Voltage Amplification Circuit - Help?

[Monkey_Beast]

Hi, I'm looking to build a circuit to act as a voltage amplifier, taking 5mV output from a load cell, and amplifying up to 5V for use with an Arduino to log values on my laptop.

Load cell output (op amp input) resistance is 350 Ohm
load cell datasheet
I'd be using a 10 V input for the load cell, producing +/- 5mV output from the load cell.

Op Amp I'm looking at is here, and tech sheet is here.

I'd be looking at connecting to a car, so ideally, the power voltage would be ~12V.

Would someone be able to help with;

a) power source diagram, taking the car voltage and rectifying to +12V, +5V and +10V?

b) Op Amp Circuit, using the requirements above.

Any advice would be greatly appreciated!

Thanks

MB

[pebe]

I assume the load cell gives diffential outputs from a bridge arrangement, but there is not enough information on the data sheet to be sure.

Have you got a link to an application sheet for the device?

[Monkey_Beast]

I assume the load cell gives diffential outputs from a bridge arrangement, but there is not enough information on the data sheet to be sure.

Have you got a link to an application sheet for the device?

Only link I have is here.
You are correct. It uses a wheetstone(sp?) bridge arrangment in the Load Cell to produce the Voltage difference. The load cell is rated for 300kg, with an excitation value between 5-12V DC. Inputting 10V would give an output upto 5mV if at max load, due to the 2mV/V ratio of the component.

I hope this helps.

[pebe]

Here is the sort of setup you need.

http://www.allaboutcircuits.com/vol_3/chpt_8/10.html

BTW, at an output of 2mV/V, won’t 10V applied to the bridge give 20mV output – not 5mV?

The LM224N is not really up to doing the job, because its 5mv input offset is comparable to the signal you are measuring. I think you need a chopper-stabilised amplifier such as the AD8574 that maintains a near zero input offset voltage. Here is a link to its datasheet.

http://www.analog.com/static/imported-f ... 2_8574.pdf

There is a circuit in its ‘applications’ section for a strain gauge amplifier.

[Monkey_Beast]

Awesome. Thanks Pebe!

[Monkey_Beast]

Right, I've had a look and I can see roughtly what I need to do. The only problem I have is that the example on the tech sheet is for a 5V input, where as I would be looking at a stabilised 10V Voltage for the Driver, and would hope for a 5V output from the setup at the end. I really could do with someone telling me exactly what I need to build - which components, etc. I would want to have a high precision, stable 10V input, so I'm not sure how to go about building that. I've seen Zener Didodes mentioned, but I can't imagine they'll provide a stable enough input voltage.

Could someone help design this circuit for me, along with a component list?

MB

[pebe]

I'll come up with some circuit details in about a week.

[Monkey_Beast]

I'll come up with some circuit details in about a week.

Epic! It'd probably take me a lot longer than that, and when I powered it up for the first time I'd probably release the magic blue smoke from every component in the set up!

[pebe]

Just a small point. When designing the cell supply I need to know the lowest voltage that your 12V battery will be supplying when running this equipment, ie. the lowest voltage it is likely to be when discharged.

[Monkey_Beast]

Just a small point. When designing the cell supply I need to know the lowest voltage that your 12V battery will be supplying when running this equipment, ie. the lowest voltage it is likely to be when discharged.

Yeah, I've been thinking about that. I think to be safe, I'd use it with the engine running, so 11V minimum, but I know a car can get as high as 14V with a decent alternator.

[pebe]

Here is a circuit that I suggest for your strain gauge amplifier – sooner that I thought!

The amplifier is the classic three-chip design using an LTC1053CN. This is a quad chopper-stabilized amplifier that has offset voltages better than 2microvolts. IC2 and IC3 together amplify the differential inputs from the cell. The gain is determined from A = (R1+R2)/R3. With a 10V supply the cell will give a maximum swing of 0 to 20mV. You need a swing of 0 to 5V so the gain needs to be 250. With the values given, A = 2 x 15K/120 = 250. Those are followed by IC3 operating at unity gain that converts the differential voltages to a single output suitable for your micro.

Even using 1% resistors, calibration will be required so, rather than fitting a trimmer as part of R3, I thought it easier to change the cell output as necessary by modifying its 10V supply voltage. So the cell (and amps) are run from an adjustable voltage regulator.

I suggest using an LM117 regulator IC. It has an adjustable output voltage and a 1.75V dropout voltage (the difference between your battery voltage and the maximum voltage you can get from the IC). So if you are using your car battery with the engine running, its voltage will be nearer 13V and you will be able to get up to 11.25V from the IC. It provides a very stable voltage, independent of load. Its output voltage is set by the potential divider R4/R5/R6. This provides a feedback voltage of 1.25V to the ADJ pin, so depending on the setting of R4 it will give between 9.75V and 11V at the output. R4 is set nominally to give +10V and adjusted from there to calibrate the cell.

Here are the datasheets for your further information.

http://www.datasheetcatalog.org/datashe ... 009063.PDF (LM117 Regulator)

http://www.datasheetcatalog.org/datashe ... zcpl7y.pdf (LTC1053CN) (CN= in small package)


Just a few points to note. Use ¼W metal film 1% resistors. Fit R1, R2, and R3 as close to their respective pins as possible. Keep leads as short as possible. Use a high-grade pot for R4. Fit the capacitors either side of the LM117 as close to the pins as possible, and be sure to use a tantalum for the 1µF. If the op-amp is more than a couple of inches from the 1µF tantalum, then fit a 100nF across the supply pins to the op-amp - as close as possible.

And finally, best of luck.

[pebe]

M_B, did you find the circuit useful?