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菂 黏崊* 墔 菂 C:\MSOFFICE\WINWORD\NORMAL.DOT 6 Practical Limitations of 2400 SourceMeter 6-Wire Ohms Gateway Authorized Customer Jonathan L. Tucker @ 0Y娀s @ Q蠅 @ `SQ藧 @ luA Microsoft Word 6.0 21 44131
A discussion of the error sources in making 4/6-Wire ohms measurements on resistor networks with the 2400 and methods of堀e = $ e $ 0 ! , j , j j, j, j, j, j, , , J , , , ,
- , / 1 - - - - - - - - - - - - - ~. R/ / T .0 v p/ 9 j, - - - - - p/ - j, j, - - - - - - j, - j, - - ~, , " j, j, j, j, - - - p - 2400 SourceMeter: Maximizing Ohms Accuracy
Chuck Cimino - Project Manager - Keithley Instruments - 28775 Aurora Rd - Solon OH, 44131
A discussion of the error sources in making 4/6-Wire ohms measurements on resistor networks with the 2400 and methods of their reduction. RUT is the Resistor Under Test, Rshunt is the parallel resistor or resistance being guarded out, Rload is the resistive load on the guard buffer.
I. Guard Offset Voltage
In order to minimize error causing Rshunt currents, the voltage offset between output high or high sense and the guard output must be reduced to a minimum. The 2400抯 offset voltage is typically 100(which will result in a 100(V/Rshunt error current. The error current as a percentage of the test current will determine the minimum error contributed by the DC offset.
For example, with a test current of 1mA, a RUT of 100 ohms and a shunt resistance of 10(, the minimum measurement error will be 100(V/10( = 10(A/1mA*100 = 1%. This error is in addition to the ohms measurement accuracy of the 2400 which is nominally ~0.05% resulting in a total error of 1.05%. It抯 evident that the majority term is that of the guard offset in this situation. With a less demanding Rshunt value, i.e. a higher value, the error contribution will be reduced correspondingly.
To minimize the contribution of guard offset voltage to the total error in the measurement, one simple technique is to simply increase the test current. For every decade increase in test current, given fixed Rshunt values, the contribution of guard offset error will be reduced correspondingly. Referring to the above example, at 10mA test current, the contribution to total error by guard offset is reduced to 0.1% (0.15% total). At 100mA, the error contribution is further reduced to 0.01% (0.06% total). Careful selection of test currents and voltages can greatly influence guard offset contribution to total error. Good 6-Wire as well as general measurement practice dictates that as high a signal level as possible be used for the best measurement quality.
Another technique to reduce guard offset error contribution is to use a 2400 feature called Offset Compensated Ohms. This feature is available as a menu selection within ohms and as a programmable function for use outside oompensation function. The added benefits of OC ohms must be carefully weighed against the potential for increased noise due to interaction of additional amplifiers and noise sources. As the OC function will effectively reduce system offsets including those generated by the guard circuits, it will also increase reading uncertainty through increased noise as a direct function of reduced Rshunt value.
Measurement noise can be periodic or random, fixed frequency or time varying. The 2400抯 A/D converter will attenuate line related noise to minimal levels assuming the integration period is an integer of the line frequency. In production applications where throughput is critical, sub-line frequency integration is often used requiring further measures to reduce noise sources like shielding, averaging, etc. These tradeoffs are fairly well understood in typical measurement situations but have an additional element in 6-Wire applications.
When a guard buffer is connected to a source through a low value of resistance like 10 ohms Rshunt, the current source and guard voltage sources begin to interact. The current source with a very high theoretical output impedance is attempting to force a programmed value of current through a resistance by varying the voltage across it in response to an error signal derived from a current measurement. The guard buffer is attempting to control a voltage on the other end of Rshunt without regard to current flow through it. When Rshunt, Rload and the RUT resistance become small in value (Rshunt <100(, Rload/Vguard > 10mA, and RUT < 500() measurement noise will be noticeably increased at sub-line frequency integration rates. The noise generated is a combination of periodic and random noise elements. The periodic element is typically ambient or power supply noise. The random element is often referred to as 搈otor-boating or that due to sporadic changes in gain or amplifier noise contributions.
Excessive reading noise can be reduced through mathematical filtering at the expense of effective throughput which can be traded off against line cycle integration filtering and through-put. Also, each source and measure range has unique frequency compensation setpoints which can allow selective 搕uning and reduction of noise by selecting different ranges for ranges of Rshunt values to minimize reading noise.
The above discussions all presume that the 2400 is being used in I-source ohms mode. Changing to V-source mode operation has advantages in 6-Wire ohms with Rshunt values below 100( as well. This is again due to beneficial compensation and the fact that the guard buffer is connected to a voltage source and not a current source resulting in less effective noise. In V-source mode with RUT抯 of 350(, Rshunt of 10(, accuracies have been demonstrated in the 0.05% range for the 2400 absent any signal switching or substantial ambient noise sources.
III. Accuracy Enhancement Techniques
In addition to the above options, additional system level options exist to further improve measurement accuracies in a production test application. The first and most obvious is often referred to as 搒tandardization. Standardization, performed on a regular basis, often daily or even before each batch run, involves comparing or referencing the measuring instrument to a standard. The standard should be representative of the ideal device under test or DUT as possible. In the case of a resistor network, the configurations and values should be selected to be close to those of the DUT to be tested. By connecting the instrument to such standards, longer term errors can be effectively canceled enabling higher precision DUT抯 to be tested.
Standardization imposes an additional burden on the manufacturer in that the references must be maintained and measured frequently with appropriate metrology analysis applied. The good news is that once reasonably accurate and stable standards are constructed, a high precision DMM can usually characterize the standards to high enough precision to allow all error budgets to be satisfhms modes. When enabled, the OC ohms mode will cause the ohms source to toggle between zero and full scale values making a measurement at each point. A calculation to use the difference in voltage divided by the difference in current has the effect of canceling any offsets present, either external or internal to the instrument including guard offset voltage. The effective minimum guard offset voltage realized through this method is ~20(V across Rshunt due to guard buffer common mode voltage and settling errors. Using the above example, with a 1mA test current, 20(V would correspond to 0.2% guard induced error (0.25% total), at 10mA test current the guard error would be 0.02% (0.07% total), at 100mA guard error is 0.002% (0.052%). Obviously increasing the test current to 100mA does not substantially decrease total error in this example.
II. Noise
In any measurement system, noise will contribute some degree of uncertainty. The 2400 is no exception, particularly in 6-Wire applications using the offset cied. The DMM becomes the primary standard upon which all downstream accuracies are referenced. This DMM can be traced back to it抯 manufacturer and ultimately to a standards bureau for guaranteed calibration trace-ability.
The CALC-ulate subsystem of the 2400 can be used to store temporary calibration constants in the form of y=MX+b. It抯 possible to setup as many as 5 separate Calc system equations and call them up on a per range basis in fixed range manual or automatic ohms modes. The output of the Calc system, in effect the corrected measurements, can drive the limits subsystem and thus the binning outputs. These 5 Calc setups can be combined with Source-Memory capability to enable multiple setups to be called up and run if values or ranges are changed during network scanning operation. Enabling the Calc subsystem for a multiply and an addition will add less than 0.5 milliseconds to the total test time enabling high throughput with improved accuracy.
Summary
The 2400 baseline level of performance is suitable to a wide range of component test applications. The rich feature set offers many additional avenues to enhance speed and/or accuracy in standalone or in system applications.
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