1 bit of ASM advice needed...

[Magnus Danielson]

>>>>> "S" == Stopp,Gene  <gene.stopp at telematics.com> writes:

 S> Magnus wrote:
 >> 
 >> Next to the negative input at the CA 3140 is a offset pin sitting very
 >> closely to -15V with only 500 ohm down to it. This migth be good to
 >> know if there is a odd frequency stick in the VCO which doesn't
 >> correlate with the current in the Q1/Q2 10k resistor...
 >> 
 >> The CA3140 has an input impedance of about 1 Tohm so some caution
 >> should be taken for low input currents... but I am not sure that the
 >> VCO get that low current for it to be significant...
 >> 

 S> Uh -huh... well the 3140's are used in the VCF as well. Good catch,
 S> Magnus! I think you might be on to something.

Well, you got me triggered!

Once you started discussing crawling currents on PCBs I recalled an
article in Analogue Devices "The Best of Analog Dialogue 1967-1991"
(full of nice articles). Especially the article "Avoiding
Passive-Component Pitfalls" originally from Analog Dialogue 17-2 1983
goes into among other usefull things PCB and their static and dynamic
behaviours and ways to avoid it. One technique is to put a guard ring
around the + and - inputs and actively drive it to an voltage near the
actual input voltages. This way will stray leakage coming from other
places meet the guard copper which is driven by a reasnoble low
impedance source and will not crawl into your 1 Tohm pins....

Now, with this in memory I started to think that maybe Ohms law still
applies "No current unless a potential difference to drive it!" so I
pulled out the ASM-1 VCO schematics (it was allready on the table
anyway) and thougth that I would track down the most nearby sources of
potential differances to the CA3140 input pins... the positive input
was just grounded, so it should not be particularly sensitive to
crawling currents of pA or fA. The negative input is intended to look
into a capacitor, a disabled JFET and the Q2. If the current from Q2
is very low we can suspect that a crawling current may become large
enougth in comparision to have a noticable effect. This will happend
in the low frequency range (I think I said high in my first notice -
was just tired, so sorry). So, now when there is a possibility that we
can be sensitive to crawling currents we must find a suitable source
that we can cause a suitable difference. We know that the negative
input will be near ground, so either of the power supply lines will be
suitable - but these are both over 5 mm away from pin 2, lets look
closer - pin 3 is also at ground (pin 2 is at ground since the
feedback forces pin 2 to land at ground where pin 3 is) but pin 1
seems to be sitting there doing nothing!
Now, pulling out the Harris databook reveals that the balancing inputs
is both tied to -Vcc (that is, -15 V) via 500 ohm resistors and that
the input stage has a total standing current of 200 uA which should
make 100 uA into each of the resistors, giving a voltage rise of only
50 mV from -Vcc, so we can look at it as being -Vcc anyway... 
Looking close at the PCB reveals that everything else near the - input
copper is ground and the G and D terminals of Q3.

So there is some possibility that crawling currents can cause unwanted
effects, and if you have a water washable defluxer and some humidity
you can probably jump several decades up in the current scale. To
measure these problems you can just try to modulate the VCO to the
lowest frequency with the coarse knob and monitor which frequency it
will land on. As it happends will a negative current cause the VCO
frequency to rise, one can view it as a constant linear FM modulation ;)

If the design would have been targeting lower input impedance, then
would the VCO has a less desirable frequency range and be less
sensitive to these crawling currents.

Oh, did I say that the CA3140 has a high amplification on diffrential
input voltages?

PS. Juergen and others, I found a nice article about Noise and
Operational Amplifier Cursuits in the same collection from AD.

Cheers,
Magnus