Advantages of F2000 Precision DC Current Source and Its Application in Low Resistance Measurement
DXF2000 precision DC current source with 1/50000 high setting resolution, 0.05% high accuracy and 0.01% high stability.
Product Description
DC current sources are widely used in the measurement of electronic parameters. Precision DC current sources with high accuracy and high stability are used in resistance/resistivity measurement, optoelectronic components such as LED and semiconductor laser testing, and industrial 4-20mA current loop transmission, etc. Used in high-precision applications to ensure high confidence in measurements.
The precision DC current sources on the market are divided into two main types according to the design structure, the traditional type for digital meter monitoring and the modern instrument with a microprocessor inside. The significant difference between the two is that the output current of microprocessor-based products, such as Dexinmag's F2000 precision DC current source, can be output according to a preset value. For meter head monitoring products, the monitoring value of the output current must slowly approach and reach the set value by adjusting the potentiometer knob.
Although the core analog circuits of these two types of current sources are similar, the difference in design structure still makes them show obvious differences in performance and operability. Taking F2000 as an example, these differences are mainly reflected in the following aspects:
1. Output current stability and repeatability
The output current of F2000 is input to the microprocessor by the user through the keyboard, and is executed by the DAC under the control of the microprocessor. Compared with the operation mode of adjusting the potentiometer to observe the meter head, there are fewer uncertain factors in this process, and there is no mechanical contact in the analog circuit link, so it has higher output current stability and repeatability. Digital meter monitoring products must rely on adjusting the potentiometer knob to complete the setting process. During the manual adjustment process, there may be more accidental factors caused by the operator himself, resulting in low repeatability. In addition, since the potentiometer is a mechanical component, its rotational position may show long-term slight changes due to internal stress, and cause long-term continuous drift of the output current, which is usually an unacceptable problem in the application of precision current sources. This problem is solved in the F2000 by a DAC without mechanical contacts.
2. Output current regulation
Digital meter monitoring current sources generally have two potentiometer knobs for coarse adjustment and fine adjustment. The adjustment fineness ratio of this combination is usually set to 100:1 to take into account the span required for coarse adjustment and the fineness of fine adjustment. F2000 provides a more flexible adjustment method. First of all, the current can be set directly by directly inputting the value. When fine-tuning, the F2000 provides a fine-tuning capability of plus/minus 1 for each setting value, which not only provides a configurable fine-tuning fineness from 1:1 to 10000:1 Compare. Where accurate large-step coarse adjustments and fine fine adjustments are required, this adjustment method provides convenience that cannot be achieved by rotating the potentiometer knob, and has a much faster operating speed.
3. Control of the output state
The controllable output terminal state can realize convenient functions such as starting up and changing the load. Before changing the load, set the output terminal of the current source to a high-impedance state, and reset the output terminal to a normal output state after the load replacement is completed. Although controlling the state of an output terminal is easy to implement on a circuit, it can only be effectively implemented in products with a microprocessor inside. The most important reason is that once entering the high-impedance state, the output current is zero, and the meter head of the meter-monitoring current source will not be able to display the set value, so the output current cannot be set in the high-impedance state of the output terminal. Therefore, this structure The product generally does not provide the output state control function, and the load must be switched off to replace it. For the same reason, even at the moment of power-on, the load has to bear uncontrollable power-on impact.
The F2000, which is equipped with a microprocessor, does not have this problem at all. Regardless of the state of the output terminal, the set value is always stored in the memory of the microprocessor and displayed on the LCD, so that the output current can be set at any time. During the power-on process, the output terminal of the F2000 always maintains a high-impedance state, thereby effectively avoiding damage to the load due to power-on impact.
4. Clamp voltage indication and alarm
Just like a voltage source has a maximum output current limit, a current source also has a maximum output voltage, that is, an open-circuit voltage limit, and the accuracy of the output current may decrease when it is close to the open-circuit voltage. It is therefore necessary to issue a notice that can be learned by the operator before the output voltage reaches the open circuit voltage.
In order to ensure the accuracy of the output current, usually set a voltage slightly lower than the open circuit voltage, for example, 2V lower than the clamping voltage, and issue a clamping indication when the output voltage reaches or exceeds this voltage. The clamp indication is generally a flashing alarm, which visually reminds the operator that the output current may not reach the rated accuracy at this time. However, in some applications, the operator cannot always maintain visual attention to the current source, but must ensure that the output current is accurate. For example, in the case of manual inspection lines in the production process, the inspector must concentrate on the connection of the object under test. To ensure the correctness and speed of operation. In view of this, the F2000 provides an optional sound alarm besides the flashing indication. The audible alarm may need to be turned on, and may be turned off in laboratory environments where quiet is required.
5. The set value remains after shutdown
In many applications, the output current of the precision current source is set at a certain fixed value for a long time, and the operator hopes to maintain this set value after shutdown to avoid repeated settings after restarting. This feature is often considered a strong point of meter-monitored products, where the mechanical rotational position of the potentiometer is inherently retentive.
Now, the F2000 with its internal microprocessor can do even better. The non-volatile memory inside the F2000 will save the last setting value before power off, and it will be restored automatically when power on again. Not only that, the setting value saved by the F2000 cannot be accidentally changed during shutdown, and the potentiometer knob of the meter monitoring product is likely to be turned by an illegal operator unintentionally, and may output excessive current to the load after the startup It will cause damage to the hub, and this situation cannot be observed on the meter head of the meter head monitoring product before the current is output, so it cannot be effectively avoided. After the F2000 is turned on, the output end always enters the high-impedance state first, so that the operator can provide current to the load after completely confirming that the settings are correct. Similar to the clamp voltage audible alarm, the setting value hold function of the F2000 can also be turned off, and the setting value after power-on is fixed at zero.
6. Industrial 4-20mA current loop transmission application
In the current loop application, the current signal lower than 4mA is an invalid signal, but it is used to detect the disconnection fault. F2000 provides a special current loop mode. In this mode, the lowest current that can be set is 4mA, and the highest current is 20mA, so as to avoid the invalid operation of the controlled equipment by the output current beyond the working current range of the current loop. In addition, F2000 can also simulate short-term faults by controlling the state of the output terminal, so as to provide users with a fully functional 4-20mA current loop transmission detection solution.
The F2000 precision DC current source has an extremely low temperature coefficient
Dexinmag precision DC current source F2000 provides users with an extremely low output current temperature coefficient through temperature compensation on the premise of maintaining low cost. In the rated ambient temperature range of 15-35°C, when the full-scale output is 50mA, the F2000 can provide an average temperature coefficient (box-shaped measurement) lower than 5ppm/°C, and the full-scale output current is 50mA when the temperature changes by 1°C. The average change is at most 0.0005%, that is, the change value does not exceed 0.25uA. Since zero current also changes slightly with temperature, the overall equivalent temperature coefficient may increase slightly at lower output currents.
For a precision current source, low temperature coefficient is an extremely important feature, otherwise, no matter how it is calibrated, the output current will have a slow drift error due to temperature changes, and the error is so large that the output current cannot reach the nominal rated accuracy.
Each F2000 has been calibrated for temperature compensation before leaving the factory, and then the accuracy is calibrated. The temperature compensation calibration is carried out in a high and low temperature simulation environment where the temperature changes slowly, to ensure that the output current shows a flat temperature change relationship within the temperature range of 15-35°C, so as to ensure the validity of the subsequent accuracy calibration.
At the same time, the extremely low temperature coefficient makes the full-scale output current of the F2000 have a high stability of 0.01%, which effectively improves the reliability of the measurement and improves the repeatability of the measurement in long-term applications.
Like all precision instruments, F2000 must be fully preheated before official use to ensure the lowest temperature coefficient, best accuracy and highest stability. Usually the preheating time is not less than 5 minutes, and the recommended optimal preheating time should be more than 30 minutes.
Application of F2000 Precision DC Current Source in Low Resistance Measurement
The biggest problem with low resistance measurements is the connectivity between the test fixture and the physical resistance under test. The general application of the four-wire connection method in low resistance measurement eliminates the contribution of the lead resistance and the contact resistance between the fixture and the resistance to be measured to the measurement error in principle.
Even so, however, the four-wire connection still cannot solve the ubiquitous problem of contact thermal EMF. The contact term electromotive force exists between conductors of two different materials in contact with each other, and becomes more and more significant as the temperature difference between them increases. The contact thermal electromotive force is related to the materials of the two conductors in contact with each other, and usually has a temperature coefficient of the order of 1uV/°C, that is, for every 1°C increase in temperature difference, the contact thermal electromotive force increases by about 1uV.
Low resistance measurement belongs to low voltage measurement, usually in the order of 1mV or 10mV, so it has high sensitivity to contact thermal electromotive force. Unless you wait for a long enough time under constant temperature conditions to reach the same temperature on both sides of the contact point between the fixture and the resistance under test, the contact thermal electromotive force will inevitably show significant errors and affect the measurement accuracy. On the other hand, the oxidation of the contact surface will change the material properties on both sides of the contact surface, and cause the temperature coefficient of the contact term electromotive force to increase by an order of magnitude. measurement accuracy.
The contact term electromotive force can be regarded as a voltage source existing at the contact interface, which is connected in series between the materials on both sides of the contact surface. One of its important characteristics is that the polarity of this voltage source is only determined by the material of the conductors on both sides of the contact surface, and has nothing to do with the direction of the current flowing through it, and when the temperature difference remains constant, its magnitude remains constant.
When the four-wire connection is adopted, the contact thermal electromotive force exists at the contact point between the two voltage measurement lines and the measured resistance, which can be equivalent to two voltage sources connected in series with the voltage at both ends of the measured resistance. When it is consistent with the polarity of the voltage across the measured resistance, it will show a positive error in the measurement result, otherwise it will show a negative error.
If the contact temperature difference at the time of positive and negative error measurement is equal, the error caused by the contact thermal electromotive force can be eliminated by subtracting the measurement results of two applied currents in opposite directions. In this process, the two most critical conditions are the same temperature difference, and the applied currents are exactly equal in magnitude and opposite in direction. For the former, since the temperature is a relatively slow-changing physical quantity, the best way to ensure that the temperature difference remains unchanged is fast, and the fixture and the measured resistance cannot be operated, otherwise the heat carried by the hand will destroy the initial condition of the temperature difference. For current conditions, if there is a difference in the magnitude of the two applied currents, this difference will appear as an error in the measurement result.
Dexinmag's F2000 Precision DC Current Source is designed to perform manual measurements for this process. F2000 can output more than 50mA DC current, can generate 5mV DC voltage on 100mOhm resistance, and the accuracy can reach 0.05%, so as to improve the reliability of measurement. In addition, F2000 provides positive and negative output currents in both directions through internal relay switching. This design produces two currents of opposite polarity that are exactly the same magnitude compared to the bipolar scheme, which tends to produce measurable errors. The switching process of the current direction is controlled by the internal circuit of F2000 within 2 seconds, so as to ensure the smallest possible temperature difference change, and the switching of the current direction does not require the operator to directly contact the fixture and the measured resistance. By measuring the two voltages with a precision voltmeter and subtracting the results, most of the influence of the contact thermal electromotive force can be eliminated, and by dividing the result by 2, the accurate resistance value of the measured resistance can be obtained.
Another advantage of using the F2000 is that during the current polarity switching process, the F2000 will keep the output terminal in a high-impedance state until the switching is completed. This feature can effectively avoid the instantaneous high voltage generated by the short-term open circuit of the output terminal during the switching process. The impact of the resistance to be measured at all times. In many self-made current direction switching devices, especially when switching manually, this instantaneous high voltage is usually the main and hidden cause of damage to the measured resistance.
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