ads1230 interfacing 8052

Hi Ajith,

This sounds like it may be a thermal equilibrium issue.  Can you share the details of your layout and packaging with us?

Hi Sir,

   As per your information,pcb layout file has been attached with this mail.

L1=100mH

L2=100ohm,(resistor)

L3=100ohms(resistor)

C8,C17,C18,C19=0.1 uF(box type capacitor)

Please go through that and give me necessary instruction.I m expecting your valuable reply.

     Thank you

     Ajit.. 

0218.silk.pdf8322.bottom.pdf7563.top.pdf

Hi Ajith,

There are a couple of things that are confusing to me.  The layout doesn't appear to match the actual pinout of the ADS1230.  Can you send me your schematic?  One thing that might be a problem is your cap values.  If you use both differential (cap across the inputs) and common mode caps (caps from inputs to ground) the common mode caps must be at least 1/10 the value of the differential caps or you will see some dramatic drift issues.  So if the differential cap is 0.1uF, the common mode caps should not be larger than 0.01uF.

Best regards,

Bob B

Dear Bob,

As per your information,schematic has been attached with this mail.I m facing the following issues :

1) The counts are gradually increasing upto 10 counts within 5 - 6 hours.So this reflects on the weight value.But i observed that, its due to temperature variation inside the weigh scale body(Regulator Ic temperature variation,when it connected to main supply).Because when the weigh scale operates on battery there is no such type of drifting.

 And if i connect electrolytic capacitor(100uF or 10uF) across the AVDD and DVDD for filtering,the same drifting issue was happening(during battery operation also).So later i removed those electrolytic capacitor and placed with 0.1uF capacitor.Now the counts are maintained in between + or -   1 or 2 counts drifting.....Is it possible to arrest this small drifting completely.

2)After calibrating using standard weight,the weight shows on display is lesser than the standard weight.

For Example:After calibrating a 10kg/1gm scale(load cell capacity is 15kg) ,if i put 1kg on the load cell it shows only 0.999gm on the display.Is this due to the counts fluctuating + or - 1 count.Also I m little bit confused that,my averaging process is perfect  or not.Below there shows the outline how i m taking the values,Please verify..

i)After offset calibration,i  take 20 bits of useful data.

ii)After taking 20 bits of data,i  divide this with 4.

Therefore display weight=(ADC_count - Zero_count - TARE_count) * ((Cal_weight)/(span_count - zero_count))

So in practical, display weight=(6000-2000)*500/(6000-2000)=500gm  (TARE_count not mentioned)

so if the ADC_count is fluctuating 1 count and hence the value is 5999,the value seems

                            display weight=(5999-2000) * 500/(6000 - 2000) = 499.875 gm, So if i take the integer value i get the value 499gm only.So 1 gm is missing.

How to fix this problem...Please suggest me some ideas...I m expecting your valuable reply...

Thank you..

By Ajith..

4812.sch_adc.pdf

Hi Ajith,

Thanks for the schematic.  You might want to consider placing a 0.01 to 0.1uF cap close to the reference input pins.

I'm not sure about your averaging.  Did you mean to say that you are taking four readings (each 20 bits) adding them together then dividing by four?  Are you doing a running average?

When calibrating, have you waited for the input to settle for both analog and digital filters before averaging the readings?  You must wait for the input to settle before averaging.  Averaging helps with noise, but will not correct settling error.

Can you clarify one other thing...in your equation, is the span_count the count with the calibrated weight applied?  I normally think of the span as being the number of counts adjusted by zero, or the measurement range of counts.  You have to be really careful with TARE and ZERO counts so that your equation is adjusting correctly.

It appears that noise or settling may be an issue initially.  For example, if the reference is not stable or the filtering not settled when calibrating you will see an error after the reference is stable or the filter is settled.  I think that is why you see a difference with the larger caps.  With the smaller caps the supply settled faster.  As the supply is being used for your reference, this will shift slightly over time.

Best regards,

Bob B

Dear Bob,

Thank you for your reply..I had provided a 0.1uF capacitor,but it is not closer to reference pin(around 1 inch distance).would this affect the adc performance..?

Could you please explain, how to perform averaging,Actually i m not doing any averaging.I m directly taking the value after 20 clock pulses and process it,to display the weight and also to achieve good speed at 10SPS.(was it a wrong procedure).

Here after getting the value (after 20 clock pulses) i  divide that value by 4,to view the counts in display before doing calibration..

Does Averaging reduces the processing speed of the controller .?

What is running average ?,could you please give me the details..!

I m expecting your valuable reply..

Thank you 

by Ajit..