ADS127L11: Not Getting resolution in microvolts.

Hi Haresh Prajapati,

Can you please provide a schematic for your system so we can identify any issues?

Also, can you describe in detail the source you are using to generate the 10mV signal?

Please share the data you are receiving from the converter as well so we can see how the output changes when you apply a 1V signal and the 10mV signal, for example

-Bryan

Basically we are generating current signal of 250 micro amp. from cypress microcontroller,  cy5688AXI-LP099. and passing this current to a 15 ohm resistance. Then we are measuring voltage across the resistor by using ADS127L11, and controller through SPI communication. We are using reference voltage 1 V. The voltage across the resistance is 250 micro amp * 15 = 3.75 millivolt . So the count should be According to this. But we are not getting the count for this. We are generating voltage from programmable DC power supply, 72-13360.

When we apply direct voltage 1 V to this ADC , we received count = 8224124

When we apply 500 mV to ADC, we received count = 4078166

When we apply 250 mV to ADC, we received count = 2039538

When we apply 100 mV to ADC, we received count = 789516ADC_Reading.pdfADS_PCB.zip

When we apply 50 mV to ADC, we received count = 394758

When we apply 25 mV to ADC, we received count = 197122

When we apply 10 mV to ADC, we received count = Not receiving value.

I am attaching schematic for connecting the 15 ohm resistance, passing 250 micro amp current through it, read the voltage across this resistor.

I am also attaching the code for giving 1 V, 500 mv , 10 mV directly, and reading ADC count from SPI communication for cypress microcontroller.

now we are given voltage from programmable dc power supply directly to the ADC pin. we are using ref voltage 4.829 V. and reading the voltage by cypress controller. But we are unable to get the resolution even in millivolts. I am sending to you the input voltage , getting counts and calculated voltage to you. i am using same code as I sent to you. Please give reply as soon as possible. we want resolution in microvolts for our application.

input voltage               count           calculated output  voltage

4829 millivolt          8355711          4829 millivolt

4730                       8158332          4714.929133858268 

4720                       8158332          4714.929133858268 

4620                       7960953          4600.858267716536

4600                       7960953          4600.858267716536

3670                        6316128          3650.267716535433

3650                       6316128           3650.267716535433

3600                       6184542           3574.220472440945

2360                       4079166          2357.464566929134

2350                      4013373           2319.4409448818897

45                          65793               38.023622047244096

40                          65793               38.023622047244096

38                          65793               38.023622047244096

35                          0                        0

30                           0                       0  

20                           0                       0

Hello Haresh,

This appears to be a problem with the driver/code, since you are getting the exact same reading from the ADC 3 times each.  With the same input voltage, you should always read a slightly different value due to noise.

Please connect a logic analyzer to your board and capture the following signals over a full SPI frame, similar to figure 8-33 in the ADS127L11 datasheet.

/DRDY, /CS, SCLK, SDI, SDO

Also, I took a second look at your schematic and noticed an error: you have IOVDD and CLK pins connected incorrectly.

Also, according to your schematic notes, VCC_3 is a 5V supply.  I assume you intended to connect this to IOVDD, in which case your IO pins will operate at 5V logic levels.  Please confirm that this is compatible with your MCU logic level requirements. 

Regards,
Keith

Thank you for your great support. We find out that(by inspection at logic analyzer) we are receiving only 8 bytes because Cypress microcontroller supports only 8 bits data frame. That's why we are receiving only MSB 8 bits. Can you help us to find out the way how can  receive 24 bits from Cypress microcontroller. We have attached code for it. Can you please see that to find the solution?