You forget that the AD7730 expects a bridge configuration being connected to the inputs! This means, that not only a differential voltage must be present between AIN1(+) and AIN1(-), but also a common mode voltage between both these inputs and AGND. So, to provide the inputs with a suited test voltage you must also simulate the common mode voltage of bridge.

You can achieve this by the following procedure:

Connect REF IN(+) and AVDD to DVDD. Connect a 470R resistor from this point to AIN1(+). Connect a 1R resistor from AIN1(+) to AIN1(-). Connect a second 470R resistor from AIN1(-) to AGND. Connect REF IN(-) and AGND to DGND. Mount these parts and connections in closest distance to the AD7730 and route them along the analog side of chip!

If DVDD is now 5.0V, then a common mode voltage of about 2.5V is generated, being well in the allowed common mode input voltage range of AD7730. Also, a differential voltage of about 5.3mV between AIN1(+) and AIN1(-) is produced.

Now, take a look what your AD7730 outputs...

By the way, you haven't omitted the 10µF tantal in parallel to 100nF ceramics at DVDD and AVDD?

Connect a scope probe between REF IN(+) and REF IN(-). If there's the least amount of noise, then REF IN(+) and its "load cell" connection must be low pass filtered!


Kai