Hardware circuit timing calculation method and application examples

The measurement error is caused by the accuracy of the test system and unexpected transient processes during the signal and measurement of the OLED. When performing rapid production tests, the ability to perform DC measurements in a steady state is constrained by the need to complete the test as quickly as possible. The length of the test cycle is composed of source/measurement and switching operations, and this cycle time can have a very large range of variation. For example, if the 2400 is set to use a short test time interval (aperture) to complete the operation, that is, 0.01 NPLC, then the source/measurement process can be completed in 1ms. If you increase the integration period or measurement time to 1.0 NPLC, the measurement time will increase to about 17ms. The advantage of increasing the test time interval at the expense of test speed is that you can get excellent noise suppression, that is, in comparison. Test in a quiet" state.
In order to obtain stable and repeatable measurements, the key point is that the measured parameter reaches and maintains a stable value during the source/measurement period. This concept is particularly important for OLED testing. The electrical and optical characteristics of OLEDs are time-dependent, and exhibit a hysteresis effect1,2. Compared with the more familiar semiconductor-based photoelectric emitters, the electrical characteristics of OLEDs are very different. For this reason, we must have a complete understanding of the transient behavior of test parameters before attempting to design and implement an automated test system. The characteristics of the transient process also contribute to the development of test protocols, simplify the analysis of test data, and increase the credibility of the test system. The signal source delay time, that is, the variable time delay from when the signal is applied to the OLED to the start of the measurement, may be used to reduce transient effects. Figure 1 shows the leakage current of each pixel when the test system is set to NPLC = 10 and the signal source delay is changed from 0.0005 to 10 seconds. In order to achieve a steady-state leakage current of less than 1nA, it takes at least a few seconds.

Figure 1. The reverse bias current of each pixel in the four-pixel test, where the source/measurement time delay varies from 0.0005 to 10 seconds, and a 6V bias voltage is used.
The test performance of the test system depends on the basic accuracy of the test instrument and the source of error caused by other components in the system. Leakage currents from cables and switch cards are a source of error in current measurement. For the test fixture and cable connection, this error will increase as the measured current decreases. Select the correct scan card, that is, the rated leakage current of the scan card should be at least an order of magnitude lower than the measured current. This indicator is very critical. For a test system designed to perform 10-8A measurements with the 2400, no protection circuit is required.
The voltage measurement error of the two-wire sensing structure is caused by the "on" resistance of the relay used on the scan card and the resistance voltage drop loss of the cable. The combination of the two relays on the 7015-C card will produce a resistance of <300Ω to the signal path. For the current measurement of less than 50μA, the voltage error including the typical resistance voltage drop loss will be very small, and its typical value is <15μV. For larger current measurements, such as when a whole column of the display is activated, the error will be proportional to the current of the OLED. This value may be calculated with Verror = 2 * (Rrelay)×IOLED(s). Those applications that require very high voltage measurement accuracy, that is, voltage measurement that is not affected by the DUT current, require a four-wire test structure.
The mechanical relay on the 7158 and 7058 scan cards has a contact resistance of approximately equal to or less than 1Ω, and the voltage error caused by this is negligible, even at high currents. For this application, the error caused by the contact potential of the following scan cards may also be regarded as negligible; the contact potential of these scan cards is: 7015-C is <5mV, 7058 and 7158 are respectively <250μV and <200μV.