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Problem in Daisy-Chaining of 2 ADS1278 Devices

Mahesh Hegde
Intellectual 400 points
Other Parts Discussed in Thread: ADS1278, DAC8568

Hello All,

I have connected 2 ADS 1278 in a daisy chain format. I am facing the some channels digital data problem in my 2nd 1278 device. The problem is explained below. 

The configuration pin status are like this.

mode0                  =             '0';

mode1                  =             '1';

format0                =             '1';

format1                =             '0';

format2                =             '0';

clkdiv                     =             '1';

Test0                     =             ‘0’;

Test1                     =             ‘0’;

The voltage details for the IC’s are as follows.

VREFP                   =             2.5 V

VREFN                  =             GND

VCOM                   =             2.5 V (The output voltage from both the IC’s)

AVDD                    =             5 V

IOVDD                  =             3.3 V

DVDD                    =             1.8 V

 

DRDY & DOUT pins are connected to FPGA. I am reading all the channels through SPI format mode. The clock frequency 3MHz (FCLK) & SCLK switching frequency is also 3MHz. The ratio I have used is 1:1;

 

I have carried out the below tests to verify the 1st ADS1278 channels. 8 channels from 1st ADC’s are working as mentioned above.

  • The working channel digital data is as follows

 

AINP                      AINN                     Vin=(AINP-AINN)        Digital data(Theoretical)              Digital data(Practical)

3.75 V                    1.25 V                    2.5 V                                   0x7FFFFF                                       0x1FAEA7(MSB is constant & LSB is varying one example data is taken)

2.5 V                      2.5 V                      0 V                                       0x000000                                       0x000234(MSB is constant & LSB is varying one example data is taken)

1.25 V                    3.75 V                    -2.5 V                                  0x800000                                      0xE04643(MSB is constant & LSB is varying, one example data is taken)

 

  • I have feed the DAC (DAC8568) output to ADS 1278 channels. I have generated the sine wave and that is feed to the ADS1278 channels and I am able to see the sine wave in my ADS1278 channels (1st ADS1278 IC), when I plot the graph

 

But 2nd ADS1278 IC channels behavior is not constant and its varying from board to board. In some of the channel data is not varying if I feed the analog input also. It will be always straight line in my graph plot(digital data is also constant).

The non-working channel digital data is as follows.

 

AINP                      AINN                     Vin=(AINP-AINN)         Digital data(Theoretical)                 Digital data(Practical)

1.25 V                    3.75 V                    -2.5 V                                  0x800000                                      0xE00000(Here LSB & MSB both are constant it’s not varying)

1.5 V                      3.5 V                      -2 V                       Digital  Value < 0x800000                               0xE00000 (Here LSB & MSB both are constant it’s not varying)            

 

This kind of behavior I have observed in 2nd 1278 channels. As of now I have not seen the misbehavior in the 1st 1278 channels(first 8 channels).

 

Please help me out to solve this problem..



Regards,

Mahesh Hegde