Accuracy of Resistor Modules

Pickering Interfaces resistor modules are used in a great many different applications, some requiring great precision to emulate accurate resistors while others only require a relatively low degree of precision.

More recently introduced resistor modules have very precise descriptions of the accuracy of the resistor that is being emulated while older modules are specified in different ways. This page explains the differences and how they arise.

Precision Resistor Modules

40-260 Series Precision Resistor Range

Diagram showing the flow of resistor channels to the calibration mux

The Precision Resistor Range, at this time consisting of the 40-260, 40-261, 40-262 and 40-265 offer users the very best precision in emulating a resistor in a system.

Each channel is fully characterised for its resistance accuracy at fixed calibration points. Each channel includes both a coarser setting mechanism and a fine setting mechanism which is guaranteed to provide overlapping setting possibilities. Using the combination of the coarse and fine settings the module guarantees that ANY resistance value can be achieved within the operating resistance range to the resolution of the fine control. The calibration system then ensures that both the fine and the coarse controls are fully characterised, the characteristics being stored in a calibration file on the module - ensuring that the driver always has access to the file relevant to all the channels on that module. The characterisation of the channels is supported by the use of a Calibration Port. The driver then accepts a resistance call and uses an algorithm to find the setting it needs to use (coarse and fine) to produce the requested resistance. The method used is patent protected.

The accuracy of the resistance these modules provide is stated explicitly on the data sheet in terms of a % error ± an absolute resistance. The accuracy achievable is limited by a number factors:

  • The accuracy of the measurements used in generating the calibration file

  • Temperature coefficient of the resistor channel - mainly influenced by the temperature coefficient of the copper tracks at lower resistance values (temperature coefficient of copper is approximately 0.4% per C)
    • the highest stability resistors are used on 40-26x series

  • Stability of the relays used to switch resistance

  • How well the calibration model translates to actual resistance values

  • Test equipment uncertainty

  • Resolution setting of the fine control

The architecture of the Precision Resistor Range ensures the very best performance can be achieved from this range.

40-297 and 50-297 Precision Resistor Modules

Diagram of a binary resistor chain on 40-290 series 

The 40-297 uses a traditional "binary" resistor chain where the usual factors of two between resistor values is replaced by a factor of a little less than a factor of two. This guarantees that at higher resistance settings resistor tolerances do not allow gaps to appear in the range of the module's cover, for example when the chain changes from 0111111 to 10000000 (the mid point on a 8 bit resistor chain) the first 7 resistors being all biased to low values and the 8th bit being biased high would produce a gap. The 40-297 avoids this issue and ensures there are no gaps in cover greater than the stated resolution.

As with the 40-260 series the module has a calibration table which characterises the resistors, relays and the PCB tracks. The driver uses this calibration table to calculate the nearest value of resistance that the user can obtain to a requested value (in Ohms). Since there is no fine resistance control this resistance value is unlikely to be as close to the requested value as on 40-260 series

The 40-297 has the simplicity of a binary resistor chain but with much improved accuracy and no missing settings created by the resistor tolerances. 

40-293 and 50-293

These products use a similar system to 40-297 but use a binary sequence and are fitted with resistors with reduced accuracy and temperature stability to suit cost sensitive applications. An estimated value of actual resistance can be read back based on the stored calibration information in the module.

Programmable Resistor Modules

This range includes the 40-280, 40-290, 40-294 and 40-295 series of PXI modules and PCI equivalents where available.

The data sheet for these modules specifies the accuracy of the resistor fitted and the typical residual path resistance of the resistor channel with all relays closed - it does not specify the overall accuracy of the channels (although for higher values the resistor accuracy will be the dominant factor).

The range uses reed relays as the switching element to ensure a long mechanical service life and faster response time than the EMR's used in 40-297 and 40-260 series. Resistance is varied by controlling the relays directly - the modules do not respond to resistance requests in ohms.

Reed relay based designs have higher levels of thermoelectric EMF than carefully designed EMR models, in some applications this effect can impact the accuracy of the resistance set as measured by the measuring circuits used. There is more on this issue here.

The residual path resistance is a combination of two primary contributions, the PCB track resistance and the reed relay contact resistance which can vary typically between 80 and 130 mohm (but is stable with time for a given reed relay). As the number of relays in circuit changes so does the residual path resistance (excluding the resistors fitted), the variation in the residual resistance being dependent on the the number of relays and the track lengths on the PCB. At the major changes of the relay patterns (for example 01111111 to 10000000) this can lead to significant changes in resistance compared to the expected value because of the changing residual resistance.

These modules are an excellent solution for applications when precision is not as important as speed of operation. As noted in Selecting a Resistor Module, the 40-297 is a cost effective solution that provides more traceable resistance results but with slower operation time.

The 40-294 and 50-294 (PCI) provide an alternative reed relay based solution for lower density applications. These modules are fitted with less precise resistors and are binary based, so at changeover positions gaps may appear in cover due to resistor tolerance.

External Wire Resistance

When the resistance of any of the above modules is measured it is taken across the module terminals (usually by a 4 wire resistance measurement). It does not include the external resistance of any connecting wires the user may have in the system. For precision applications the user needs to account o the external wiring resistance and its temperature coefficient (approximately 0.39% per °C for copper).

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