Ask The Applications Engineer24 by Steve Guinta
RESISTANCE Q. I’d like to understand the differences between available resistor types and how to select the right one for a particular application. A. Sure, let’s talk first about the familiar “discrete” or axiallead type resistors we’re used to working with in the lab; then we’ll compare cost and performance tradeoffs of the discretes and thin or thickfilm networks. Axial Lead Types: The three most common types of axiallead resistors we’ll talk about are carbon composition, or carbon film, metal film and wirewound: • carbon compositionor carbon filmtype resistors are used in generalpurpose circuits where initial accuracy and stability with variations of temperature aren’t deemed critical. Typical applications include their use as a collector or emitter load, in transistor/FET biasing networks, as a discharge path for charged capacitors, and as pullup and/or pulldown elements in digital logic circuits. Carbontype resistors are assigned a series of standard values (Table 1) in a quasilogarithmic sequence, from 1 ohm to 22 megohms, with tolerances from 2% (carbon film) to 5% up to 20% (carbon composition). Power dissipation ratings range from 1/8 watt up to 2 watts. The 1/4watt and 1/2watt, 5% and 10% types tend to be the most popular. Carbontype resistors have a poor temperature coefficient (typically 5, 000 ppm/°C); so they are not well suited for precision applications requiring little resistance change over temperature, but they are inexpensiveas little as 3 cents [USD 0.03] each in 1, 000 quantities. Table 1 lists a decade (10:1 range) of standard resistance values for 2% and 5% tolerances, spaced 10% apart. The smaller subset in lightface denote the only values available with 10% or 20% tolerances; they are spaced 20% apart.
Carbontype resistors use colorcoded bands to identify the resistor’s ohmic value and tolerance:
• Metal film resistors are chosen for precision applications where initial accuracy, low temperature coefficient, and lower noise are required. Metal film resistors are generally composed of Nichrome, tin oxide or tantalum nitride, and are available in either a hermetically sealed or molded phenolic body. Typical applications include bridge circuits, RC oscillators and active filters. Initial accuracies range from 0.1 to 1.0 %, with temperature coefficients ranging between 10 and 100 ppm/°C. Standard values range from 10.0 ohms to 301 kohms in discrete increments of 2% (for 0.5% and 1% rated tolerances).
Metal film resistors use a 4 digit numbering sequence to identify the resistor value instead of the color band scheme used for carbon types: • Wirewound precision resistors are extremely accurate and stable (0.05%, <10 ppm/°C); they are used in demanding applications, such as tuning networks and precision attenuator circuits. Typical resistance values run from 0.1 ohms to 1.2 Mohms. High Frequency Effects: Unlike its “ideal” counterpart, a “real” resistor, like a real capacitor (Analog Dialogue 302), suffers from parasitics. (Actually, any twoterminal element may look like a resistor, capacitor, inductor, or damped resonant circuit, depending on the frequency it’s tested at.) Factors such as resistor base material and the ratio of length to crosssectional area determine the extent to which the parasitic L and C affect the constancy of a resistor’s effective dc resistance at high frequencies. Film type resistors generally have excellent highfrequency response; the best maintain their accuracy to about 100 MHz. Carbon types are useful to about 1 MHz. Wirewound resistors have the highest inductance, and hence the poorest frequency response. Even if they are noninductively wound, they tend to have high capacitance and are likely to be unsuitable for use above 50 kHz. Q. What about temperature effects? Should I always use resistors with the lowest temperature coefficients (TCRs)? A. Not necessarily. A lot depends on the application. For the single resistor shown here, measuring current in a loop, the current produces a voltage across the resistor equal to I x R. In this application, the absolute accuracy of resistance at any temperature would be critical to the accuracy of the current measurement, so a resistor with a very low TC would be used. A different example is the behavior of gainsetting resistors in a gainof100 op amp circuit, shown below. In this type of application, where gain accuracy depends on the ratio of resistances (a ratiometric configuration), resistance matching, and the tracking of the resistance temperature coefficients (TCRs), is more critical than absolute accuracy. Here are a couple of examples that make the point. 1. Assume both resistors have an actual TC of 100 ppm/°C (i.e., 0.01%/°C). The resistance following a temperature change, T, is
For a 10°C temperature rise, both R_{f} and R_{i} increase by 0.01%/°C x 10°C = 0.1%. Op amp gains are [to a very good approximation] 1 + R_{F}/R_{I}. Since both resistance values, though quite different (99:1), have increased by the same percentage, their ratiohence the gainis unchanged. Note that the gain accuracy depends just on the resistance ratio, independently of the absolute values. 2. Assume that R_{I} has a TC of 100 ppm/°C, but R_{F}’s TC is only 75 ppm/°C. For a 10°C change, R_{I} increases by 0.1% to 1.001 times its initial value, and R_{F} increases by 0.075% to 1.00075 times its initial value. The new value of gain is
For an ambient temperature change of 10°C, the amplifier circuit’s gain has decreased by 0.025% (equivalent to 1 LSB in a 12bit system). Another parameter that’s not often understood is the selfheating effect in a resistor. Q. What’s that? A. Selfheating causes a change in resistance because of the increase in temperature when the dissipated power increases. Most manufacturers’ data sheets will include a specification called “thermal resistance” or “thermal derating”, expressed in degrees C per watt (°C/W). For a 1/4watt resistor of typical size, the thermal resistance is about 125°C/W. Let’s apply this to the example of the above op amp circuit for fullscale input: Power dissipated by R_{I} isE^{2}/R = (100 mV)^{2}/100 ohms = 100 µW, leading to a temperature change of 100 µW x 125°C/W = 0.0125°C, and a negligible 1ppm resistance change (0.00012%). Power dissipated by R_{F} isE^{2}/R = (9.9 V)^{2}/9900 ohms = 9.9 mW, leading to a temperature change of 0.0099 W x 125°C/W = 1.24°C, and a resistance change of 0.0124%, which translates directly into a 0.012% gain change. Thermocouple Effects: Wirewound precision resistors have another problem. The junction of the resistance wire and the resistor lead forms a thermocouple which has a thermoelectric EMF of 42 µV/°C for the standard “Alloy 180”/Nichrome junction of an ordinary wirewound resistor. If a resistor is chosen with the [more expensive] copper/nichrome junction, the value is 2.5 µV/°C. (“Alloy 180” is the standard component lead alloy of 77% copper and 23% nickel.) Such thermocouple effects are unimportant in ac applications, and they cancel out when both ends of the resistor are at the same temperature; however if one end is warmer than the other, either because of the power being dissipated in the resistor, or its location with respect to heat sources, the net thermoelectric EMF will introduce an erroneous dc voltage into the circuit. With an ordinary wirewound resistor, a temperature differential of only 4°C will introduce a dc error of 168 µVwhich is greater than 1 LSB in a 10V/16bit system! This problem can be fixed by mounting wirewound resistors so as to insure that temperature differentials are minimized. This may be done by keeping both leads of equal length, to equalize thermal conduction through them, by insuring that any airflow (whether forced or natural convection) is normal to the resistor body, and by taking care that both ends of the resistor are at the same thermal distance (i.e., receive equal heat flow) from any heat source on the PC board. Q. What are the differences between “thinfilm” and “thickfilm” networks, and what are the advantages/disadvantages of using a resistor network over discrete parts? A. Besides the obvious advantage of taking up considerably less real estate, resistor networkswhether as a separate entity, or part of a monolithic ICoffer the advantages of high accuracy via laser trimming, tight TC matching, and good temperature tracking. Typical applications for discrete networks are in precision attenuators and gain setting stages. Thin film networks are also used in the design of monolithic (IC) and hybrid instrumentation amplifiers, and in CMOS D/A and A/D converters that employ an R2R Ladder network topology. Thick film resistors are the lowestcost typethey have fair matching (<0.1%), but poor TC performance (<100 ppm/°C) and tracking (<10 ppm/°C).They are produced by screening or electroplating the resistive element onto a substrate material, such as glass or ceramic. Thin film networks are moderately priced and offer good matching (0.01%), plus good TC (<100 ppm/°C) and tracking (<10 ppm/°C). All are laser trimmable. Thin film networks are manufactured using vapor deposition. Tables 4 compares the advantages/disadvantages of a thick film and several types of thinfilm resistor networks. Table 5 compares substrate materials.
In the example of the IC instrumentation amplifier shown below, tight matching between resistors R1R1', R2R2', R3R3' insures high commonmode rejection (as much as 120 dB, dc to 60 Hz). While it is possible to achieve higher commonmode rejection using discrete op amps and resistors, the arduous task of matching the resistor elements is undesirable in a production environment. Matching, rather than absolute accuracy, is also important in R2R ladder networks (including the feedback resistor) of the type used in CMOS D/A converters. To achieve nbit performance, the resistors have to be matched to within 1/2^{n}, which is easily achieved through laser trimming. Absolute accuracy error, however, can be as much as ±20%. Shown here is a typical R2R ladder network used in a CMOS digital analog converter.
