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Trouble Configuring AD7738

Hi all,

First time poster here, long time chip user. 

I'm having some trouble getting my AD7738 to work (I do not have the eval kit).

I am not seeing anything on my DOUT.  Below is an view of DOUT, DIN, RDY, and CS.

to configure it I am sending in

0x00, 0xFF, 0xFF, 0xFF, 0xFF to reset it and then

0x01   IO port register, write

0xF0   Configure IO port register

0x2F   Channel setup register, AIN7, write

0x1C  Configure Channel setup register

0x37   Channel conversion time register, AIN7, write

0x91   Std. value

0x3F   Mode register, write (0x3F)

0x38   Continuous conversion, no clamp (0x38)

and then 0x4F every time RDY is polled.

but all I see is 0x00 in my rxbuffer, occasionally it goes 0xFF.

and the DOUT looks terrible.  I'm at a loss.

my pin configurations are also




CS -> SS


AVdd -> 5V

AINCOM/P0 -> OPEN (is this correct?)

SYNC/p1 -> OPEN (is this correct?)

AIN0-AIN7 -> AIN7 connected to signal generator



REFIN(+) -> 2.5V (about.. actually 2.38V)



RDY -> ISR set to detect inverted rising edge and send out 0x4F



DVdd -> 5V


I attached a graph for my 0x4F polling and for my initial setup.  the DOUT is sporadically working.. leading me to think I may have wired something wrong. 

Even more confusing, half the time the rxBuffer ISR won't be triggered and other times it will.. with identical code.

Any help at all would be great.  I am new to this chip and its kicking my **** and leaving me scratching my head.

thanks in advance,

  • Hi Scott,

    AINCOM/P0 -> OPEN (is this correct?). No, this is configured as an analog input and is the GND reference for the analog inputs. Connect to GND.

    SYNC/p1 -> OPEN (is this correct?). No, tie all logic inputs to high or low voltage level, never leave input floating.

    Regards,  Brendan.

  • Ok thank you,

    I also figured out I wasn't using SPI correctly.  I have to send out zeros to receive the DOUT.. although I now get much more consistent data on DOUT I still do get the decaying effects after about 3 or 4 DOUT recieves..

    I'll post more later tonight to confirm if connecting these pins fixes my problems.



  • I have tried the above, but I am not seeing any difference..

    something I did notice however, is my SPIM does not let SS go high between transfers smaller than 4 Bytes.. does the SS pin HAVE to toggle inbetween bytes?  I am transferring as many bytes as I expect to see (3 in the 16 bit with DUMP on case) everytime the RDY ISR is triggered.

    Thanks for the help,


  • Hi Scott,

    The decaying effect on DOUT is due to the pin going to the high impedance state and if not pulled up.

    The CS pin can be used to select the AD7738 as one of several circuits connected to the host serial interface. When the CS is high, the AD7738 ignores the SCLK and DIN signals and the DOUT pin goes to the high impedance state. When the CS signal is not used, connect the CS pin to DGND.

    Regards, Brendan.

  • Thanks Brendan! That makes sense, I studied the data being received and it seems to be correct now.  I was finally able to get my circuit to work correctly. 

    Although I think I may have found my efforts to be in vain..  I was looking for a chip that could do three 18-bit channels with a Nyquist minimum of 1kHz...

    After further studying the data sheet I think this chip cannot do what I require.  Am I right?

    Could anyone recommend a similar ADC that could do that?  I would prefer one capable of doing three (or more) 18-bit channels with a Nyquist minimum of 5kHz if possible.

    I found the AD7176-2 but it seems significantly different.  Not that I'm lazy, but time is of the essence with this project and this is only the beginning and there's already a set back.

    Any input on this would be a tremendous help!

    Thanks again,


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