Hello all,

I am working in a weight scale for persons, that is a range of aprox 3kgr-120Kgr.

I am using AD7796 with exactly the same schematic that can be found in Fig. 17 of pag. 23 of the datasheet. Power supply is 5V.

The load-cell sensor has a sensibility of 2mV/V +-10% and a range 0-200Kgr and powering at 5V.

I am not getting the results that I would expect. I think the problem comes in the process to calculate the weight.

I think I should do:

(1) Reset AD7796 (writing 32 consecutive 1s to the device)

(2) Check if the device is there reading from ID Register 0x5A. If correct I continue with the process.

(3) Set the Configuration register to 0x1710, that is unipolar and channel AIN(+)-AIN(-)

(4) Develop an internal zero calibration, setting Mode Register to 0x800A (internal clock 64Khz, update rate 16.7 Hz)

(5) I take 10 measurements with the weighing hopper empty. Calculate the average of the counts

and store these to rest this quantity to other measurements. This is an offset.

(6) I put on the hopper a known weight of, for example 40Kgr, an take another 10 measurements, calculate the average, rest offset of point (5) and store it. I have so a point (X1,Y1), (weight, ADC output)

(7) I understand that to calculate any unknown weight I need another point, (X2, Y2) and then calculate the slope of the line

passing for these two points. M= Y2-Y1/X2-X1

(8) Having the M, I can calculate any unknow weight (X, Y) with Y=MX +B, becaue X is the ADC out for the unknow weight Y,

M is the slope and B is the weight when the ADC out is zero, so weight 0, B=0.

So di I need another measure with other known weight (for exmple 20Kgr) to get (X2,Y2) or is there any other method?

Can anyone tell me what is wrong? Should I use another method to calculate the weight? Is there any Application Note describing this process fgoing from ADC count to weiths?

Thanks in advance,

Joaquin.

Hi, Joaquin.

Usually, we calibrate the system or load cell using the maximum weight followed by the minimum weight that your load cell can measure and use these known weight values as your two points for calculating the equation for weight measurements. So measured the maximum weight ( i.e. Full scale value, FS) and then measured the minimum weight which is an empty pan ( i.e. Zero Scale value, ZS) and then using the two-point formula for linear equation you will come up with the equation: Weight = (Maxweight/(FS - ZS))*(Code - ZS).

Thanks,

Jellenie