[sdiy] OTA woes

[Ian Fritz]

Hi group --

Just in the past week I tried making a VCO core using a 13600 as a switched 
current source combined with a high-performance opamp integrator and an 
LM311-based hysteretic switch.  My goal was to use the other half of the 
13600 as a Sin shaper.

This core had extremely poor tracking.  I tried different input drive 
levels to the OTA as well as using/not using the linearizing 
diodes.  Replacing the 13600 with a 3080 immediately fixed the problem.

This problem seemed similar to some problems I had many years ago using the 
13600 as 2Q or 4Q multipliers.  So yesterday I took some time to look more 
carefully for the nonlinearity.

What I did was to look at the OTA output (through either an opamp I/V 
converter or a buffered resistor to ground) as a function of programming 
current (Iabc), with fixed input voltage.  I used Iabc values from 0 to 
0.5mA.  As a rough characterization of the nonlinearity, I compare below 
the output for Iabc of .25 mA and .5 mA.

For the 13600 the results are as follows.  With an input voltage of 17 mV 
and an opamp I/V converter at the output, the midpoint error is 8%.  Using 
a buffered resistor to ground increases this error to 9%.  Increasing the 
input voltage to 170 mV lowers the error slightly to about 7%.  In all 
cases the midpoint output is too high compared to the endpoint (downward 
curvature).

For the 3080 the error is much smaller, about 2%.  Also, the error is in 
the opposite direction (upward curvature).  In contrast to the 13600, the 
error increases with input drive voltage.

Linearity vs Iabc doesn't seem to be spec'ed for either device, but the 
results I got seem to be at odds with the curves in the data sheet.  (Hard 
to tell on a log plot, though.)

The bottom line is that the 13600 seems to be a pretty crummy 
multiplier.  It *cannot* replace the 3080 in applications requiring good 
linearity.  That's a good reason for everyone to grab some 3080's while 
they are still available.  If you need the linearized input, you can simply 
add a couple of diodes.  (The 13600 input simply has diodes connected to 
the inputs at one end and to each other at the other.)

   Ian