IET Labs Inc., PRS-370 DMM-Mode Uncertainties

The IET Labs PRS-370 Decade Resistor has a very interesting built-in feature to increase its short-term accuracy. 

If you connect your 8.5 Digit DMM to the rear terminal of the PRS-370 Decade Resistor, the PRS-370 will at the touch of a button, adjust the desired resistance to any value from the range 0.1 Ω – 20 MΩ based upon measurements from the DMM.

The PRS-370 works in a loop to; control the DMM, acquire measurements from the DMM and adjust it's internal source resistoance to achieve an output resistance value as close as possible to the nominal value.  The PRS-370 stops this adjustment process when it is able to source a resistance that is as close a possible to the nominal resistance, within the uncertainty of the DMM. 

Once the process is complete, approx 15-60 seconds, the PRS-370 diplays; nominal resistance, difference from nominal and the measurement accuracy of the DMM, and switches the source resistance from the rear terminals to the front terminals of the PRS-370.

This PRS-DMM feature basically turns your PRS-370 into a short-term transfer standard that is able to source resistance to virtually the same uncertainties as your 8.5 Digit DMM.  It transforms the Fluke 8508A or Keysight 3458A, which you already own, from having the ability to measure to also being able to also source resistance. 

See Chapter 4 of the Manual for More Info on this DMM Feature

When used with a Keysight 3458A or Fluke 8508A your PRS-370 can have the following measurement accuracies shown below.

Note the exact accuracy is displayed on the front panel of the PRS-370 after the loop is completed, so you do not have to look at this table after you own the unit.

The numbers provided are the basic measurement accuracy of the 8508A or 3458A in (ppm) from their respective manuals for a 1 year calibration interval of the DMM at a 95% confidence level. 

This does not include short-term stability of PRS-370.  You would need to add ±1 or 2 ppm/<1 minute or  ±4 ppm/10 minutes to the respective numbers.

Using Fluke 8508A

Resistance Setting (Ω)	Measurement Accuracy of 8508A in (ppm) does not include short-term stability of PRS-370
1	20
2	21
3	17
4	15
5	14
6	13
7	12
8	12
9	12
10	11
Decade Step  10Ω to 10kΩ	Measurement Accuracy of 8508A in (ppm) does not include short-term stability of PRS-370
1	9
2	15
3	13
4	11
5	11
6	10
7	10
8	9
9	9
10	9
Resistance Setting (Ω)	Measurement Accuracy of 8508A in (ppm) does not include short-term stability of PRS-370
100,000	9
200,000	19
300,000	15
400,000	14
500,000	13
600,000	12
700,000	11
800,000	11
900,000	11
1,000,000	11
Resistance Setting (Ω)	Measurement Accuracy of 8508A in (ppm) does not include short-term stability of PRS-370
1,000,000	11
2,000,000	25
3,000,000	22
4,000,000	20
5,000,000	19
6,000,000	18
7,000,000	18
8,000,000	18
9,000,000	17
10,000,000	17

Using Keysight/Agilent 3458A

Resistance Setting (Ω)	Measurement Accuracy of 3458A in ppm does not include short-term stability of PRS-370
1	65
2	40
3	32
4	28
5	25
6	23
7	22
8	21
9	21
10	20
Resistance Setting (Ω)	Measurement Accuracy of 3458A in ppm does not include short-term stability of PRS-370
10	20
20	37
30	29
40	25
50	22
60	20
70	19
80	18
90	18
100	17
Decade Step 100 Ω to 10 kΩ	Measurement Accuracy of 3458A in ppm does not include short-term stability of PRS-370
1	17
2	13
3	12
4	11
5	11
6	11
7	11
8	11
9	11
10	11
Resistance Setting (Ω)	Measurement Accuracy of 3458A in ppm does not include short-term stability of PRS-370
100,000	11
200,000	25
300,000	22
400,000	20
500,000	19
600,000	18
700,000	18
800,000	18
900,000	17
1,000,000	17
Resistance Setting (Ω)	Measurement Accuracy of 3458A in ppm does not include short-term stability of PRS-370
1,000,000	11
2,000,000	25
3,000,000	22
4,000,000	20
5,000,000	19
6,000,000	18
7,000,000	18
8,000,000	18
9,000,000	17
10,000,000	17