op-amps, bias, and integration circuits
op-amps, bias, and integration circuits
(OP)
Hello all,
I have a signal, of which the integral represents a process variable. (Fig. 1 of references.pdf) I designed a gated integrator (Fig. 2) to isolate the usable portion of the signal. I then sample this output with an ADC and scale appropriately. This works well, however the input signal can be bi-directional (Fig. 3) and the circuit doesn't support this. My first thought was to simply make everything bipolar. As it turns out though, my task of selecting an appropriate and reasonably priced ADC is much simpler if the output remains unipolar. My next thought was to apply both a DC offset to the input signal and an appropriate bias to the integrator. The idea was to get an output similar to the simulated signal in Fig. 4. I'm not sure if this is even possible. Typical non-gated integrators produce a triangle wave output from a square wave input. The gated variety produce a sawtooth wave. Here in lies the problem with my concept. Resetting the gate on the integrator sends the output to the bias point. I've yet to be able to figure out a way to send the output below the bias point. Maybe I have tunnel vision and am too focused on a single approach to the problem. I was wondering if anyone can provide some insight as to alternative solutions?
Brandon
http: //home.com cast.net/~ keystonecl imber/imag ehost/refe rences.pdf
I have a signal, of which the integral represents a process variable. (Fig. 1 of references.pdf) I designed a gated integrator (Fig. 2) to isolate the usable portion of the signal. I then sample this output with an ADC and scale appropriately. This works well, however the input signal can be bi-directional (Fig. 3) and the circuit doesn't support this. My first thought was to simply make everything bipolar. As it turns out though, my task of selecting an appropriate and reasonably priced ADC is much simpler if the output remains unipolar. My next thought was to apply both a DC offset to the input signal and an appropriate bias to the integrator. The idea was to get an output similar to the simulated signal in Fig. 4. I'm not sure if this is even possible. Typical non-gated integrators produce a triangle wave output from a square wave input. The gated variety produce a sawtooth wave. Here in lies the problem with my concept. Resetting the gate on the integrator sends the output to the bias point. I've yet to be able to figure out a way to send the output below the bias point. Maybe I have tunnel vision and am too focused on a single approach to the problem. I was wondering if anyone can provide some insight as to alternative solutions?
Brandon
http:





RE: op-amps, bias, and integration circuits
A lot of amplifiers will take a reference input. This is the signal that they give if there is zero input. For you application, this means, that you could use one of these amplifiers as a unity gain follower with a 2V reference. This would then offset your signal onto the 2V reference and giving you a +/- 2V input signal range that would run between 0 and 4V to the ADC. If this is too much or too little swing, you could use the amplifier with a gain or attenuation factor in addition to the reference offset.
RE: op-amps, bias, and integration circuits
RE: op-amps, bias, and integration circuits
If you need to integrate both pos and neg, I'd go with the suggestion from Noway2.
RE: op-amps, bias, and integration circuits
RE: op-amps, bias, and integration circuits
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RE: op-amps, bias, and integration circuits
In figure 3 it looks like the output is at a 0.0V and a positve 100mv from the transducer would produce a -4V which is out of the rail to rail voltage of the AD8626. Negative integration is possible but it looks like you are on the bottom rail with the output in figures 1 and 3.
Again, I am not sure where the 2V offset has been produced in the circuit on the output with a zero voltage signal in.
Best Regards
RE: op-amps, bias, and integration circuits
RE: op-amps, bias, and integration circuits
Sorry to be confusing but fig. 2 is the original circuit I built before attempting any bi-polar stuff. It has a 0 volt bias. The traces in fig. 1 are actual measurements from that circuit, and you are correct with your interpretation.
I can add a negative supply rail to that circuit but I'm not sure it will help because the lower limits of integration have been (in my experience) somehow limited by the bias. I used a negative rail in the simulation. The bias is set via the non-inverting input with a voltage divider, or by simply grounding if no DC bias is required. In the case of the first circuit, I use some feedback from the output for this which allows a linearity correction of sorts for distorted input signals. In the more recent circuit simulation, I've removed this and opted for the voltage divider configuration for simplicity while working out the bi-polar issues.
What I've found is two fold. First I've been unable to drive a signal more than about 0.5V below the bias point. Second, whenever the integrator is reset, the output is driven to the bias point. Both of these conditions can be seen in the simulation output on the red trace.
I'm not sure the AD628 will work in this situation since it is internally configured as a difference amplifier. It seems like it would be difficult to configure as gated integrator.
Again, thanks for the help. Any more ideas?
RE: op-amps, bias, and integration circuits
I think your whole setup is fine and a gated integrator is working. I think that you must have a 2V bias on the output of the ad628 with no input signal or 0v input. Instead of grounding your linerarity 50 pot terminate it into a 2V regulated supply.
Best Regards
RE: op-amps, bias, and integration circuits
I am curious, is there any reason you don't do the integration in software?
There are plenty of very good integration algorithms, even for dirt cheap 8 bit micros.
RE: op-amps, bias, and integration circuits
RE: op-amps, bias, and integration circuits
Instead of answering my question as to whether or not you had considered performing the integration in software, which personally I think has several advantages, you responded by suggesting an even more complex circuit.
Your latest post has you looking at dual ADCs, and dual analog switches, in addition to a integrator stage. This sounds a lot worse that the bipolar ADC that you were so desperately trying to avoid.
I don't have enough information on exactly what it is you are trying to accomplish to fully understand your application. However, from what you have described, it seems to me that it would be much simpler to physically offset the transducer output onto a bias that represents the midpoint of your ADC and then process it digitally in software. Otherwise, as I suspect you are starting to see, you will be running into a complex circuit full of precision components that will more than likely suffer from parasitic effects and noise loops.
RE: op-amps, bias, and integration circuits
Thanks for your insight. Sorry for not addressing your question. This could be done in software and that is a good suggestion. I would however like to avoid that approach. I don't consider any of the circuitry that I've presented to be complex, and certainly not enough to justify coding calculus algorithms in firmware. Also, their are some timing restraints that make this approach undesirable. In a different scenario though, I would certainly consider this approach. Thanks again.
RE: op-amps, bias, and integration circuits
If you change your mind and decide to look at a software approach, I would recommend reading Jack Crenshaw's book Real Time Math Programming. Once I read it, I was truly amazed at how little processing power (read few instructions) it takes to "code calculus algorithms" to a high degree of accuracy.
RE: op-amps, bias, and integration circuits
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com