Electronics > Projects, Designs, and Technical Stuff
Load cell (Wheatstone bridge) excitation voltage hurdles
wraper:
--- Quote from: V_King on July 13, 2020, 09:47:11 am ---Following David Hess's comment it got me thinking whether I could use two V-ref outputs from the INA125. The 10V for the load cell excitation and 2.5 to my current ADC.
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Note that you need an opamp buffer unless ADC has one built in.
--- Quote ---The voltage measured with transistor was 10.01V sharp, the overheads seem to be sufficient. The INA125 datasheet recommends min of 1.25V above the reference voltage for correct operation.
I would like to keep the 12V rail, so I will get some of the transistors you recommend and see if there is any improvement.
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I didn't say you need to keep it this way. But checking if it will solve the problem can be very useful. Also, say, your MC78L12 (4% tolerance) has output voltage 2% on the low side (11.76V). And INA125 is already out of dropout voltage spec.
David Hess:
--- Quote from: V_King on July 12, 2020, 03:17:27 pm ---
--- Quote from: David Hess on July 11, 2020, 08:50:28 pm ---The usual and better solution is to make a ratiometric conversion which has the advantage of removing excitation noise and drift. To do this, the reference for the analog-to-digital converter is made the same as the excitation for the load cell, or the reverse. With this done, there is no particular need for the load cell excitation and analog-to-digital converter reference to be either noise free or low drift; the output from a common integrated fixed voltage regulator like a 7812 is perfectly adequate.
If you use a separate load cell excitation and load cell reference, then not only does this contribute another gain error term and noise source, but it prevents rejection of flicker noise which now must also be accounted for so a low drift and low noise design becomes more important.
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Thanks. I will keep that in mind.
The ADC uses 2.5Vref, but ADC Vin is +-10V, so I would loose a lot of resolution that way. But I will try such idea in the future.
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The whetstone bridge excitation just needs to use the same reference as the ADC. Nothing prevents multiplying the 2.5 volt ADC reference to produce a higher excitation voltage, or dividing the excitation voltage to produce a lower voltage reference for the ADC. This introduces a gain error term but removes the difference in excitation and ADC reference terms and rejects low frequency noise.
Check out Linear Technology application note 43 for the various bridge measurement configurations. I particularly like the last one shown in figure 4 on page 4 because of simplicity and doing away with the instrumentation amplifier while allowing ratiometric operation and several examples of this configuration are shown later in the application note.
gogoman:
hello David, can you help me understand the statement:
"If you use a separate load cell excitation and load cell reference, then not only does this contribute another gain error term and noise source, but it prevents rejection of flicker noise which now must also be accounted for so a low drift and low noise design becomes more important.[/i]"
What is the reason that the flicker noise is not being rejected and is the drift and noise degeneration the result of the load cell reference?
Is the Ratio-metric an improvement over the absolute measurement technique.
thanks gogo
David Hess:
--- Quote ---Is the Ratio-metric an improvement over the absolute measurement technique.
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A ratiometric measurement is absolutely an improvement over an absolute measurement technique, but it is not always feasible.
--- Quote from: gogoman on August 09, 2020, 04:51:47 am ---"If you use a separate load cell excitation and load cell reference, then not only does this contribute another gain error term and noise source, but it prevents rejection of flicker noise which now must also be accounted for so a low drift and low noise design becomes more important.[/i]"
What is the reason that the flicker noise is not being rejected and is the drift and noise degeneration the result of the load cell reference?
--- End quote ---
If the load cell excitation is separate from the analog-to-digital converter reference, then errors from the two references including drift and noise are uncorrelated so they add. If they both use the same reference source, then drift and noise from the reference are correlated so gain errors cancel out. In practice this means that using the same reference for excitation and the analog-to-digital converter places much less of a demand on the performance of the reference, so even the drifty and noisy output from say an integrated regulator like a 7815 is sufficient. Of course there are still noise and drift errors from the circuits between the reference and excitation and reference and analog-to-digital converter, but those errors would exist anyway with separate references. And the reference is often the largest source of drift and noise anyway.
Flicker noise is especially a problem because it limits performance by increasing at lower frequencies which also makes it difficult to filter, so rejecting it is a big advantage. It is so named because it literally causes "flicker" in the least significant digit in a system which should otherwise be "noiseless". People are often used to the last digit flickering but it is the symptom of a real limitation if it varies by more than one quantization level.
Kleinstein:
The INA125 is not such a good choice for a 350 Ohms bridge, as it is quite noisy.
Also most 350 Ohms bridged don't like 10 V excitation. This would be just too much power and thus significant heating and thermal drift.
For 350 Ohms bridge I would more think about 1 V, maybe 2.5 excitation and an amplifier that is really low noise. There are also ADCs that include bridge amplification that is often better than the INA125.
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