ADS1220EVM: AIN1-AIN0 Negative Voltage Reading

Sercan Kerp

Part Number: ADS1220EVM

Hello;

We configured the EVM to be able to work with 3-wire PT1000 RTD. We used Fluke 712B RTD Simulator instead of a real sensor.

Here is the resistance and capacitance values:

RF1=RF2= R6=R7=4k7 Cdıff=100nF Ccm=10nF

RF3=RF4=R28=R29=1k Cdiff=100nF Ccm=10nF

Rref=R3=8k2

We routed IDAC1 from AIN2 to AIN0 and IDAC2 from AIN3 to AIN1. In order to have a 3rd lead we populated the R30 resistor as 0 ohm.

After that, common leads of RTD have been connected to RREFP0 and AIN1. The other one has been connected to AIN0.

When we measured the voltage between AIN1-AIN0, somehow it was negative but the digital conversions were looking good. Datasheet says AIN1 should be larger than AIN0, if Im not mistaken. We wonder how it is possible. What do you think ?

over 1 year ago

0 Bob Benjamin over 1 year ago

TI__Guru** 113765 points

Hi Sercan Kerp,

Can you provide the AVDD voltage you are using and the complete register settings?

Thanks,

Bob B

0 Sercan Kerp over 1 year ago in reply to Bob Benjamin

Prodigy 130 points

Hello Bob;

AVDD is 3.3V and the register settings as follows;

CONFIG_REG_0 : std_logic_vector (7 downto 0) :="00000110"

CONFIG_REG_1 : std_logic_vector (7 downto 0) :="11000100"

CONFIG_REG_2 : std_logic_vector (7 downto 0) :="01000011"

CONFIG_REG_3 : std_logic_vector (7 downto 0) :="01110000"

Regards

0 Bryan Lizon86 over 1 year ago in reply to Sercan Kerp

TI__Mastermind 42801 points

Hi Sercan Kerp,

Bob is out of the office until Tuesday, please expect a reply on Tuesday or Wednesday (local time) next week. Thanks for your patience

-Bryan

+1 Bob Benjamin over 1 year ago in reply to Bryan Lizon86

TI__Guru** 113765 points

Sercan Kerp,

Based on the configuration information the current should be flowing from AIN0 to AIN1 and not from AIN1 to AIN0. The ADC mux configuration is also connecting AIN0 to AINP and AIN1 to AINN, so the ADC conversion measurement should be correct.

The datasheet uses a slightly different connection scheme, but for the EVM you should follow the connection as shown in Table 7 of the ADS1220EVM user's guide.

Note in above that AIN1 is the negative input pin and AIN0 is the positive input pin and so for your external measurement you should be measuring from AIN0/AIN1.

Best regards,

Bob B

0 Sercan Kerp over 1 year ago in reply to Bob Benjamin

Prodigy 130 points

Thanks for the help Bob. Changing the config register as you said solved the issue. Now we're trying to improve the accuracy, It looks like we got up to +/- %1 C° accuracy over the EVM. Before designing our own board, we'd like to know what can be done in order to have a better accuracy(at least 0.5 C° as stated in Datasheet). Could you please lead us ?

Regards

+1 Bob Benjamin over 1 year ago in reply to Sercan Kerp

TI__Guru** 113765 points

Hi Sercan Kerp,

A good reference guide is A Basic Guide to RTD Measurements. This guide has lots of useful information for making RTD measurements.

One thing to verify is the level of noise in the measurement. The datasheet application uses a PT100 which allows for larger current to be used that can be helpful in overcoming noise. When using a PT1000, the larger resistance requires lower current to be used. Even with a ratiometric measurement the noise may be difficult to overcome without some additional filtering.

Lastly I would recommend calibration. First remove any offset of the ADC by using the internal mux short. You can then use a 2-point calibration to determine the gain slope for the sensor measurement.

Best regards,

Bob B

0 Sercan Kerp over 1 year ago in reply to Bob Benjamin

Prodigy 130 points

Thanks for your guidance Bob.

Kind Regards
Sercan.

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ADS1220EVM: AIN1-AIN0 Negative Voltage Reading