April 12, 2010

Vision Version 4.4.1 is now available for download from Radiant's web site at www.ferrodevices.com. There are two major changes to Vision 4.4.1. The first is the new ability for Vision to create block diagrams of the Test Definitions you create for use in your documentation. The second is the ability of Vision to measure and cancel small voltage offsets originating from the DRIVE output of our testers. The block diagrams were described in the previous issue of "Something New". The offset cancellation is discussed below.

DRIVE Offset Correction :

As each Precision tester is assembled at Radiant, we put it through a long series of tests to calibrate the output voltage and the input measurements. As part of that calibration, the DRIVE output voltage at zero volts is measured with a NIST-traceable volt meter. Potentiometers in the DRIVE circuit are adjusted to force the DRIVE output to a value less than a millivolt from zero volts as determined by the volt meter. For most of the tester models available from Radiant, this calibration remains unchanged for the life of the tester. However, several of the tester models, in particular the RT66B and the Premier II, change the configuration of the DRIVE output circuitry duing each test to optimize performance based on the sample being tested. The single potentiometer setting is not enough in this situation to maintain the low output offset voltage calibration for all DRIVE circuitry configurations. The result is that for some test situations, the DRIVE output may develop an offset voltage when zero volts is assigned. Usually, the offset voltage is less than 23 millivolts, a value which should not affect a thin ferroelectric film capacitor. The situation is completely different when the tester is driving an HVA for high voltage measurements. For a 10kV amplifier with an amplification factor of x1000, the 23mV becomes 23 volts!

To correct for this situation, a self-calibration algorithm has been added to Vision and the RTS_USB_DRIVER.dll driver. When Vision starts or when the ALT-W function is activated to re-load an attached tester, the driver will cycle through all DRIVE circuitry output configurations and measure any voltage offset from zero present on the DRIVE output. After the start-up calibration period is complete, Vision will automatically offset any test or stress waveform with the appropriate self-calibration value to eliminate any unintended bias voltage originating from the DRIVE output. The DRIVE offset compensation will always be active on start-up but may be turned OFF by the user from the TOOLS menu.

Important Points:

1. The DRIVE offset compensation function will not add a correction larger than 100mV. Instead,if the correction gets that large, Vision will report a hardware error to the user. An offset that large indicates a problem with the tester and it should be returned to Radiant for evaluation.

2. The accuracy of the offset compensation will be determined by the resolution of the DRIVE circuitry of the tester being used. Ideally, the compensation should be within 1 resolution bit for that tester but measurement noise during the calibration period will extend that limit to 2 bits. The maximum offsets that should for each tester with the DRIVE offset compensation enabled are given below

Premier II =>600uV

FH & FH100 => 2.44mV

LC, SC, RT66B, &RT66I => 9.77mV

3. The original Precision Premier, Precision Workstation, and EDU do not have the DRIVE offset compensation capability.

4. Normally, Vision will inform the user if a voltage too large for the tester configuration is requested by the user. Since the DRIVE offset compensation function adds a small value to the user requested voltages, a situation may arise when the user requests a Vmax very close to the voltage limit of the tester that the additional compensation voltage may put the Vmax over the limit. If this happens, Vision will give the user a warning about the situation. The user may either reduce the requested Vmax slightly or turn off the DRIVE offset compensation.

If you experience any problems with the DRIVE offset compensation function, please contact Radiant Technologies, Inc.

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