Temperature Stability of Resistor Modules
We are occasionally asked about the temperature stability of our wide range of both PXI & PCI resistor modules. The temperature stability is dependent on a number of factors, no simple statement can give a complete answer so this page is intended to provide some useful guidance.
What determines temperature stability?
There are a number of contributing factors to temperature stability of resistance as measured by the voltage drop across the resistor when excited by a constant current source (the most common scenario, including use of a DMM). The resistors are made from a chain of devices (including relays, discrete resistors and sometimes digital potentiometers) and copper tracks interconnecting them. Typical structures for resistor modules can be found Accuracy of Resistor Modules.
The principle contributing factors are:
- Stability of the resistors used in the resistor chain
- The PCB tracks that interconnect the devices in the chain
- Stability of contact resistance of relays
- Stability of any digital potentiometers used in the design
- Thermoelectric effects
The specification for the Pickering Interfaces resistor call based modules includes an allowance for temperature stability of a temperature variation of +/-10C from a 21C ambient.
Stability of Resistors
Our range of resistor modules is graded by their accuracy targets, the most accurate being our Precision Resistor range; the 40-260 series (for example 40-262 and 40-265), then the 40-297 family and the least accurate being the 40-295, 40-293, 40-294. The better the accuracy target the more stable the resistors that are fitted. So a short summary would be:
- 40-260 series use the highest stability resistors throughout their design with the resistors being chosen to provide temperature coefficients of 15ppm/K or less.
- 40-297 series use high stability resistors for values above about 60 Ohms and less than about 700kOhms. Above 700kOhms the resistors are 50ppm/K. Below 60 Ohms the resistors are dominated by PCB track effects.
- 40-295, 40-293, 40-294 us resistors of 50ppm/K or higher throughout their design
At very high values resistors can also be effected by leakage resistance on the PCB which tends to be both humidity and temperature dependent. This effect is generally only seen above 1MOhm.
PCB tracks that interconnect the resistors are made primarily of copper which has a temperature coefficient of approximately +3900 ppm/K, although this seems a high number the copper resistance is a small proportion of the total resistance and only has an impact at lower resistance settings. If a particular resistor module has a track resistance of 1 Ohm then for a set resistance of 100 Ohms the track contributes +39ppm/K, at 1kOhm it contributes just +3.9ppm/K. The 40-260 series modules tend to have the lowest track resistance as they are designed for precision rather than density.
It should also be noted that many switching products (matrices, multiplexers) track resistance is also a dominant factor in their path resistance when the relays are new, as a result path resistance on switching systems also tends to have a temperature coefficient similar to copper.
Stability of contact resistance of relays
The lowest contact resistance is for designs that use EMR's, the resistance of reed relays tends to be higher and the materials used have more temperature dependency. For EMR designs there is no great impact from the relays on the overall temperature coefficient beyond that expected from the equivalent copper resistance.
Stability of Digital Potentiometers
Digital potentiometers are fabricated using FET's and resistor chains and are used on designs such as 40-262 and 40-265. These have their own temperature coefficients. However the design of the product ensures they are only used for fine control of resistance and as a result they contribute very little to temperature coefficient of the complete resistor module.
The impact of thermoelectric EMF's on resistance measurements is described in Importance of Thermoelectric EMF ins resistor modules and Thermo Electric EMF in relays. Designs based on EMR's use an arrangement which is designed to minimise thermoelectric EMF. In most cases thermoelectric EMF has minimal impact on measured temperature stability of Pickering Interfaces resistor modules.
Reed relays tend to have higher levels of thermal EMF and this can create similar time varying effects on measured resistance because the operation of the coils change the local temperatures. It is impossible to quantify the impact of this on a measurement without knowledge of the measurement conditions, and the effect is time dependent.
The temperature stability of the resistor modules is dependent on the module for a variety of reasons. The best stability is obtained using the 40-260 series resistor modules. For modules like 40-297 the stability will be dependent on the resistance setting, being worse at low setting because of PCB (copper) connections and at very high resistances because of the resistors used and the impact of board leakage.