Application of AGILENT 34970A in Product Parameter Verification (Fig.)

For high products such as standard resistors, they need to be verified one by one after production. The manual verification method is cumbersome and easy to introduce errors. Using AGILENT 34970A (data acquisition/switch unit), connected with PC host through HPIB bus, an automatic test virtual instrument is formed to test it. After testing, the verification report is automatically generated by DDE (Dynamic Data Exchange). In this way, not only the measurement process is simplified, but also the measurement is improved.

Introduction

At present, the method of direct measurement or substitution of the same nominal value is generally used to verify the parameters of standard resistance after production in China, and the measuring instruments are usually bridges. Although this method is mature, it uses a lot of matching equipment and is cumbersome to operate, and the whole verification process is carried out under manual intervention. It is unavoidable to introduce bias (such as artificial reading and other factors), plus some non-technical factors (such as human mood), the quality of verification is difficult to guarantee. With the rapid development of computer technology and measurement technology, test automation has become an inevitable trend.

Based on the above reasons, we have developed a standard resistance auto-verification system using AGILENT 34970A of Agilent and connected to the computer host via GPIB bus. Under AGILENT VEE software platform, it enables verification to proceed in an automatic state, including the verification report that is generated automatically in EXCEL by DDE (Dynamic Data Exchange), so the system is reliable and efficient.

Test System Architecture

1. Discussion on Verification Method

There are many methods to calibrate standard resistance, such as direct measurement, equivalent nominal value substitution, and transition transfer. The advantage of direct measurement is that it is simple, but it requires a resistance measuring instrument to be two levels higher than the measured resistance. For a standard resistance of more levels, the cost is to use a higher, more expensive testing instrument. Transition transfer method achieves the transfer of resistance value by measuring the ratio of two different resistance values. This method is often used for the measurement of high resistance. It requires a high test instrument and the test process is complex. The method of substitution for the same nominal value is to measure the resistance values of standard resistors (those that have been measured and met the requirements) and the tested resistors RX in turn with a resistance measuring instrument. The verification results are obtained after the following calculation.

RX = RS + (AX - AS)

AS in Type - Sample Value of Measuring Instrument when Measuring RS

AX - Examples of measuring instruments when measuring RX

It can be seen from the above that the method of substitution of the same nominal value requires less resistance measuring instruments than that of direct measurement, and the measurement method is simpler than that of both methods. According to the code JJG166-93, when the resistance measuring instrument is less than two levels higher than the RX level of the standard resistance being tested, and there is a resistance standard RS with the same nominal value as the RX level of the resistor being tested, the same nominal value substitution method can be used to verify the resistance value of the resistor being tested. What we are now achieving is to verify the nominal resistance of level 0.01. The existing measuring instrument is AGILENT 34970A, which is known by AGILENT 34970A. Although it does not meet the requirements of two higher levels for directly measuring the standard resistance of level 0.01, we have finished using it as a measuring instrument of the same nominal value substitution method. Based on the above analysis, the system uses the same nominal value substitution method and uses AGILENT 34970A as the test instrument.

2. AGILENT 34970A and its application

AGILENT 34970A is a multi-function measuring instrument with 61/2 A/D converter and three module slots in the rear. Agilent offers eight module options to complete a variety of testing functions according to different measurement modes. This system is mainly used for standard resistance measurement. Therefore, AGILENT 34901 is selected to provide 20 analog channels (40 input channels at one end). We select two of them to connect the standard resistor RS and the measured circuit RX. Because the test uses the same nominal value substitution method, RS and RX will be measured in turn during the test process, and a switch is required for switching, AGILENT 34904 module is selected, which is a 4 × 8 Double-line matrix transformation switch, which can be programmed freely to switch the measured object. Figure 1 is a diagram of a standard resistance test connection (four-wire connection).

Fig. 1

According to the requirements of JJG166-93, the error introduced by the resistance measuring instrument for resistance substitution comparison should not be more than 10 × 10-6 (1/10 of the grade index), to achieve this goal, a four-wire measurement method is used when measuring low resistance values (AGILENT 34970A provides this function); In order to eliminate the error caused by zero potential, such as contact potential, in resistance measurement, the Offset Compensation (OFFSET COMPENSATION) technology provided by AGILENT 34970A is applied: the second is to measure the voltage value of resistance under the action of a constant current source, the second is to measure the zero voltage value of resistance under the condition of switching off the constant current source, and the second is to eliminate the effect of zero potential caused by the Substitution Measurement by the voltage value of the resistance measured by two measurements. Make it conform to the rules.

3. System Structure

This system uses AGILENT 34970A as the measuring instrument, connects the measured resistance through AGILENT 34901 and AGILENT 34904 modules, uses AGILENT VEE as the testing system software operation platform, and connects with AGILENT 34970A through HPIB bus interface. AGILENT 34970A supports SCPI commands, and all actions of AGILENT 34970A are implemented by the AGILENT VEE platform by sending SCPI commands to it.

Software implementation

AGILENT VEE (AGILENT Visual Engineering Environment) is a graphical programming language developed by Agilent. It has the following features:

Fig. 2

A. Easy to program;

B. Powerful GUI features: No effort is required to create a powerful and user-friendly user interface;

C. Powerful instrument control function: A large number of instrument drive functions, allowing users to control the instrument freely.

Because of the above features, the system uses AGILENT VEE as the test software, and Figure 2 is the flowchart of the test program.

When the software is running, the operator is prompted to enter some of the most necessary data, such as the number of resistors checked, the average number of measurements expected, the nominal value of the resistor being measured, etc. These interactions can be avoided if the objects being tested are of a single type. Figure 3 shows the test program operation interface.

Fig. 3

From Figure 3, you can see an overview of the human-machine interface. The curve dynamically reflects the real-time status of each test, and real-time reflects the average number value of each test above the curve graph. The colors of these numbers correspond to the colors of the curves. Move a small triangle on the curve to reflect the numerical difference between any two points below the curve. It provides operators with a simple but necessary means of data analysis. At the end of the test, the DDE function provided by AGILENT VEE is passed directly to Excel (spreadsheet software). The data is analyzed, synthesized by EXCEL, and the verification report is generated. This process is completed by the program.

Error synthesis

Standard resistance is calibrated by substitution method. The main sources of standard resistance errors measured are standard instrument RS and calibration device. The main calibration instrument in this system is AGILENT 34970A. According to the regulations JJG166-93, the standard RS selected by us is the second-class standard resistance, its grade index is 0.001%, which completely meets the requirements of the regulations and is not greater than 1/4-1/5 of the resistance detected. By analyzing the verification device comprehensively, the possible error sources are as follows:

A. Error caused by the annual variation of second-class standard resistor RS, i.e. grade index CS Δ U1;

B. Transfer error caused by second-class standard resistor RS being verified Δ U2;

C. Errors introduced by the repeatability of measuring instruments Δ U3;

D. Error caused by inadequate resolution of the measuring instrument Δ U4;

E. Errors caused by ambient temperature changes Δ U5;

F. Errors caused by lead resistance, parasitic potential, zero current, etc. Δ U6.

The error caused by the lead resistance in the above six errors is mainly for small resistors. For the objects we examined, the main errors affected are 100_and 1000_resistors. Based on the characteristics of AGILENT 34970A, a four-wire measurement method is used to eliminate the effect of lead resistance. Due to the influence of parasitic potential and zero current, another characteristic of 34970A - offset compensation method is used to reduce its impact on measurement and make errors caused by lead resistance, parasitic potential, zero current, etc. Δ U6 is not greater than 0.05CX, that is Δ U6=5 × 10-6. Use AGILENT 34970A 6-bit half display capability to increase the integral time to 200PLC, when the resolution reaches 0.00000022X range, the resolution error will occur Δ U4=1.1 × 10-7.

The repeatability of the measuring instrument AGILENT 34970A according to the 24-hour index of its resistance unit, we know that among the three ranges of 100_~10 K used, the resistance of 100_is 0.0065% in 24-hour index, and the deviation limit of 0.01% in standard resistance of 0.01% according to the regulations. According to our actual test, the repeatability error of AGILENT 34970A conforms to the regulations, that is Δ U3 < 10 × 10-6.

Errors caused by changes in ambient temperature Δ U5, because of the limitation of the experimental conditions, it is considered to be in conformity with the regulations, i.e. Δ U5=8 × 10-6.

The combined error of the above six errors Δ U:

Δ U= Δ U12+ Δ U22+ Δ U32+ Δ U42+ Δ U52+ Δ U62

= 17.7 × 10-6 < 33 × 10-6

This value is less than the total uncertainty caused by the calibration device and environmental conditions specified in the JJG166-93 specification and is not greater than the measured resistor 33 × Requirements for 10-6 (1/3 of the hierarchical index).

The specific error allocation and the combined error are shown in the table below (the values are expressed in relative errors):

Concluding remarks

The development of this system shows that it is entirely feasible to verify the standard resistance of 0.01 grade and below in the range of 102~104_with the same nominal value substitution method. If a higher resolution measuring instrument, such as 7 1/2 resolution, is used, the range of detected resistance values can be wider. The automation of testing not only improves the measurement, but also greatly improves the efficiency. We can also see that virtual instrument is a brand new concept, and its application prospects are very attractive.

Source:Xiang Xueqin