ADC for strain gauge measurement

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I have strain gauge of 1K, 50000 microstarin , & Gf = 2, so for 1 microstarin change in resistance is 2milliohm.
I am using external reference created by IDAC current on reference pins

1. Currently i am using ADS1248, by directly passing 50uA through strain gauge without any bridge, with PGA = 32

So ideally at 1000ohm i.e 0 strain, voltage is 50000 microvolt

& 1 microstain voltage is 50000.1 microvolt. So change of 100nV.

Will ADS1248 will be able to sense such small changes?

2. If not, is there any ADC which can do this job better

3. Is is correct method to directly send current across strain gauge & measure change in resistance rather than brigde

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That is why op-amps were invented.

It all starts with a mental vision.

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After trying several models, we settled on AD7190:
https://www.avrfreaks.net/index.p...

Quote:
BTW, a decade ago we'd use an instrumentation amp and gain control and filtering and software filtering and ... Now we build in a chip basically made for load cell apps. ...

We always use a bridge.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Atmega1280 wrote:
I have strain gauge of 1K, 50000 microstarin , & Gf = 2, so for 1 microstarin change in resistance is 2milliohm.
I am using external reference created by IDAC current on reference pins

1. Currently i am using ADS1248, by directly passing 50uA through strain gauge without any bridge, with PGA = 32

So ideally at 1000ohm i.e 0 strain, voltage is 50000 microvolt

& 1 microstain voltage is 50000.1 microvolt. So change of 100nV.

Will ADS1248 will be able to sense such small changes?

2. If not, is there any ADC which can do this job better

3. Is is correct method to directly send current across strain gauge & measure change in resistance rather than brigde

Why are you using such a low excitation current? If you want to conserve power then pulse the excitation so it is only applied briefly for the duration of the measurement. You can also apply the switching to reverse the polarity to compensate for thermoelectric error voltages.

kevin

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You really want to use a bridge configuration to compensate for expansion/contraction of the base metal with temperature. This also compensates for some thermoelectric effects. At such low voltages - just about every connection is a thermocouple.

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@theusch,

how do you measure such small changes.

A) Like for 1K strain, Gf = 2 , strain range = 5%
1 unit change in strain = change in resistance is 2miiliohm.

B) Like for 350ohm strain, Gf = 2 , strain range = 5%
1 unit change in strain = change in resistance is 750microohm.

Since I have battery powered application, so I used constant current source of 50uA. It gives very low voltage drop across them.

What excitation voltage you use.
Do you use any opamp in front

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Quote:

how do you measure such small changes.

Dunno in our current (pun intended) applications, as they use a load cell. All we provide is an excitation voltage and a ground. The bridge and the like are in the load cell. The output goes directly into the AD7190 which is configured as needed.

I think the type is a "shear beam load cell". We get them OEM so I don't have a part number or datasheet for you to examine.

That said, typical output is 3mV/V. So that would be a full-scale swing of 15mV. Let's say 75kg and 16-bit/64k counts. That would be 1 count per gram. 200uV per kilogram; 0.2uV per gram.

In practice, the app shows US pounds. I measure to centi-pounds, or about 1/6 ounce, and then log totals to the ounce.

Another app with the same sensor is scaled to record to 0.1 ounces and the AD7190 is giving values to 0.01 ounce. IIRC I'm throwing away the low four bits (might be six) in that app.

Getting back to your latest post, I'd have to find a datasheet to see what bridge values and resistance changes result in 3mV/V. But you can see from the above that I can indeed resolve small changes with a pre-packaged load cell along with the AD9190, suitably configured.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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A cheapo 20kg eBay listing, about US$5, is 1mV/V:
http://www.ebay.com/itm/Electron...
Rated Load: 20Kg
Rated Output: 1.0mV/V±0.15mV/V
Zero Output: ±0.1mV/V
Creep: 0.03% F.S./30min
Input End: Red+(power), Black-(power)
Output End: Green+(signal), White-(signal)
Recommended operating voltage: 3 ~ 12 VDC
Maximum operating voltage: 15 VDC
Input Impedance: 1115+/-10%
Output Impedance: 1000+/-10%
Protection class: IP65

I haven't done a straight "strain gauge" app in over 10 years so I probably cannot help much more.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.