Signal Conditioning Problem
Signal Conditioning Problem
(OP)
I developed a circuit for a module that transforms several types of signals into a digitally compatible signal. I tested this circuit on a breadboard in January and it worked perfectly over the required range of amplitudes and frequencies. I had some PCBs fabricated about a month ago with the signal conditioning circuit and a bunch of digital components. The signal conditioning circuit does not work for a variety of signals now. Low amplitude, low frequency sine waves do not generate a clean signal like I was seeing with the breadboard. So I used the circuit on the breadboard again to make sure everything was the same and that circuit no longer worked! I originally tested the circuit in two locations and it worked perfectly in both locations. I tested it in two locations again and it failed in both locations. I'm completely dumbfounded.





RE: Signal Conditioning Problem
Gunnar Englund
www.gke.org
RE: Signal Conditioning Problem
TTFN
RE: Signal Conditioning Problem
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Signal Conditioning Problem
RE: Signal Conditioning Problem
RE: Signal Conditioning Problem
Been there too..
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Signal Conditioning Problem
The original circuit I built was placed into a cabinet and left alone. Not a part on the circuit was touched. When I used the circuit a few months later I had problems. Thats what I don't understand.
The circuit is comprised of a differential amplifier being used as a differentiator and a comparator. There are limiting zener diodes before the differential amplifier to prevent large voltages from destroying the components. The next stage has a comparator with a feedback resistor to provide some hysteresis.
Anybody have any particular problems with these types of circuits?
RE: Signal Conditioning Problem
Power supplies never cause problems . . . .
Dan
RE: Signal Conditioning Problem
Gunnar Englund
www.gke.org
RE: Signal Conditioning Problem
RE: Signal Conditioning Problem
Gunnar Englund
www.gke.org
RE: Signal Conditioning Problem
Differentiator has very high gain at very high freq.
so you must limit the bandwidth or else it will be VERY
noisy !!! Make sure the BW is not wider than necessary.
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Signal Conditioning Problem
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Signal Conditioning Problem
Gunnar Englund
www.gke.org
RE: Signal Conditioning Problem
Possibilities:
* The "untouched breadboard" was mounted on a chassis plate or put in a case.
* The "untouched breadboard" had its power supply/input leads removed and reconnected differently.
* The "untouched breadboard" was powered from a different power supply and/or signal generator.
* The power supply, signal generator or sciope developed a fault.
* Someone took the earth lead off the scope or connected local earthing on either the signal generator or the power supply.
* An electrolytic capaitor aged and no longer functions correctly.
* The board was "zapped" by an ESD event due to mishandling.
If none of those are true it is probably something else!
RE: Signal Conditioning Problem
* The power leads changed oreintation or length.
* There is a new RF emmitter nearby.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Signal Conditioning Problem
Superstition.
It is an unsuitable design.
A design that is sensitive to parameters like those mentioned above is bad. We do not help this guy if we give him excuses not to go over the design and make it better.
One thing that would help is if we could see the schematics. There is an FAQ on how to publish drawings/pictures in FAQ238-1161 (thanks Itsmoked). Use that to show us your diagram.
Gunnar Englund
www.gke.org
RE: Signal Conditioning Problem
RE: Signal Conditioning Problem
Gunnar Englund
www.gke.org
RE: Signal Conditioning Problem
RE: Signal Conditioning Problem
When you say I should put a cap across the zeners, do you mean in parallel?
The requirements for the circuit are that it has to operate at 27 Hz and 100 mV for the low amplitude sine waves. The upper limit is not of concern as 20K or so would be fine and the circuit has no trouble at high frequency high amplitude.
RE: Signal Conditioning Problem
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Signal Conditioning Problem
Yes. Parallel to the zeners. It will reduce the high-frequency NMR.
Gunnar Englund
www.gke.org
RE: Signal Conditioning Problem
RE: Signal Conditioning Problem
RE: Signal Conditioning Problem
It is now that you have the problem. Isn't it? Or have you verified that the circuit works in a car? Please answer how your signal source is connected. Or do you not understand the question?
Gunnar Englund
www.gke.org
RE: Signal Conditioning Problem
I don't see how the connection of the signal source affects anything. In my last post I stated that the signal out of the differentiator is fine. This would point to a problem with the comparator circuit would it not?
But to answer your question, the signals are coming from sensors on a vehicle. These sensors are either passive or active. The active sensors are connected with the signal being V+ and the ground being V-. The passive sensors do not have a reference to ground that I know of, that was the point of having the differential amplifier.
RE: Signal Conditioning Problem
The fact that the signal "looks good" after the differentiator is interesting. You have a comparator with only 4 mV hysterisis. And it is referenced to GND. Do you really not see anything on the differentiator output? Using 5 mV/div on the scope?
Gunnar Englund
www.gke.org
RE: Signal Conditioning Problem
The active sensors produce square waves that have a steady amplitude as the frequency changes. Those work well for the most part.
The comparator should have 30 mV of hysteresis.
(200K/1K)*15V = 30 mV
It's a recognizable signal on the differentiator output.
RE: Signal Conditioning Problem
Gunnar Englund
www.gke.org
RE: Signal Conditioning Problem
Also, as shown, the AMP02 is configured for a gain of 1. You need a resistor across the gain set terminals so Gain = (50000/Rgain +1). No resistor equals infinite ohms, giving gain as 1. So as drawn, your differentiator is essentially built around a unity gain buffer.
RE: Signal Conditioning Problem
Especially at the pullup resistor at the comparator output. If that 15 V is not decoupled close to the circuit, your comparator will freak.
Benta.
RE: Signal Conditioning Problem
If thats what you mean, the PCB version of the circuit has those caps.
RE: Signal Conditioning Problem
The day before yesterday I had an oscillator producing spurious tones, but only when run from one particular bench power supply. When I changed to a different model of power supply the tones disappeared. I changed them back and forth a couple of times to make sure.
I was going to investigate what was wrong with the "noisy" power supply yesterday but the effect no longer occurs! I tried for an hour to reproduce the noise by moving wires around, turning on adjacent equipment and so forth, but to no avail.
Now this is a relatively complicated test in that I am having to mix two oscillators and amplify the difference frequency of a few gigahertz in order to see anything on the spectrum analyser. Nevertheless the bench setup had not been changed overnight. I don’t have a week to try to reproduce the faulty condition. It might have been some external interference to which only one power supply was susceptible and may never repeat whilst I am watching the display. In these cases you just have to move on. …
RE: Signal Conditioning Problem
RE: Signal Conditioning Problem
I have not examined the design in detail and cannot say what is failing. One thing that I suspect is that lespaul has a poor understanding of reference levels, operating points, circuits swinging close to rails and such things. It has also been questioned if he at all is using the components as intended.
There is a (fuzzy) limit to what we can and shall do when helping a poster. A poster that starts with a declaration that a circuit was thoroughly tested in January and that it doesn't work in May shall not be told that there are lots of subtle influences that may or may not make his circuit work. That is why I call it superstition.
Instead, we shall make it clear to him that he has to design with the right topology and components, enough margins, filtering, shielding and whatever is needed to make the design reliable and reproducible.
Giving him excuses not to improve his design is not to help him.
Gunnar Englund
www.gke.org
RE: Signal Conditioning Problem
thanks for posting the circuit. On closer inspection I am worried about the power rails. Do you actually have a negative power rail for U1?
If U1 were an ordinary opamp (as drawn), and if it had a negative power rail, then the output would naturally want to swing symmetrically up and down around ground (0V).
However U1 is not an ordinary opamp, it is in fact an AMP02 instrumentation amplifier. This also needs a negative rail; in fact it needs at least -5V on the negative rail according to the data sheet. Without a negative rail this circuit cannot function correctly, even with a "rail-to-rail" opamp.
I am suspicious of your requirement for a differentiator. Why do you want to have a gain that increase continuously with increasing frequency? In actual fact R1 is already in series with C1 so the high frequency gain is only around 7 anyway.
RE: Signal Conditioning Problem
For trouble-shooting, don't forget to twist the time scale knob on your o'scope to check for RF oscillations. I've seen cases where the only clue was a slight fuzziness of the traces in the audio range. Twist the knob to find out that the fuzziness is a low level 40MHz RF oscillation on the desired audio. Added a cap in the negative feedback loop to nuke it. You can even choose the cap value to series resonate with its own lead lengths when installed.
Of course, the problems with your circuit might be something else entirely...
RE: Signal Conditioning Problem
Log, The AMP is powered by +/- 15 V.
Like I said before I think the problem is with the comparator circuit. I wired up a few sample circuits with a comparator to see how it functioned without the front part of the circuit. I tried using several different LM311s, but none of them worked at all. They were all brand new from the bin. There must be some sort of interference somewhere. I think I'm going to scrap the second half of the circuit and try something else.
RE: Signal Conditioning Problem
Redflag my postings if you do not value them.
But before you do. Read my last post again. Try to understand what I am saying - and wy I am saying it. "Attacking" is not my usual behaviour even if I often is very open and direct in my comments. I think that other members can verify that.
Gunnar Englund
www.gke.org
RE: Signal Conditioning Problem
How are you setting the internal gain of the AMP2?
You need a resistor connected between the gain set pins.
There is no resistor shown on your schematic.
Without a resistor, the instrumentation amp's gain is UNITY.
Also, I generally try to avoid the practice of having positive feedback element driving into what feeding it. The high frequency output impedance of the previous stage (the AMP02) is seeing some very high frequencies driven into its input through 201 Kohms. Doesn't sound like much, but I've seen issues with that topology.
You might try grounding the left hand side of the 1 K resistor and appling the input to the inverting input of the 311. Try adding a 1 K resistor to balance impedances and input bias currents as well. Good bypassing is a must.
If phase polarity is important, just swap the V+ and V- inputs to your circuit.
Glad to hear that there's a +/-15 volt power supply, but it does point out that the schematic you supplied doesn't have all the required information. (A common problem. I hate schematics that assume power supplies. Sometimes the software itself makes it overly difficult to include power on a schematic.)
Another question about your schematic...The LM311 needs a connection to ground for the output transistor, as logbook said. This isn't shown either. Just assumed.
RE: Signal Conditioning Problem
If I underrstand it, you want to convert a sine of variable
frequency and impedace to digital.
I think there is a better way. Can you please tell us
1.) min/max frequency your circuit must work with?
2.) Min/max. amplitude ?
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Signal Conditioning Problem
This circuit must also be able to convert square waves with amplitudes .7-1.54 V, 0-4.2V, and 0-12 V. Same frequenecy requirements.
RE: Signal Conditioning Problem
I would suggest:
clamp the input with back to back Schottky diodes and amplify 10X. Digitize it with a comparator with about
100 to 200 mV hysteresis. Trigger with output a non-retriggerable single-shot with about 8 uS and use this to reclock a FF. The D input of the FF is the comparator's
output with an RC delay of about 10 uS.
Refinements:
1.) Change the gain, SS delay and/or hysteresis with the frequency
2.) Add a PLL.
i.e. Digitize first and then debounce it. Do not differentiate !!!
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Signal Conditioning Problem
RE: Signal Conditioning Problem
differentiate.
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Signal Conditioning Problem
Put a scope on the output of the first stage and see how much noise is present. If the noise is bigger than the hysteresis in the comparator you will of course get noise spikes on your digitised waveform. So in this case you either have to reduce the noise or increase the hysteresis.
PS: I think scogs was having a go at me rather than you ... and I could see piece-wise true parts in his criticism (of me)
RE: Signal Conditioning Problem
RE: Signal Conditioning Problem
Maybe I missed something in the quick look at the datasheet.