possible tempco with KTY81
Rene Schmitz
uzs159 at uni-bonn.de
Thu Nov 18 01:57:57 CET 1999
Hi all!
At 07:58 17.11.99 -0700, Ian Fritz wrote:
>UNLESS R(T0)=C1*T0, which I believe is pretty close for the wirewound
>Q81. (Terry Michaels showed some results from a Q81 spec sheet once, but
>I don't have them any more.) Different kinds of tempco's would have to
>be checked separately.
I think this is correct.
I've done some calculations comparing an ideal tempco resistor and
the KTY81 + series 1350 Ohm. I've chosen the Ohms/K factor for the wirewound
so that both devices have equal resistance at 25 deg.
T R1(KTY+1.35k) Relative error in percent
R2=C*T
-55 1825 1.712,13 -4,8236
-50 1850 1.751,37 -4,2150
-40 1902 1.829,85 -3,0832
-30 1959 1.908,34 -2,1650
-20 2019 1.986,82 -1,3751
-10 2083 2.065,31 -0,7562
0 2152 2.143,79 -0,3509
10 2224 2.222,27 -0,0738
20 2300 2.300,76 0,0324
25 2340 2.340,00 0,0000
30 2379 2.379,24 0,0103
40 2458 2.457,73 -0,0117
50 2542 2.536,21 -0,2474
60 2628 2.614,69 -0,5686
70 2719 2.693,18 -1,1035
80 2812 2.771,66 -1,7238
90 2909 2.850,15 -2,5151
100 3009 2.928,63 -3,4346
110 3112 3.007,11 -4,4823
120 3217 3.085,60 -5,6155
125 3269 3.124,84 -6,1607
130 3320 3.164,08 -6,6632
140 3415 3.242,57 -7,3690
150 3495 3.321,05 -7,4338
C was chosen to be 2340 ohms/298.15 K (as for a 2.34K TC)
The value for R1 is the value for typical R of the KTY81-1xx from the
datasheet plus a series resistor of 1340 ohms.
The relative error calculates to ((R2-R1)/2340)*100
Now what we can see here is that the KTY+R combination behaves pretty much
like the wirewound within resonable limits. (-10..60 degrees, less than 1%
error.) This shows that the KTY arrangement will pretty much work like the
wirewound, within that range. Assumed you never leave that range it will
behave like a wirewound resistor.
I think if you'd take your measurements only within that range and you'd
fit a line into the values it will also go thru the origin of your R vs. T
graph. It is not required that the values outside the ususal range fit on
that straight line.
I think the -10..60 degrees range is even more than we actually require.
The 1% error
is 1% error of the total compensation (i.e. 33ppm!). This means the drift
will be compensated to >99% within that range. So we get >99% of the
performance for 10% of the price of a Pt resistor!
That remaining error probably gets swamped by other factors, like opamp
drift, capacitor drift, mismatch of temperatures (which also applies to
Q81!)...
What do you think?
Bye
René
opinionsexpressedher | uzs159 at uni-bonn.de
einarethatofmyemploy | http://www.uni-bonn.de/~uzs159
erandnotofmyself.... | http://members.xoom.com/Rene_Schmitz
More information about the Synth-diy
mailing list