capacitive loading

Once in a while you see a circuit that has an opamp with a humongous
capacitor from its output to ground, not one of those cool C-Load
National things, just an ordinary opamp. I've even seen this in Analog
Devices and other "respectable" appnotes.

Today's problem is that I have one of those notoriously fickle
National LM45 temperature sensors in a temperature controller, used as
the thermocouple reference junction temp sensor.

http://www.national.com/pf/LM/LM45.html

I got lazy and connected it directly into the ADC mux, and the charge
injection from the mux makes the LM45 go nuts for longer than the
available settling time, a bit under a millisecond. So I wrote an ECO
to add a 1K resistor to ground, which fixed some units and made others
break into oscillation. Grrrrr.

So, not wanting to cut traces if I can avoid it, I was thinking of
hanging a biggish tantalum cap from the LM45 output to ground, 47 uF
maybe. If the resulting pole crushes the internal frequency
compensation (if it actually has any, which is problematic) the whole
mess ought to be stable. The charge injection thing will sure be
fixed, and there won't be much oscillation at the output, whatever
happens inside.

Incidentally, the resistors from the output emitter to ground add up
to about 40k. Measured small-sig AC impedances of the unloaded LM45
are roughly

1 KHz 40 ohms? hard to tell on current setup
10k 200 r
20k 400 r
50k 1k
100k 2k
200k 3.4k <-- peak Z freq
500k 1.4k


So the equivalent circuit is roughly


sig---20 ohms-----3.2 mH--------+-------out
|
|
200 pF
|
|
gnd


I can't easily measure the DC source impedance because of self-heating
effects. It is a temperature sensor after all.

It seems to work OK with the tantalum.

I had to write this up anyhow, so may as well post.

John

.

Relevant Pages

  • Re: capacitive loading
    ... National LM45 temperature sensors in a temperature controller, ... KHz 40 ohms? ...
    (sci.electronics.design)
  • Re: capacitive loading
    ... National LM45 temperature sensors in a temperature controller, ... It seems to work OK with the tantalum. ... Do you have room to connect a .1 uF ceramic in series with a 10 ohm resistor? ...
    (sci.electronics.design)