Calibration tools part 1 : Selection of DMM

Last 6 months I was busy to collect and to make some calibration tools. Because of this I was not able to update the blog and to make progress on my multimeter schematics and layout. Not sure even will I have enough time to finish it till end of this year.

So, at the end of March I bought a second hand HP 34401a 6½ digits DMM with idea to use it as my temporary primary DMM for calibration of my open source mutlimeter, evaluation of the LT2440 linearity and to have something better than my 3½ digits KEW 1012 DMM. Temporary, because my goal is to have for that task 8½ digits metrology grade DMM. I'm still looking into the ebay.com for affordable second hand HP 3458a or Advantest 6581. I prefer the 3458a/6581 because they can be adjusted only with 10V Voltage and 10KOhms resistance references. Hope that till end of this year I will have a metrology grade DMM.

I think that the HP 34401a DMM was the best for the money which I spent. I bought from UK based seller for 245GBP + 15GBP shipping which cost me 306 EUR (~363USD). For that money I got :

- 6½ digits DMM with possibility to extract the 7th digit from the RS-232/GPIB interface/Min-Max statistics if the unit has low noise.
- Mechanically switched two inputs (front and rear).

- 2 ppm linearity, which in the reality is better that 1 ppm based on shared eevblog tests of member Dr. Frank. May be this is based on the fact that design of HP 34401a was based on HP 3458a which has defacto the best linear ADC in the world. Recently Keysight seems made the same trick again with releasing HP 34470a (7½ with LTZ1000) and HP 34465a (6½ with LM399) which share almost the same main PCB, but have different daughter boards with the voltage reference.

- around 3-4x money saving if I would like to buy a new 6½ from Keysight/Tek/Fluke/Rigol/Siglent. 

- may be the worst user interface :-), but I had idea how to improve it.

- 22 years old DMM which was produced somewhere in 1996 based on IC and transformer date stamps. This give very stable voltage reference and indeed initial measurements shows about 3 ppm difference regarding calibrated (within 3 months) Keithley 2002 for 10V range using the 2.5 LTC6655 and 7V LTZ1000 references. For resistances I got between 104-143ppm difference for 100R/1K/10K/100K ranges using Vishay sealed Z-Foil resistors as references. I was able to adjust the resistance ranges and now they have few ppm difference with calibrated Keithley 2002.

The last calibration of this unit was in December 2005 year (12.5 year from now). Which was very impressive for me. By spec the 1 year relative accuracy of the 10V range is 35 ppm and 1 year relative accuracy of 100R/1K/10K/100K is 100 ppm. Now my HP 34401a have absolute uncertainty below 13 ppm for the reading and 0.65 ppm for the range based on the following data:
-> HP 34401a relative accuracy of 3 ppm regarding the Keithley 2002 for 10V range measured in 2.5V and 7.16V points using 100 NPLC, High input resistance, 5 minute period, average value from statistics, 23 +/- 2°C accuracy.
-> The relative accuracy of the Keithley 2002 : 6ppm reading + 0.15 ppm range for 10 PLC + 10DFLT for 3 month period.
-> the absolute uncertainty of the used calibrator Fluke 5720a : 4 ppm reading + 5 uV range for 1 year period.

HP 34401a remote control

To get data from the HP34401a can be used RS-232 or GPIB interface. I do not have GPIB card, so the only way was to use the RS-232 interface. There are 2 ways to get measurements results:
- Talk mode using HP-IB address 31 (Menu->IO->HP Address) and connecting the HP34401 to the computer with USB to RS-232 cable. Used pins from the RS-232 port of the HP34401a are 2 (Rx), 3 (Tx), 5 (GND) and 6 (DSR). They have to be connected to the USB/RS232 cable by the following way:

DMM	RX	TX	DSR	GND
Pin	2	3	6	5
USB to RS-232	TX	RX	DTR	GND
Pin	3	2	4	5

RS232 setting are performed via the front panel (RS232/8bit/2stop/No parity) and using Cutecom application under Ubuntu 14.04, the measurements results can be saved into a file.

- Full remote control mode using HP-IB address different than 31 (I'm using number 30). In this mode the full control of the DMM functionality is possible via SCPI language. However to make this possible was not so easy. I was not able to find any information in the Internet about this and It took me almost two weeks to figure out how to make it. I used all available technical documentation from HP and my Siglent SDS 1202X-E scope which was very helpful in this case.
If computer with USB to RS-232 cable is used, the same connection as the talk mode between the multimeter and the computer should be done. After setup of the RS-232 settings from the front panel, command SYST:REM have to be send via cutecom or other RS-232 terminal application.

If microcontroller with Max-3232 board is used for connection to the DMM, the following connection have to be done:

Connect Rx pin of the microcontroller to the Tx of the MAX3232 board and Tx pin of the microcontroller to the Rx of the MAX3232 board.
Connect MAX3232 pin1 (7V) to pin 6 DSR of the DB9.

Do not connect DTR 4 pin from Max-3232 to DSR pin 6 to DMM.
The last 2 changes are coming from the HP 34401A UserGuide page 151:
"To disable the DTR/DSR handshake, do not connect the DTR line and tie the DSR line to logic TRUE. If you disable the DTR/DSR handshake, also select a slower baud rate (300, 600, or 1200 baud) to ensure that the data is transmitted correctly."
But in fact 9600 baud rate worked fine.
After setup of the RS-232 settings from the front panel, command SYST:REM have to be send via cutecom or other RS-232 terminal application. This will enable the remote control mode.

With the full SCPI remote control, the HP 34401a can be improved significantly :
- Using microcontroller with external display and buttons can make better user interface than the old one made around 30 years ago. Buttons can be made for shortcuts to Slow 6 digit mode (NPLC 100 + AutoZero) , DISPLAY OFF, High/Low input resistance, showing 7 digits if the noise of the DMM is low, statistic information like Min/Max/Standard deviation/ppm difference from given reference value, very accurate temperature measurement using RTD PT100 sensor. If the VFD display is very dim or partially broken, this can save the HP 34401a from the dumpster as well.

- Using WiFi based microcontroller, environment sensors and micro SD card,  complete automatic test equipment station can be done. I already started to work on this and using ESP 8266 I was able to make HTTP Server which control some of the DMM settings and store measurement and sensors data into micro SD card for post analyses.

The noise

The first measurement which I made was for the noise floor of the HP 34401a. With this measurement I hoped to use the 7th digit which can be extracted from the RS-232 interface. According Agilent's document named "Understanding Reading Accuracy and Resolution in a 6 1/2-Digit DMM" the effective number of digits can be calculated for 68.3% probability as follows:
           Digits = Log10( PositiveOverRange / StandardDeviation of the noise)

For 99.7% probability the formula is:

           Digits = Log10( FullScale / Peak-Peak of the noise)

The range which I'm interesting is the 10V, which have 12V overrange. 7 digits on this range means that my HP 34401a should have noise less than 1.2uV measured with standard deviation and less than 2.4uV measured as peak to peak value. 

I made several measurements in different days which last for at least 8 hours and based on the collected data I'm confident that exact this unit can use the 7th digit. The tests were performed under the following conditions : shorting inputs in 10 V range, 10 GOhm input resistance, with 100 NPLC and AutoZero enabled (Slow 6 digit mode which duration is 4 seconds). I got near 74% of the samples below the 2.4uV threshold and near 95% of the samples were under 3uV which is 6.9 digits. Using standard deviation for 1 sigma confidence the result was 7.3 digits. Here are the data:

Percent of measured samples	Peak to Peak thresholds / stdev	Digits	Sigma delta	Based on:
	0.56-0.62uV stdev	7.3	1	STDEV calculation
73.9-76.4%	<2.4uV	7.00	1	Vp-p measurements
94.4-95.8%	<3uV	6.90	2	Vp-p measurements
98.2-99.7%	<3.6uV	6.82	3	Vp-p measurements
<2%	>3.6uV up to 4.3uV	6.75		Worst case : max noise

In ~75 % of the measurements, the last digit will fluctuated by +/- 1 digit due the noise (+/- 1.2uV, 2.4uV peak to peak). This is equal to 7 effective digits.
In ~95 % of the measurements, the last digit will fluctuated from +/- 1 digit to +/- 2 digit due the noise (+/- 1.5uV, 3uV peak to peak). This is equal to 6.9 effective digits.
In ~99 % of the measurements, the last digit will fluctuated by +/- 2 digit due the noise (+/- 1.8uV, 3.6uV peak to peak). This is equal to 6.82 effective digits.
In the worst case of less than 2% of the samples, the noise will reduce the effective number of digits to 6.75.
The minimum measured value was -4.1uV and maximum was 0.2uV, min-max difference is 4.3uV and average value was -2uV.