Here is our first “Ask the experts” tech brief in advance of our Resistor chat on Wednesday, August 20 at 1:00 p.m. ET (10:00 a.m. PT). I hope this stirs up some questions/comments for our chat.
This is what our expert Edwin G. Pettis says:
In general, I am comparing leaded versions of both types described below; SMT types tend to have noticeably differing specifications compared to their leaded forms.
Traditionally, precision wire wound resistors (PWW) have been used in DC to relatively low frequency applications due to parasitics and film/foil have been used in DC to relatively high frequency circuits due to their low parasitics. To some degree, the PWW parasitics are overstated; the capacitance tends to be quite small and the inductance parasitic, having a very low Q, often below 1 except for low ohmic values. The film/foil resistors have very low parasitics due to their small compact design. PWW resistors potentially have the lowest noise figure approaching closely the theoretical limits, foils are slightly noisier due to their design and have “shot” noise as well, and PWWs do not. PWWs also can have superior stability and reliability over film and foils. In general, TCR in both types are very similar, foils only exceed PWWs TCR capability in hermetic cans which are quite expensive. For the best PWW resistors, hermetic sealing has less pronounced effects although it improves the performance of inferior PWWs.
While similar size PWWs are available to match the standard film/foil square form (1/4″ square), the film and particularly foil types out perform their similar PWWs in parasitics and frequency performance, other characteristics between the two types tends to be better matched. The parasitics of the PWW form suffers from having only one pi, therefore there is no cancellation effects of the parasitics. In pricing, PWWs tend to be competitive and in some cases, particularly in high performance areas such as TCR, stability and reliability can provide similar characteristics to the hermetic types at a lower price point. In the area of sub- 1 PPM/o C TCR characteristics, a lot depends on the actual requirements of the specification and PWWs can be competitive in this area as well as at least one manufacturer can provide sub- 1 PPM/o C C TCR without hermetic sealing. Both types of resistors have made considerable advances in performance over the years and some PWW resistors have made significant advances to challenge foils in many characteristics.