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AC-DC Difference of Millivolt Thermal Standards

By: De-Xiang Huang, Ballantine Laboratories, Inc.

Abstract:

The error sources of ac-dc differences of millivolt thermal standards- Micropotentiometers (micropots) and the step down calibrations of micropots from 20 Hz to 1 MHz are analyzed. New step down method for sub-mV ranges significantly reduces the micropots uncertainty.

Introduction:

Thermal voltage converters ( TVCs ) are widely used as primary standards for ac-dc differences from 0.5 V to 1000 V. The new 0.25 V TVC reduces the ac-dc differences and the contact error from connections, and is suitable as 100 mV to 250 mV primary standard up to 10 MHz. Below 100 mV, microports are used as primary standards from 20 Hz up to 1 GHz.

Micropots have been used as RF mV standards for 40 years [1], but as low frequency(LF), mV standards only for a few years[2]. The ac-dc differences of thermoelements (TE) as thermal current converters (TCC) are usually a few ppm at audio frequencies, the disk resistor has very small inductance due to its radial film structure[1], so the pot has very small and stable ac-dc differences at LF. Down to sub-mV range, pot's might be better than we can certify, therefore it is necessary to investigate the error sources of pot and new calibration method.

Error Sources of Micropotentiometer

1. The error sources of TE:

a. The ac-dc differences of TEs as TCCs at audio frequency are due to Thompson effect, Peltier effect, etc.. The ac-dc differences of many TEs as TCCs at audio frequencies are less than 10 ppm.

b. Skin effect of TE as TCC: The skin effect of heater and inner leads will increase the ac resistance and generate more heat at ac than at dc[2]. The estimated ac-dc differences of one type TE as TCC, dTCC , are shown in Table 1.

c. Transmission line effect of ac current measurements: The TE is used to measure the ac current passing through the annular resistor. For Ballantine pot's, the transmission line error of ac current measurement is less than 0.2 ppm up to 1 MHz.

Table 1. d_TCC Caused by Skin Effect (ppm)

2. Error sources of annular resistor:

a. Inductance error:
The ac-dc difference caused by the inductance of the output resistor is as follows:
when , dL = - 1 =
where L is the inductance of the annular resistor Ro. The inductance of the resistor is extremely small. The ac-dc difference of annular resistor caused by the inductance is less than 0.2 ppm up to 1 MHz.

b. Transfer impedance error:
When the resistance value is low and the film is thick, the transfer impedance, that is the output voltage of annular resistor versus the input current of same resistor will vary with frequency[1], [2]. The connecting metals of annular resistor will have similar effect. The calculated transfer impedance error of resistor film is less than 1 ppm up to 1 MHz[2].

c. Dielectric loss:
The dielectric loss of film material and protection coating will reduce the ac resistance value. Since the resistance value of annular resistor is low, the error caused by dielectric loss is very small.

3. Each error above is very small up to 1 MHz. Therefore the error of TE and the error of annular resistor can be treated and tested separately and then combined together.

Error Sources of Measurement

The 100 mV pot is certified by 0.25V TVC. Then use step down method, namely use pot to certify lower range pot using TVC with amplifier (ATVC) as transfer device. The triangle measurements were performed to verify the accuracy of step down method. Use 0.25 V TVC to test pot and ATVC, then also use ATVC to test pot, the results are shown in Table 2.

1. Skin Effect of Adapter:
The skin effect of adapters is large due to the low input impedance of 0.25 V TVC. N male to GR874 adapter plus GR 874 to N female adapter are inserted between pot and 0.25 V 45 ohm TVC to test the influence of the skin effect of the adapters, see Table 3.

Table 2 Triangle Error (ppm )

Table 3. AC-DC Difference of upot ( ppm )

2. Load Effect:

a. Load Effect of input capacitance of TVC or ATVC:
The input capacitance of TVC or ATVC is in parallel with the annular resistor, it will low down the output ac voltage, and causes the ac-dc difference error.

Let
If , Then

b. Load effect of the skin effect and inductance effect of TVC:
The ac-dc differences, dTVC of 0.25 V TVC at higher frequencies are mainly caused by the skin effect and inductance effect of TVC which will increase the input impedance of TVC, and raise the output ac voltage of pot.
dLoad =
where Rh is the heater resistance of TVC.

3. Round trip error of ATVC:

ATVC has offset voltage, and cause large DC reversal error and ac-dc difference, this error and other errors become larger down to mV region. The round trip errors between ATVC and pot are shown in Table 4.

Table 4 Round Trip Error of ATVC ( ppm )

In order to reduce this error, the test condition between standard pot and test pot should be kept as close as possible, and 1 to 3 ratio is better than 1 to 2 ratio for step down.

New Test Method for 0.5 - 5 mV Ranges

Down to sub-mV and a few mV range, the ATVC is not suitable as transfer device for high accuracy. The new way is to treat pot as lumped elements at LF.

dupot = dTE + dR (4)

The current ac-dc differences dTE of 2.5 mA to 25 mA TEs are tested using TCC standard, and are usually less than 20 ppm up to 1 MHz for Ballantine pot's TEs.

The 0.5 ohm and 1 ohm resistor can be inserted into 10 mA or 25 mA TE housing and tested at 10 mV, then ac-dc difference, dR of resistor can be calculated from (4). Thus dupot of low mV pot can be calculated from (4) easily. Table 5 shows the dupot of 1 mV pot using different step down methods. Path 4 is described as above and has smaller random error. In path 1, 2 and 3, dR is calibrated at 1 to 4.5 mV.

Table 5 AC-DC Difference of 1 mV pot (ppm)

Conclusion

The pot's are suitable as LF mV primary standards. The Ballantine microports had been employed as primary standards by national laboratories. The new step down method has greatly improved the pot's accuracy.

References

[1] M. Selby, "Accurate Radio-Frequency Microvoltages", AIEE Trans., Vol. 72, pp. 158-164, May 1953.

[2] D.X. Huang, G. Rebuldela, and J. Harper, "High Frequency Response of Micropotentiometers" NCSL Workshop & Symposium, pp. 547-555, 1992.

[3] Nile Oldham and Robert
Henderson, "New Low-Voltage Standards in the DC to 1-MHz Frequency Range", IEEE Trans. Instrum. Meas. vol. 40, pp. 368-372, Apr. 1991.