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 = (Tmax – TderatingStart)/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
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