the voltmeter is used in the current and resistance measurements, the better accuracy in DC Voltage measurement, means also better accuracy of current and resistance measurements.