The Design of a Calculable AC voltage reference using digital waveform generation

16th July 2013 NCSLI 2013, Nashville 1

Abstract

The design of an AC voltage reference source using a digital to analogue converter controlled by a microcontroller to produce a calculable RMS AC voltage reference with accuracy suitable for calibrating high performance Digital multimeters.


Technical Objectives

• To provide a method for secondary and below laboratories with the ability to generate high accuracy AC Voltages

• Investigate errors associated with digitally generated AC Voltages, as well as the practical application of digitally generated waveforms in commercial calibration


Learning Objectives

• To investigate alternatives to traditional AC voltage verification methods

• To enhance calibration laboratories understanding of verification of high performance modern DMM’s


The calibration of Today’s Modern high performance Digital Meters which offer ACV Accuracy in the order of 80-100 ppm

To achieve a stand off ratio of just 4 to 1requires an accuracy of around 20 ppm. This is not possible with a multi-product calibrator


The Requirement

The Solutions

1) Multi–Junction Thermal Transfer Standard

2) Calculable AC voltage reference


The Solutions : Multi-Junction Thermal Transfer Standard


Advantages Disadvantages

Wide Frequency Range Requires both a stable DC and AC voltage source

Proven Accuracy Equipment is expensive

Calibration is costly, especially when a wide range of points is required

Even with protection, older Thermal Transfer devices are easy to damage by accidently over-ranging

The Solutions : Calculable AC Voltage Reference


Advantages Disadvantages

Due to the nature of the device, does not require an AC or DC source

Limited frequency range

Instrument can be internally verified with a DC volt meter

Output voltages are limited

Due to solid state design, instrument is hard to damage with incorrect connections

Low cost due to ‘simple’ hardware

The Concept of theCalculable waveform

• Digitally create an AC waveform using a digital to analogue (DAC) converter. In our experiments we chose a waveform based on 256 steps.

• Single step the waveform to allow the level of each step to be measured as a DC level.


The Concept of theCalculable waveform

The RMS value of the waveform is then calculated using the following formula


Where :

Vrms = RMS Ac voltage outputV = measured DC voltage of ‘step’n = number of steps

History

This is not a new idea.

This technique was pioneered as

long ago as 1988 where results against

Thermal converters gave uncertainties at

the 7 volt level of just 5ppm.


What has changed

There is now a much greater need for high accuracy ACV. In 1988 we were only seeing the first developments in High accuracy AC DMM’s. Now almost every laboratory has a high performance DMM


What has changed

Digital Electronics along with the rapid development of digital to analogue converters for high quality audio has now made it much easier to implement this concept.


The Circuit

The design uses a PIC micro controller, programmed with the waveform to directly drive a DAC. The PIC is directly

controlled from an RS232 interface.


Control Interface

Micro ControllerPIC 16F84

Clock

DAC

10V Reference Input

AC Voltage Output

The DC Reference

The D to A converter needs a short term stable 10Volt DC reference.

The Linear Technology LTZ100 reference can easily provide better than 1ppm stability over the

short term 2/3 hours required.

As the LTZ1000 is temperature stabilised

temperature variations over the measurement period will not add any significant contributions to an

uncertainty budget16th July 2013 NCSLI 2013, Nashville 16

Improvements Vs Performance

The circuit used is extremely simple. Early design’s used two converters, with the output being switched between them

to remove glitches.

With modern converters this is not necessary.

The PIC micro controller directly drives the converter, loading the code into the converter directly from it’s

memory, also clocking the converter after each data load. This very simple approach requires some machine code for the PIC to make it run as fast as possible. All timing comes

from the crystal oscillator driving the PIC.


The Software

The AC reference is very simple to control, with just a few commands needed.

1 - 15Hz

2 - 60Hz

3 - 200Hz

4 - 1000Hz

S - Stop waveform and set to zero

X - Advance 1 step

Y - Advance 10 steps

Z - Advance 64 steps


Taking the DCV Measurements

The DCV measurements can be taken with a high performance 8 digit DMM.

This will typically give full scale & linearity uncertainties of less than 5ppm.

As there are over 256 measurements to be made it is recommend that the process be automated by using a PC &

software.


Measuring Software

A procedure to measure each step was written using Procal. As each step is measured it is squared and

added to a running total.

The procedure being 256 steps long, takes about 40 minutes to run automatically.

Although the measurement procedure could be written in any language Procal has the advantage to

work with any DMM without changing code.


Basic Measurement Diagram


Practical Set up


Voltage Range

With a 10V input the peak output

of the converter will be 10Volts;

giving an RMS voltage of 7.07V

To get a 1V output the 10Volt output

can be resistively divided down allowing the output to still be single stepped and measured on DC.

Note an IVD cannot be used to divide down the DC output. However it can be used on the AC output


Errors Switching Glitches and transition

This error is most difficult to evaluate. It is dependent on the DAC used. The DAC we have chosen is typically used for audio will be very ‘clean’. The approach used by Transmille has been to look at an individual step on a scope and estimate an error based on the size and duration of the transitions.


Errors Switching Glitches and transition

Typical figures for this, at a frequency of 50Hz, where there is a step every 80uS, worst case measurements give glitch times of around 8nS, with a glitch size of around 10% of the step. This gives transition errors around the 10ppm level. In practice as some glitches will add and some will subtract the real error will be much less.


Errors from Rise / fall times & DC offsets

Rise and Fall time

Providing the rise and fall times are equal they do not contribute to errors. The error caused by a difference in rise/fall times can be mathematically calculated

DC offsets

Any DC offsets in the DAC will be measured and added to the calculated RMS figure. However if the device being calibrated is AC coupled DC offsets in the AC output will give an error and it may be best to trim any offsets out to avoid this problem.


Errors from Loading effects

Output Loading

When measuring the output with a thermal converter the load effect on the 2 wire output will need to be considered.

When using measuring the output with a typical DMM loading effects will be negligible


Errors from drift in DC Reference Source & DAC

As the AC source is a transfer device it is only the short term drift between the DC measurement of the system and the subsequent use on AC that contributes to the error.

Long term changes in the reference or the DAC linearity or gain do not contribute as the system is calibrated before use.


Uncertainties

Typical laboratory figures for uncertainty

Imported DC Voltage Measurement 5ppm

Resolution of DC voltage Measurement 0.1ppm

Switching transition errors 10ppm

Short term stability of DC reference and DAC* 2ppm

Combining for 95% (K=2) gives 13ppm

* Includes effects of temperature for +/- 2’C from Tcal


Verifying the accuracy

Measurements made by TransmilleTransmille best AC measurement capability uses

our Fluke 540b thermal transfer, the accuracy of this

Instrument is enhanced by measuring the output of the thermal converter directly. Loading effects on both AC and

DC measurements were compensated for. Tests performed against a Wavetek 4920, Transmille 8081,

Agilent 3458A and Fluke 8508A provided similar results.

Typical readings obtainedSingle step calculation = 7.055681

Measurement by thermal Transfer 540B = 7.055779

Error = 14 ppm16th July 2013 NCSLI 2013, Nashville 30

Frequency Range

To date Transmille’s interest has been

In the frequency range 40Hz to 1kHz.

The design could generate frequencies

below 1Hz with no additional errors.

Frequencies up to 10kHz could also be

generated with increased error from switching.


Improvements……

Extending the Voltage range.

A simple resistive divider

to provide 1V and 100mV outputs.

A DC voltage amplifier to provide a 100V output.

Both could be again ‘calibrated’ at DC using a known DMM to provide the calculated AC RMS value


Other Functions……

Non sine wave outputs

With this method it would be very easy to generate other wave shapes to evaluate performance of

converters including crest factor.


Power and Phase…..

Future development of this technique could be to add additional converters to allow for several

outputs. Up to 6 outputs could easily be provided for 3 phase simulation of power, with an accurate

phase, or delay between the outputs.

If all the outputs were at the same 7V level this would provide a very affordable solution for a phase

and power reference.


References

[1] R. Kinard, and L. A. Harris “ A digitally synthesized sine wave source with +/- 1 ppm amplitude stability. “ in Euromeas ’77 Conf

Digest. Institute Elect. Eng. Conf. Pub no 152, Sept. 1977

[2] Oldham N., Hetrick P., Zeng X., “A Calculable, Transportable Audio Frequency AC Reference Standard”. IEEE Transactions on I & M, April

1989, Vol. 38.

[3] Oldham N., Bruce W., FU C., Cohee A., Smith A., “An Intercomparison of AC Voltage Using A Digitally Synthesized Source”.

IEEE Transactions on I & M Vol. 39 No. 1, February 1990


Conclusions

Digital synthesized waveforms can

provide a low cost and easy to use solution for DC to AC voltage transfer.

Copies of our paper and presentation are available on our booth on USB memory

sticks


The Design of a Calculable AC voltage reference using digital waveform generation

Documents

Transcript of The Design of a Calculable AC voltage reference using digital waveform generation