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In below circuit I pointed(red arrow) the part with "power supply rails with two caps".

As you see, it is drawn seperately from the main circuit.

enter image description here

1-) Does that mean the capacitors C1 and C2 are optional?

2-) I'm having some days spikes from the circuit output(ADC). I was planning to investigate the power supply. Below is the sampled data from a noisy day. Is there a possiblity that this can be power supply noise?:

enter image description here

edit:

Here AC coupled scope view of 15V to gnd rail:

enter image description here

Here in freq. domain:

enter image description here

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    \$\begingroup\$ It's not optional. The 2 capacitors are drawn in between power rails (+15, GND and -15) and that means you have to connect the capacitors that way. And the purpose of those is to reduce the supply noise. \$\endgroup\$ – chamod Nov 23 '16 at 11:22
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    \$\begingroup\$ And before you ask why is it drawn that way - It's because the capacitors aren't part of the amplifier circuit, they are part of the power supply. By putting them separate it avoids cluttering the the signal path part of the schematic with unrelated parts. \$\endgroup\$ – Andrew Nov 23 '16 at 11:26
  • \$\begingroup\$ A single question per post would get you clearer answers and would help future researchers. \$\endgroup\$ – UnconditionallyReinstateMonica Nov 23 '16 at 13:13
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1-) Does that mean the capacitors C1 and C2 are optional?

no!

I really don't like the schematic for several reasons (e.g. with a slight reorganization, U3 wouldn't have the ground "above", other cosmetic reasons), but the fact that they chose to dangle the 100nF stabilizing capacitors just "anywhere" on the board really makes it a bad schematic, in my humble opinion.

I was wondering where the designer of the circuit intended them to be.

I came to the conclusion that they were probably meant to be as close as possible to the lower side of R4, essentially forming a low-pass filter with the "zero adjust" voltage divider. Strange choice, by the way, to introduce yet another name for this ground net, and call it -OUT.

I personally dislike this style of drawing capacitors just "somewhere in the schematic, so we don't forget to include them in the list of components to buy". The placement of such capacitors is critical to their function and performance (long trace = high inductance = less effective to swallow noise)

2-) I'm having some days spikes from the circuit output(ADC). I was planning to investigate the power supply. Below is the sampled data from a noisy day. Is there a possiblity that this can be power supply noise?

Yes.

But it's impossible to say, because you seem to be sampling at a very low rate (looks like < 1kS/s), and there's nothing indicating you limit the bandwidth. So this might just be some proper signal aliasing. Or Megahertzes of noise power density concentrating in your Nyquist band. When using an ADC, you must always make sure you apply the Nyquist sampling theorem and limit your signal bandwidth with an analog anti-aliasing filter!


Word of advice:

The INA110 is an early/mid 1980's instrumentation amplifier, and U3 is a gain adjustment mechanism, and U2 is just a voltage buffer built from an ancient OP! The OP07 used comes from an era when Operation amplifiers were still commonly sold in these circular-pin packages, resembling tube pentodes!

I'm pretty sure you can build something with at least the same performance pretty easily these days with less components and less hassle. Maybe a redesign would be a good thing, assuming you know what the device is going to do.

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  • \$\begingroup\$ Thanks for the answer. Some more information. That circuit is from this pressure_scanner module: ohtegiken.co.jp/wp/wp-content/uploads/manual_zoc33man_1007.pdf And the power supply is: ohtegiken.co.jp/wp/wp-content/uploads/manual_zoc33man_1007.pdf You can see the electronics components detailed at the end of the manual. The thing is that spiky noise was there last week but not this week. If it is power supply noise, can it behave like that(some days exists some days not)? \$\endgroup\$ – floppy380 Nov 23 '16 at 11:59
  • \$\begingroup\$ sorry the link for the power supply would be: pdk.co.kr/uploads/Scanivalve/rpm1000.pdf \$\endgroup\$ – floppy380 Nov 23 '16 at 12:07
  • \$\begingroup\$ I still can't guess what is behaving randomly in an undersampled scenario! \$\endgroup\$ – Marcus Müller Nov 23 '16 at 12:15
  • \$\begingroup\$ You are right and I now edited my question. Please see my edit. I added the scope screen shot of power supply's 15V to Gnd in AC coupled mode. The noise is around 500kHz and 150mV peak to peak. Is this a bad regulation? Could that be the source of the noise? \$\endgroup\$ – floppy380 Nov 23 '16 at 13:16
  • \$\begingroup\$ again, whether this is bad or not depends on the signal filtering. I'll stop commenting until you explain what your analog signal chain does with the signal before it hits the ADC. \$\endgroup\$ – Marcus Müller Nov 23 '16 at 13:17

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