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Weird (to me) 60 Hz pickup

So this is still a thermal circuit. I posted a simple schematic and 'scope shots here.

formatting link

(Let me know if that doesn't work. I linked to a folder and not individual images.)

I'm looking at the error signal out of the second instrument amp. There's a lot of pickup from the room lights and such, but when I get right near zero in the error signal I get these big spikey things with alternating polarity at 60 Hz. (AC line.) The second 'scope shot shows the same pickup, but I've now added a 1uF filter cap across the first int. amp input. (about a 2ms TC.. impedance of diode about 2.5k ohm.)

As the error signal moves away from zero the spikes shift in time such that there is always 16.6 ms between the negative spikes, with the position of the positive spikes moving around. (Like some zero crossing spike.)

Any ideas of where I should look? Or what is going on? The diode is a diode connected transistor. When I increased the diode current to 100 uA the spikes became much smaller.

Reply to
George Herold
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Oh I should add that this is all open loop, I don't have the heater circuit energized. Though when it is energized I still see similar type pickup as the error signal moves across zero... and of course the idea of the loop is to keep the error signal near zero. :^)

GH

Reply to
George Herold

dl=0

Uh, that's weird. Have you looked at what's happening at the input to the _first_ instrumentation amp, and the output of that amp?

You're sure there's no dirt in any of your supplies?

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!
Reply to
Tim Wescott

Sure looks like reactively coupled Triac noise. I trust you have shielded twisted pair.

Reply to
Anthony Stewart

Well the power supply is not particularly "clean" but there's little sign of

60 Hz. My first guess was that it's something to do with the instrument amp. ('cause it happens it's at zero, but then why does changing the diode bias current do anything.) It is a diode connected transistor, I'm going to unhook the base and collector (measure b-e diode) and see what that does.

George H.

Reply to
George Herold

The output of the first int amp is doing the same thing. ~10 mV p-p Looks like it's there on the diode too, but now it's 1mVp-p, and kinda lost in the noise. I uploaded more 'scope shots. (IA1-output-chan2. and diode-chan2.)

Changing to a regular "diode" did nothing. Maybe it's my current source?

One very frustrating thing, is that it seems intermittent. I move the error signal around zero and nothing... then it's back... grumble. Maybe I'll try making some of my own big 60 Hz fields... bringing out the Weller soldering gun!

George H.

Reply to
George Herold

Did you look at the power supplies? Where is Vref coming from? A variable supply with dry capacitors maybe?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Maybe somebody in the next room is using the microwave oven? If the supplies are really clean, it sounds like rectified RF to me, and microwaves put out a lot of nasty line-frequency-modulated stuff.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Hmm well no shielding nor twisted pair on the cable. (It's still in the disgusting looking prototype stage.)

George H.

Reply to
George Herold

Yup supplies are OK. Vref is from a 10 volt reference and 10 k pot. (REF102)

George H.

Reply to
George Herold

Hmm, being rectified in the diode? Dang interference has gone away... (The Weller gun just made bigger sine waves.. no spikey stuff.) If it comes back I'll try shielding the diode.

I think I can afford to run the diode at 100uA (~0.1mW max power dissipation, having the collector clamped to ground/hot plate should keep self heating to a minimum.)

George H.

George H.

Reply to
George Herold

Doesn't have to be a microwave. Electronic ballasts in celing mount fluorescents and compact fluorescents can be potent noise sources that are strongly modulated by (usually) 120 Hz in the US. And, don't trust anything else in the room, either. PCs and all sorts of other gear with switching power supplies can generate a lot of noise.

Jon

Reply to
Jon Elson

If it's the diode rectifying RF, then it would only be in one direction. And wouldn't a TIP-30 be slooooooow?

If it is RF, then a cap across the "diode", sized to not slow the measurement down too much, would be fix and diagnostic all in one.

There's probably some clever way of ignoring high-amplitude short pulses going into a low-pass filter (probably involving more diodes). I'd try to suggest something, but I currently have my head in Mathemagic Land and can't think practically -- I'd give you a suggestion with 27 diodes* that all had to be precisely matched, or something stupid like that.

  • 8 x 10 color glossy diodes. I need to lay of the math, but it's what I'm getting paid for at the moment.
--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!
Reply to
Tim Wescott

And why does the diode care whether the error voltage is close to zero? That's only happening at the input to the 2nd instrumentation amp.

I just had an odd thought -- could it be caused by that 2nd amp, but then either getting into the power supply leads or getting fed back through the I-amp? If it's happening fast enough it may not be getting attenuated by the usual "feedback and tons-o-gain" method of op-amps. It would still require that the instrumentation amp do something odd at zero common-mode voltage that would make it susceptible to weird things happening, though.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!
Reply to
Tim Wescott

dl=0

What's the instrumentation amp? Your drawing makes it look like "AD670", but Analog Devices says that's a DAC.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!
Reply to
Tim Wescott

Yep, that's the most likely. Initiating the current in a fluorescent lamp is just a little less abrupt than a spark gap, and (depending on warmup and other odd factors) can come and go into radiative modes.

Occasionally, a fluorescent tube starts to rectify. Sometimes, reversing the tube heals a bad light fixture, which means the whole ballast was starting to saturate... spike city when THAT happens.

Reply to
whit3rd

I'll try a cap across the diode. Thanks George H.

Reply to
George Herold

Yeah, that may be my mistake, futzing around I found the spikey stuff at other levels... I guess I just notice it more near zero. :^) I have no idea why it's level dependent... I could make up some stories.

I've got 4 feet of antenna on the tip32, and get ~1mV * 2 ms (TC my diode-1uF cap low pass) ~= 2uJ of energy. (The spike rails, things w/o the LP)

The florescent lights over the bench make more at 25kHz. (CW)

Oh my, I've got a cluster "f" of electronics. (I'll take a picture, it's not pretty.) It started out OK, but then I wanted gain here, and something else over there,

George H.

Reply to
George Herold

AD620 ... GH

Reply to
George Herold

Hmmm I'll try some power supply filters on the current source. (Volt ref->R->out/opamp feedback)

George H.

Reply to
George Herold

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