OPamp

Hi.

I am in need of some assistants, and was hoping that you guys might be able to help. I'm trying to find a suitable OPamp for a 45KG load cell, I've spent the last couple of days surfing the web hoping to find an answer and I sort of did, many of you suggest using the INA125 instrumentation amplifier, but here's the thing, I don't really need an accurate readout.

I'm in more of a need of weight detection than weight measuring, with a resolution of say 25KG. So I was hoping that a more basic OPamp would do. Or perhaps another device altogether if there is such a thing?

I do have some requirements for the amp, I'd like for it to work of 5-9V and have a adjustable gain using a resistor, if the output was fairly noise free.

Thanks in advance.

What is the output of your particular 45KG load cell?

I assume the load cell has no amplifier. If it has typical ratings, a MCP3422 or MCP3424 would work well.

The LSB (Least Significant Bit) at full gain corresponds to about 2 microvolts.

http://ww1.microchip.com/downloads/en/devicedoc/22088c.pdf

This is an 18-bit ?? ADC with Differential Inputs. It has these features:

On-Board Programmable Gain Amplifier (PGA) with gains of 1, 2, 4 or 8.

On-Board Voltage Reference (VREF) Accuracy: 2.048V ± 0.05%.

I2C interface.

Here is an old post with some test code http://forums.adafruit.com/viewtopic.php?f=31&t=12269

Thanks a lot for the tips, I'm in no real need of a ADC though, since I'll be hooking it up to the arduino.

EDIT: Hmm, haven given it more than 5 seconds of thought, I kinda see how it would work, maybe I should mention that I plan on using it in a Wheatstone Bridge configuration, measuring the 2 voltage differences between the resistors and amplifying that,

The load cell or strain gauge "not entirely sure which is which" have been scavenged from a bathroom scale, and is very similar to the ones that Sparkfun sell.

The load cell is a wheatstone bridge. Not sure why you'd be adding resistors?

If the cell has the typical four wire colors (red, black, green, white) then you would connect red and black to an appropriate voltage (typically 5V) and the remaining two into the Arduino analog pins. The difference between the analog readings varies with the weight applied. It's a very small difference; you might not be able to do this without some kind of amp.

what about a cheap rail2rail opamp? e. g.: ISL28130CEZ ( http://www.digikey.com/product-search/en/integrated-circuits-ics/linear-amplifiers-instrumentation-op-amps-buffer-amps/2556125 )

wired up as an amplifier... http://en.wikipedia.org/wiki/Opamp#Non-inverting_amplifier

The Arduino ADC just isn't a good choice for measuring a bridge, even with an amplifier.

The MCP3422 was designed as a single part solution for low voltage differential measurements.

The part costs a little over $3.00. Here are typical applications:

Temperature Sensing with RTD, Thermistor, and Thermocouple

Bridge Sensing for Pressure, Strain, and Force

Weigh Scales

why doesnt an amplifier help the arduino? it would be 2 times cheaper... the ISL28130CEZ can do G=10000 with 100nV sensitivity... according to the datasheet...

Chagrin: The load cell is a wheatstone bridge. Not sure why you'd be adding resistors?

I need to add resistors as a "dummy" load cell since the ones I have aren't "full-bridge" I could also combine 2 or more cells into a bridge but that seems like a waste of expensive parts to me.

Chagrin: you might not be able to do this without some kind of amp.

That's why I started this topic, it's less about the cells and more about the OPamp

RIDDICK: what about a cheap rail2rail opamp? e. g.: ISL28130CEZ ( http://www.digikey.com/product-search/en/integrated-circuits-ics/linear-amplifiers-instrumentation-op-amps-buffer-amps/2556125 )

wired up as an amplifier... http://en.wikipedia.org/wiki/Opamp#Non-inverting_amplifier

Thanks I'll look into those.

maybe u should use this topology http://en.wikipedia.org/wiki/Operational_amplifier_applications#Differential_amplifier for that bridge...?

I'm really starting to get that drowning sensation, you know water over your head :P

The ISL28130CEZ isn't "available" where I'm at, I can of course get a hold of it but I'd like to get things moving.

The LM358AN has been suggested by some, It's seems to do the things I need, Sadly I have little experience with OPamps beyond the Cmoy, so I'm not really sure.

RIDDICK: maybe u should use this topology http://en.wikipedia.org/wiki/Operational_amplifier_applications#Differential_amplifier for that bridge...?

I intend to

according to this datasheet http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000464.pdf (page 10/21 figure 22) u can use it for ur bridge...

can u show us a picture of ur assumptions about the internals of that "load cell"?

here u can do some simulations: http://www.falstad.com/circuit/

RIDDICK: can u show us a picture of ur assumptions about the internals of that "load cell"?

Something like this,

+ | R ------ Readout R

|

So my "assumption" is that it's a voltage divider circuit, You apply a voltage across the "resistors(R)" And read the voltage at the "readout",

My load cell isn't a full-bridge Wheatstone Bridge it's a half-bridge

Sorry I didn't understand you wanted to learn op-amp use.

I you just want typical bathroom scale accuracy the Arduino is fine. You won't need much amplification to get that level of accuracy.

The sensor in the scale is probably a half bridge like this https://www.sparkfun.com/products/10245.

Sparkfun has this link http://www.nerdkits.com/videos/weighscale/.

The video uses a full bridge sensor but you just need two resistors and/or a pot to complete the bridge. The video uses a true instrumentation amplifier, an AD620, which is expensive but easy to use.

18 - 24 bit delta-sigma ADCs are a better fit for this type application. A delta-sigma ADC requires less signal conditioning circuitry, less amplification, and has greater resolution than the Arduino SAR ADC.

There are lots of app notes like this on the web:

http://cds.linear.com/docs/Application%20Note/an96fa.pdf

http://ww1.microchip.com/downloads/en/appnotes/01030a.pdf

I don’t really need any higher resolution than 1 bit, I intend to use the load cell for weight increase detection not measuring, I’d like for my Arduino to detect if the base value of the load cell increases by say 10Kg not 5Kg not 1Kg, In fact I’d be very happy if it didn’t even notice a 5Kg increase.

So a high resolution or great accuracy is not something I’m in need of (Low noise is though).

The less expensive the better (not that I’m cheap I’m just thinking large scale).

I bet the ADC you talk about fat16lib, is great it might even make my life easier, And if you feel that It’s what’s best suited for my application than I’ll try it.

If you have three wires coming out, I think you have two resistor elements in the strain gage, one is on the piece of steel that strains, and the other is in another place where it is an offset for temperature correction.

They make up half of a wheatstone bridge.

growerdick: If you have three wires coming out, I think you have two resistor elements in the strain gage, one is on the piece of steel that strains, and the other is in another place where it is an offset for temperature correction.

They make up half of a wheatstone bridge.

You think?

Your assumption is sadly not correct.

The strain gauge is a voltage divider. It works alot like a potentiometer only the strain gauge only changes it's resistance value by a couple of ohms at full load. There is no resistor in there for temperature correction. However by using two Half-bridge gauges, one as a pressure sensor and the other as a reference for the amplifier you'd get your temperature correction.