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Thursday, 16 February 2012

Opamp Circuit: Gain Setting Resistor Type and Package Consideration


Opamp: Gain Setting Resistor Type and Package Consideration


What is the resistor type to use?

In a nutshell:

1.       You would need to know the requirement of your circuit
2.       You need to know what’s available and what’s not in terms of selecting resistor
3.       You need to know how real life resistor deviate from text book

Requirement of circuit:

Ok, to answer this seemingly simple question, we need to know a few aspect of the circuit function
1.       Power rating required?
2.       Accuracy required?
3.       Cost allowed?
4.       Availability?

 

Basic knowledge:

1.       Then you would need to know what’s available in the market, of course the best place to get to know this is through wiki: http://en.wikipedia.org/wiki/Resistor
2.       Some good info on Resistor 101 (www.vishay.com/docs/49873/49873.pdf)
3.       It is extremely rewarding if you understand pros and cons of basic resistor types, as they allow you to make your selection almost immediately in future. Resistor is basic ingredient in most circuit, so your time invested is going to have a pretty good ROI (return of investment J)
4.       Continue from point #2, below are some basic type and common expectation that I have on them (off my mind)
a.       Wirewound
                                                               i.      Expensive – as it takes effort to wind those wire, and just imagine how long the wire to use to get at 1MOhm (to state the obvious wire is always in mOhm range)
                                                             ii.      High inductance – hey, wound wire is almost like a inductor, so if the circuit is of high frequency nature, I would avoid it
                                                            iii.      Precision – not much drift
b.      Thin film
                                                               i.      Accurate
                                                             ii.      Low inductance – no winding of conductor
                                                            iii.      Low noise
                                                           iv.      Low Resistance – how much resistance can a piece of film has
c.       Thick film
                                                               i.      Wide range of value
                                                             ii.      Cannot be as accurate as thin film
d.      Carbon
                                                               i.      Easily available
                                                             ii.      Noisy – compare to other resistor types, the question is how relevant this noise is to application



Real life resistor

1.       Temperature coefficient:
a.       what is means is that the resistance value will change according to temperature, stated as ppm per unit temperate change,
b.      note that temperature of the resistor is in turn depend on the selfheat of the resistor and ambient temperature change. Which means the better the temperature coefficient is, the better the accuracy is (more expensive as well)
c.       The temp coef is readily available in the datasheet, so you are generally know what you are getting

2.       Breakdown voltage
a.       If you have a 0.1Watt 10MOhm resistor, you would expect that the voltage level can go up to P = V^2/R = sqrt (P*R) = sqrt (1M) = 1kV
b.      But the fact is that all resistor will eventually breakdown above certain voltage, and for a typical part, 1kV certainly is too much to bear with. Use of common sense tells you that all 0805, 0603, 0402 has too little clearance between terminals and should not be able to handle it kV – either resistor breakdown or arcing will occur.
c.       This value is generally available in the datasheet, ensure no violation on this spec.



3.       Voltage coefficient
a.       One of the most likely mistake in choosing resistor is that fresh engineer overlook the fact that resistance changes when voltage across them changes. If you are using a voltage divider, this will give you some serious error depend of the resistor type
b.      Resistor that meant for high voltage (above 100V I think) will generally have this spec in the datasheet. If you can’t find this spec, and you intend to use the resistor for 100V or more, good luck.

4.       Power dissipation
a.       Given enough power, you can raise temperature of resistor into > 100degC, as long as that does not go beyond resistor’s rating you are fine, but your resistance will change like crazy
b.      Given too much power, you can burn the resistor, literally.
c.       So always size up the resistor size for the expected power level.
d.      To give an example: a 0.1W 0603 resistor will have power derating curve as below: to get the sense of how much temperate rises for a given operating condition, let’s calculate the thermal impedance of this part.
                                                               i.      Rated power = 0.1Watt
                                                             ii.      Tmax = 155degC (refer to any resistor datasheet for this, in 0603, the curve is labelled as 3E in this particular example)
                                                            iii.      TderatingStart = 70degC
                                                           iv.      Rthermal = (TmaxTderatingStart)/Prated = (155-70)/0.1 = 85degC per Watt






Application of knowledge:

Now that you know about what you can choose from, non-ideal characteristic of resistor and the requirement from your circuit, let’s go through some of it
1.       Power rating required?
a.       Calculate worst case power dissipation
b.      Ensure factor in the ambient temperature of end product
                                                               i.      If your product is going to be used in outdoor of Africa, then be prepare for the maximum ambient temperate
2.       Accuracy required?
a.       If your circuit has calibration, then initial tolerance is less critical, only drift and calibration period matters.
b.      Remember temperature changes of the resistor due to power dissipation or environmental temperature will change the accuracy. For example, a 100ppm part with negligible power dissipation but having 10degC operating temperature changes will have 1000ppm of changes, which is 0.1% of error.
c.       Consider to mount the gain setting resistor close by, such as both will see the same temperature changes. By doing so, both resistor values will drift together, and since the gain is depend on the ratio of the two, total error will be much less. Remember certain part of the PCB will always be hotter than the rest, as high power devices always heat up the region around it.
3.       Cost allowed?
a.       Know how much you are supposed to spend
4.       Availability?
a.       It is useless if you cannot get the resistor in time , or you can’t buy them
b.      If your design cycle is short, always pick those parts that distributor has plenty of stock. Some part lead time can be as long as 16weeks – 4months!!
5.       Others
a.       If you are going to handle the assembly, pick resistor size that is manageable. Personally, it is extremely tough to handle 0402 part
b.      For prototyping or hand build board, 0603 part is extremely useful, as the standard prototyping breadboard will have holes just about 60mils, so you can solder them neatly and tightly.

 

References

www.vishay.com/docs/49873/49873.pdf

1 comment:

  1. Nice blog… Thanks for sharing very useful information about electrical circuits.
    Learn Electronic Circuits

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