AD7731 via Arduino SPI

Brendan,

     I've attached a diagram showing our connections to the AD7731.  We are pretty much following Figure 21 on p. 39 of the datasheet.  We're using the AD780 to supply the VREF and a 4.9152 MHz crystal across the MCLK pins, with 33 pF caps to ground from each.  On a scope, the MCLK out signal (wrt ground) is a quasi-sine wave with 1 V amplitude with a +2V offset from ground (goes between 1 V and 3 V).  We don't use optoisolators between the Arduino and the AD7731.  The filter register is unchanged from its power up values, which the data sheet says should give an update rate of 600 Hz if my calculations are correct (I haven't yet had the code do a read of the filter register).  I see nothing on the scope or on the logic analyzer to indicate that the /RDY pin is going low at any time in the process.  If I use a DMM to monitor /RDY, I do see a quick few mV drop that seems to be cyclical (remember, the Arduino code continuously loops), but that could be crosstalk from some other line (like the /CS).

We are using a powered breadboard, which isn't the most quiet of electronic environments, but I hesitate to solder up a board with ground planes and everything if we can't make it work at all under these conditions.

2441fa37fe78dfa939b5c82be308f798.tiff

Thanks,

George

Brendan,

     I've attached a diagram showing our connections to the AD7731.  We are pretty much following Figure 21 on p. 39 of the datasheet.  We're using the AD780 to supply the VREF and a 4.9152 MHz crystal across the MCLK pins, with 33 pF caps to ground from each.  On a scope, the MCLK out signal (wrt ground) is a quasi-sine wave with 1 V amplitude with a +2V offset from ground (goes between 1 V and 3 V).  We don't use optoisolators between the Arduino and the AD7731.  The filter register is unchanged from its power up values, which the data sheet says should give an update rate of 600 Hz if my calculations are correct (I haven't yet had the code do a read of the filter register).  I see nothing on the scope or on the logic analyzer to indicate that the /RDY pin is going low at any time in the process.  If I use a DMM to monitor /RDY, I do see a quick few mV drop that seems to be cyclical (remember, the Arduino code continuously loops), but that could be crosstalk from some other line (like the /CS).

We are using a powered breadboard, which isn't the most quiet of electronic environments, but I hesitate to solder up a board with ground planes and everything if we can't make it work at all under these conditions.

2441fa37fe78dfa939b5c82be308f798.tiff

Thanks,

George