PGA309: Offset

Hi Guillaume,

The input referred offset of the PGA309 is 50uV with a typical drift of +/-2uV/C. Over a temperature change from 25C to 125C, the typical input referred offset error change should be in the range of ~200uV.

What are you gain Settings for the Front End PGA , the Output Amp Gain settings? What is the coarse offset DAC setting and Reference settings? Do you have an schematic of the prototype?

If you can detail the steps you have followed during the calibration process, this may be helpful

Most issues we encounter are due to issues related to the calibration sequence and/or software. If everything works properly and the error only occurs once in a while, a noise disturbance or glitch could be occurring during the calibration process.

Thank you,

Best Regards,

Luis

Hello, 

I can't send our schematic on this forum, but these are the register values used and the EEPROM values.

For the moment there is no program. I compensate manualy (using sboc320c.zip to change the registers manualy and to program the EEPROM)

The steps I do :

1/ Program EEPROM with standard values (coarse offset, Input Gain, Out put Gain, Temperature mode....)

2/ PGA Test Mode

3/ Temperature @-55, tune Gain DAC and Offste DAC (pressuse : 0 and FS) and record temperature Hex value, Gain DAC and Offste DAC

4/ Temperature @-10, tune Gain DAC and Offste DAC (pressuse : 0 and FS) and record Temperature Hex value, Gain DAC and Offste DAC

5/ Temperature @35, tune Gain DAC and Offste DAC (pressuse : 0 and FS) and record Temperature Hex value, Gain DAC and Offste DAC

6/ Temperature @80, tune Gain DAC and Offste DAC (pressuse : 0 and FS) and record Temperature Hex value, Gain DAC and Offste DAC

7/ Temperature @125, tune Gain DAC and Offste DAC (pressuse : 0 and FS) and record Temperature Hex value, Gain DAC and Offste DAC

8/ Calculate with exel a polynome for the Offset DAC vs Temperature and a polynome for the Gain DAC vs Temperature in order to calculate intermediate gain and offset values between each tuned temperature (to use all temperature steps of the EEPROM table.

9/ Calculate with exel all the values to send to the EEPROM

10/ PGA Stand-Alone Mode 

11/ Program the EEPROM

12/ Measure the sensor 

When I tune the sensor in test mode, the error is less than 1mV for each temperature or pressure step. But when  I measure the sensor a few days after the result is 10mV lower ...

REGISTRE 0	5449	21577	TI Flag
REGISTRE 1	0000	0	ZeroDAC
			0
REGISTRE 2	0000	0	GainDAC
			0,3333333
REGISTRE 3	0700	1792	Lin Range (EXS)
			Range 0
			Vexc Enable (EXEN)
			Enable
			Internal Vref (RS)
			2,5
			Internal Vref (REN)
			Internal Ref
			Bv (%FS)
REGISTRE 4	0613	1555	OWD
			Enable
			Gain Out GO
			2
			Front-End Gain GI
			64
			Coarse Offset
			-0,006375
REGISTRE 5	0000	0	CLK mode (Noise)
			00
			EXTEN (Enable External faul comparator)
			Disable
			INTEN (Enable Internal faul comparator)
			Disable
			EXTPOL (Out polarity when external fault)
			Force Low
			INTPOL (Out polarity when internal fault)
			Force Low
			OUEN (Over/Under scale Limite)
			Disable
			Overscale Threshold
			2,427
			Overscale Threshold
			0,064
REGISTRE 6	140D	5133	ADC2X (Temp conversion speed EXTERNAL sense)
			6ms
			ADCS (Temp conversion START conversion)
			Single Start
			ISEN (TEMPin current source 7µA)
			Disable
			CEN (Temp Continous Conversion Mode)
			Enable
			TEN (Intenal Temp Mode)
			Disable
			AREN (Intenal Temp Reference)
			Disable
			RV (External Temp Reference select)
			VREF
			M (Temp pin selection)
			TEMPin,GNDa
			G (Temp Gain)
			8V/V
			R (Temp Resolution)
			Low

register.xlsx

Addr0=21577
Addr2=0
Addr4=0
Addr6=1792
Addr8=1555
Addr10=0
Addr12=5133
Addr14=35478
Addr16=3649
Addr18=15548
Addr20=61989
Addr22=3760
Addr24=355
Addr26=59814
Addr28=3854
Addr30=338
Addr32=60027
Addr34=3949
Addr36=321
Addr38=60227
Addr40=4043
Addr42=305
Addr44=60424
Addr46=4138
Addr48=291
Addr50=60623
Addr52=4232
Addr54=275
Addr56=60819
Addr58=4327
Addr60=261
Addr62=61020
Addr64=4422
Addr66=243
Addr68=61216
Addr70=4516
Addr72=229
Addr74=61415
Addr76=4611
Addr78=213
Addr80=61615
Addr82=4705
Addr84=199
Addr86=61813
Addr88=4800
Addr90=181
Addr92=62009
Addr94=4894
Addr96=169
Addr98=62208
Addr100=4989
Addr102=151
Addr104=62407
Addr106=5083
Addr108=133
Addr110=62606
Addr112=5194
Addr114=120
Addr116=62819
Addr118=32767
Addr120=0
Addr122=9331

Hi Guillaume,

  When compensating for the offsets and gain errors manually, which algorithm are you using to perform the Gain DAC and offset DAC corrections? Is it the same as the algorithm attached below?  The document attached below details the mathematics and register writes behind the calibration process on the PGA309EVM. 

PGA309_calibration_procedure.pdf

What type of meter are you using to perform the measurements during the calibration process?

Please let me know if you wish that I contact you directly via email so we can review your schematic.  I will also continue reviewing the register settings provided.

Thank you and Regards,

Luis

Hi,

The algorithm I use is quite the same your document. The only difference is that I calibrate at five temperature steps and I use GainDAC and OffsetDAC polynomes to add values in the table-lockup (EEPROM).

I use a keithley 2750 with a switch card (it s like a 6 digits +1/2 DMM)

yes, I would like to be contacted by email.

Did you find something abnormal in the registers I provided ?

Thanks

Guillaume

Hi Guillaume,

I will contact you directly via email to per your request.

Thanks and Regards,

Luis