SYNTHESIS TECHNOLOGY
PAGE 16
MOTM-510 ASSEMBLY 7/11/04
WWW.SYNTHTECH.COM
ln(4) = 1.38629
ln(4) * (3) = 4.15888
ln
-1
(4.15888) = 64 <<<taa daa!!!
Of course, step #2 above (the multiply) is done inside the AD538 by adding logarithms (as
before). Personally, I think this is pretty COOL STUFF, being able to multiply, divide AND
raise to powers using only adding circuits. And even cooler, we are able to this is real time,
using voltages as the ‘numbers’ in our little analog computer.
What’s the difference between a
root
and a
power
?
In this context, they are the same, but the implication is that a root is <1 (the other use of the
math term ‘root’ means the number that makes an equation equal to zero).
So, a ‘square root’, which is usually written like this:
√
x
Can also be written as: x
.5
The downside of the AD538 (well, a downside for use MOTM users) is that the exponential
factor is NOT a control voltage. Rather, it is designed to be an external trim pot. This is
because the ‘normal’ usage of an AD538 is to generate a specific equation that is application-
dependent. In these circuits, the exponent is a constant that is trimmed. However, the MOTM-
510 ‘abuses’ the AD538: we replace the fixed trim pot with a rotary switch and a front panel
control, so that you can tweak the exponent in real-time.
So, what was the original application(s) of the AD538 (before we started abusing it with audio)?
One widely used application was electric thermometers.
Many of you have heard the term
thermocouple
. A thermocouple is a temperature probe (there
is one in your oven) made by twisting 2 different metal wires together. This makes a small
(very small!) battery whose voltage output is based on the temperature of the ‘junction’ of the 2
metal wires. Also, you can take certain alloys of metal (usually platinum) that change
resistance to temperature, according to something called the Stainhart-Hart equation:
1/T = a + b + (lnR) + c(lnR)3
where a, b, and c are ‘fudge-factors’ based on the wire used, R is the resistance and T is the
temperature. Note that this “nasty” equation can be ‘solved’ with the AD538 (the ‘ln(R) part
and the c(ln(R)
3
) part as well). Why is THIS a big deal? Because….this chip was designed in the
pre-microprocessor, pre-ADC world, and the temperature readout was a linear analog meter.
Since the meter has a linear scale, we want a linear relationship between temperature (what
we are measuring) and voltage driving the meter (how we are displaying the temperature).
Think about it for a minute: if I came to you and said: “I need a circuit that uses this equation
to drive a meter over a 0 to +5V scale so that 1V = 100F, 2V = 200F, etc” how would you do it?
Answer: AD538! The details are left to the user, but this part was widely used for this
application. The other application was to “warp” the voltage in radar CRT terminals so that
specific shapes (square, triangles, etc) ‘drawn’ on FAA radar screens (which are NOT flat CRTs,
Summary of Contents for MOTM-510 The WaveWarper
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