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Signal Conditioning Problem

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

Me too. What do you want us to do? Believe that you tested it thoroughly in January? Or tell you that Ohm's and Kirchoff's or someone else's law have changed since then?

Gunnar Englund
www.gke.org

RE: Signal Conditioning Problem

Or that your PCB is an exact replica of your breadboard?

TTFN



RE: Signal Conditioning Problem

I had problems like this once.  It turned out that my circuitry  was very sensitive to RF. Local transmitters would feed in signals that could be rectified by the circuitry and cause unexpected results.  Turning the board, moving my hand near it, wrapping it in tinfoil, all caused changes in operation.  If you ever look at a real world high freq or high freq susceptible desigs you will find a sizable percentage of the design effort goes into shielding and other protection methods.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Signal Conditioning Problem

(OP)
Thanks for the help guys, appreciate the courtesy.

RE: Signal Conditioning Problem

Could the original circuit have been just-on-the-edge of an instability?  The slight capacitance or resistance change with the newer circuit is setting if off?

RE: Signal Conditioning Problem

(OP)
It's possible the circuit was on the edge of stability.
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

You wouldn't have a flakey power supply?  Using the same one on prototype as on board models.

Power supplies never cause problems . . . .

Dan

RE: Signal Conditioning Problem

What do the signals look like on a scope screen?

Gunnar Englund
www.gke.org

RE: Signal Conditioning Problem

(OP)
With a larger amplitude (about 500 mV or higher) the output signal is a fairly clean square wave from 0-5V no matter that the input signal. But with smaller amplitudes, the output is still from 0-5V but there are noise spikes all over the place. When the amplitude is decreased beyond a certain point the signal dies completely. The main problem is with a sine wave input because all of the other signals will be of a much larger amplitude that doesnt change.

RE: Signal Conditioning Problem

That sounds like a design thing, plus noise coming in either from your power supply, input path or oscilloscope ground. You do use the ground clip of the probe, I hope? Not using the power cord ground like bad boys do?

Gunnar Englund
www.gke.org

RE: Signal Conditioning Problem

lespaut:
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

Agree. Didn't see that differentiator word. Yes, of course. Do what nbucska says. Absolutely.

Gunnar Englund
www.gke.org

RE: Signal Conditioning Problem

Fishy. The key thing here is that the breadboard was working then stopped working later for "no reason". Let's not look at the pcb version as it clouds the issue.

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

* One of the million lousy bread board connections oxidizided into a rectifier joint.
* 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

(OP)
Here is the schematic.

RE: Signal Conditioning Problem

A suitable capacitor across the zeners to reduce bandwidth wouldn't hurt. We don't know yor requirements, so you will have to decide value yourself. Also, I wonder how the common mode signal ((V+ - V-)/2) is limited. How is the input signal referenced to your circuit's ground? Even if the amplifier is an instrumnt amplifier, it still has common mode limitations.

Gunnar Englund
www.gke.org

RE: Signal Conditioning Problem

I agree with nbucska. There is no filtering on that circuit. If you have a low amplitude low frequency signal going into that circuit, any noise on the input is going to make the output of the differentiator go nuts.

RE: Signal Conditioning Problem

(OP)
The instrument amp has a reference pin that is grounded.
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

Yes, the amplifier has a reference ground. Sure. But what about the signal source? Does it float with respect to the circuit's ground?

Yes. Parallel to the zeners. It will reduce the high-frequency NMR.

Gunnar Englund
www.gke.org

RE: Signal Conditioning Problem

(OP)
Well, this is eventually going to be used in an automotive DA system. So the signals will be coming from sensors that either have no ground or are referenced to vehicle ground. The DA box will be connected to vehicle ground through the lighter plug.

RE: Signal Conditioning Problem

(OP)
I think the problem is in the back part of the circuit after the differentiator. The signal coming out of the differentiator looks pretty good, but the signal out of the comparator is awful at low frequencies.

RE: Signal Conditioning Problem

Do you want help? Or do you think that this is a conversation piece?

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

(OP)
The circuit that was on the breadboard works in a car, no idea if the pcb version works because I don't have access to a vehicle for testing at this time.
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

Do you have a problem with both the active and the passive sensors? Or does it work with one type and not the other?

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

(OP)
The problem is mainly with the passive sensor which produces a sine wave that varies in amplitude as the frequency increases.
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

No. It is less. You have a 5.1 V zener there. But it is actually around 5/200 = 25 mV. I misread the R4 value.

Gunnar Englund
www.gke.org

RE: Signal Conditioning Problem

This may be a silly question, or I might have missed something in the discussion, but what, exactly, is powering this thing?  The only supply in the schematic is for a pull up resistor.  It can't be just the ground and 12 volt system of the car, because the comparator reference is at ground, and the AMP02 is not rail to rail.

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

Decoupling, decoupling, decoupling of the supply.
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

(OP)
By decoupling the power supply are you referring to placing a cap between the powers and ground?
If thats what you mean, the PCB version of the circuit has those caps.

RE: Signal Conditioning Problem

Skogs, it's a bit off-topic now but "superstition", I don’t think so.

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

As for the original posting, beware of the LM311 inputs/outputs. The + and - input polarity gets effectively swapped depending on how you wire the output. Looks like you are using the output emitter grounded and using the open collector drive. Have you verifed you are actually getting positive feedback?

RE: Signal Conditioning Problem

Having problems with parasitic effects in a GHz design is one thing. Trying to explain problems in a marginal low frequency design with other phenomena than design flaws is another thing. That's why I used the word superstition.

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

Lespaul,
    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 the bandwidth limitation as already suggested (a good idea), how about some nF caps in parallel with the negative feedback resistors (to knock off the unneeded high frequency gain). Might be much better there than at the zeners (input).

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

(OP)
Skogs, if you are just going to attack me than please do not post anymore in this thread. I've let it go long enough. I don't need your help that bad that I need to take flak from you.

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

lespaul,

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

lespaul,

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

LESPAUL:
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

(OP)
Max frequenecy is negotiable,  but it must be able to do at least 30K. Min frequency is 24 Hz. Max amplitude is 50V and min amplitude is 50 mV.

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

zero volts signal can't be digitized.

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

(OP)
I forgot about one of the signals. There will be a 700 mV p-p square wave signal that will be riding below vehicle voltage which is about 12-14 V. If I amplify that signal, it will run an amp right to the positive rail and never leave. Thats the reason for the differentiator.

RE: Signal Conditioning Problem

Of course it is not a "differentiator" anyway, since the HF gain is simply R3/R1. C1 could stand being a bit bigger to make the LF signals stronger.

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) smile

RE: Signal Conditioning Problem

With R6 at 1K, it means that the comparator is switching currents of 15mA.  This isn't necessary, and such transitions introduce noise in the rest of the circuit, especially since you're trying to detect low voltages.

RE: Signal Conditioning Problem

If the output of the LM311 is a NPN transistor connected to ground then the LM311 can't pull it's output negative so your positive feedback circuit that's referenced to ground (circuit built to work with input signal centered around ground) just won't work. The output MUST be capable of going negative for the positive feedback you're using to work.

Maybe I missed something in the quick look at the datasheet.

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