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Calibration, Part 2

In my first blog about calibration (Calibration, Part 1), I discussed calibrating inputs using techniques that tolerated offsets by simply including them in the measurements. In this blog I will look at nullifying the offsets.

The initial approach is to build in references to the circuitry and allow the system to self-calibrate. One issue would be that the calibration is then not traceable back to some standard, but let's ignore that and consider the circuit in Figure 1, where the input signal is first measured, and then the multiplexer is switched and the reference voltage is then checked.

Figure 1

Using a reference to nullify offset and noise errors.

Using a reference to nullify offset and noise errors.

When measuring VIN , the voltage measured at the ADC is VIN + VOFFSET1 + VNOISE1 . Similarly the voltage measured for VREF is VREF + VOFFSET2 + VNOISE2 . In some circumstances you can argue that the offset and noise are equal in both cases, so if you subtract the two readings you are left with VIN – VREF , and if VREF is known then you have VIN . Seems simple, but you have to consider a few other things.

Firstly, is the noise in fact equal? You can mitigate the noise variation by using signals that change slowly, but there is more to consider. VIN and VREF need to be that same order of magnitude, or you will need different amplifier settings, which will negate this approach. Further, unless your reference is significantly more accurate than the signal, it will contribute to inaccuracy as well, through noise and drift.

Cypress refers to this technique as “correlated double sampling” (CDS), which seems to have taken its name from a similar approach in the charge coupled device (CCD) sphere. It is possible to improve on the arrangement of Figure 1 if you can configure a setup as in Figure 2, where you measure the signal differentially and introduce the VREF into the signal measurement. This is very similar to ratiometric measurements, where the reference of the ADC also drives the sensor, and so the changes in the reference cancel out.

Figure 2

Both sides of the input are measured introducing VREF into the VIN measurement.

Both sides of the input are measured introducing VREF into the VIN measurement.

Going back to the current loop I discussed in my last blog, I have introduced a resistor in series with the load resistor. The reason is that I am trying to get the VREF input voltage above 0V. If you have had experience with so-called “rail-to-rail” analog inputs you will know that it is still a good idea to get use margin between the input voltage extremes. I could have used an amp with negative supplies, but if the amp is internal to the micro you will have to resort to some level shifting techniques. In Figure 2 the voltage measured for VIN is VSIGNAL + VREF + VOFFSET1 + VNOISE1 , and for the lower side of the input VREF + VOFFSET2 + VNOISE2 . Now if we subtract them (and provided the noise is equal) we are left with VSIGNAL only.

According to Cypress, you can apply signal processing techniques as well to reduce the effect of the 1/f noise component.

Cypress has a video — here and two application notes (AN66444 and AN2226 — that mostly deals in generalities before turning to its own products. I think it explains the concept far better than I did.

The LMP90100 ADC has built-in self-calibration that resolves most of the issues. To top it all, it does it in the background. TI does not give a name to the calibration techniques that it uses, so I have no idea if it is CDS, but whatever it is, I hope that this is the beginning of a trend.

What do you do with a non-linear relationship? You could linearize the curve (in hardware or software) before applying the calibration techniques. More simply, you could calibrate many points and do a piecewise linearization of the curve.

There is no reason you can't use CDS together with calibration. CDS does not give you the gain of the system, so you don't know what the ADC conversion actually corresponds to, so it is definitely beneficial unless you are measuring ratios.

So, what calibration techniques do you use?

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8 comments on “Calibration, Part 2

  1. eafpres
    September 20, 2013

    Hi Aubrey.  If I am understanding figure 2 correctly, I would guess that you would try to make the resistors of similar value, and similar construction, and hopefully near each other physically, to be at the same temperature, etc.  Then are you essentially saying the noise measured in Vref is the same as in the noise in Vin?  If, say, a lot of the noise is thermal in origin, this seems OK as long as the voltages are similar etc.  Is there then an optimum value for the 2nd resistor that balances the tradeoffs?

  2. Netcrawl
    September 20, 2013

    @Aubrey that was great, thanks for sharing this great article, we can't ignored it but we need to know that  CDS circuit can be completely effective, only if the FPN is intensity independent, if the circuits  are linear, and the mismatch only has an offset component,  but in reality these assumptions are not true and more elaborate CDS circuits which can compensate for gain mismatch.

  3. Davidled
    September 21, 2013

    With noise cancellation or reduction, I would think about Amp calibration to increase the input signal amplitude, up on the reach of clipping the voltage. I am wondering how to amplify input human voice as input signal through Microphone.

  4. samicksha
    September 21, 2013

    in most of cases the gain should be constant for any combination of input and output signal.

  5. Davidled
    September 21, 2013

    ->gain should be constant for any combination of input and output signal.

    Please would you explain more in detail with some case for any combination?  I am curious for human voice input signal.

  6. antedeluvian
    September 22, 2013

    Sorry to everone- I haven't been able to respond until now. I was away for the weekend.

    The DeusM implementation on Planet Analog doesn't recognize the version of Safari on my iPad. It allows me to read, but not to post a message. And the only EE Times associated forum that it does it on is Planet Analog. All the others work fine.

  7. antedeluvian
    September 22, 2013

    eafpres

    Then are you essentially saying the noise measured in Vref is the same as in the noise in Vin?

    I must admit that in all my calibration efforts I have never tried to calibrate for noise, only the offsets of the transducers/ampfliers/ADC etc. Typically I try to nullify noise effects by software or hardware filtering.

    Although you could probably take your suggestion and analyze the different effects, I don't believe that I am competent to argue that.

  8. CameronRobertson
    September 30, 2018

    Ah, isn't it wonderful to have inbuilt calibrators? I think it would be so wonderful if more things in life had some regulating mechanisms attached to it, don't you. We all need to be able to learn how to focus better so that we can do away with distractions and -ahem- noise. Pity that managing the human interactions and life systems are not as easy as a technical or electrical one.

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