AD8295 to AD7690 Reference Design Problem

I recently built the reference design included with the AD8295 datasheet, which uses the AD8295 to differentially drive an AD7690 ADC chip.  This is a very compact implementation but I can't seem to get it to work correctly.

That said, I notice that the evaluation board for the AD8295, EVAL-CN0225, implements a different approach using an AD8275 to generate the differential signal driving an AD7687. I'm not able to find any confirming implementations for the AD8295/AD7690 combination but I haven't seen any web references suggesting that the design has any problem or needs additional components.

I have attached my schematic of the amplifier section.  I have four of these channels in a compact area that is space constrained.

Can anyone comment on this in general?  I can be more specific later.

Tim

AD8295.pdf
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  • 0
    •  Analog Employees 
    on Jul 13, 2012 1:56 AM

    Hi Tim,

    The AD8275 is a G=0.2 difference amplifier. The main function in the CN-0225 circuit is to provide the attenuation needed to match the +/- 10V industrial signal range to a +/-2.5V input differential ADC. Other than this attenuation, the two circuits are very similar. The output of the AD8295 is a gain of 1 difference amplifier, and A1 in CN-0225 is connected the same way that A1 is connected in the AD8295 datasheet to provide the differential output.

    Note also that the attenuation provided by the AD8275 also attenuates the noise level of the in-amp. You may find that you need to reduce the noise bandwidth of your amplifier as well as your sampling rate by quite a bit in order to get true 18-bit performance. Increasing the resistor too much increases the distortion of the ADC, but increasing the capacitor too much can be a heavy load on your amplifier at your maximum frequency of interest.

    Regards,

    Scott

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  • 0
    •  Analog Employees 
    on Jul 13, 2012 1:56 AM

    Hi Tim,

    The AD8275 is a G=0.2 difference amplifier. The main function in the CN-0225 circuit is to provide the attenuation needed to match the +/- 10V industrial signal range to a +/-2.5V input differential ADC. Other than this attenuation, the two circuits are very similar. The output of the AD8295 is a gain of 1 difference amplifier, and A1 in CN-0225 is connected the same way that A1 is connected in the AD8295 datasheet to provide the differential output.

    Note also that the attenuation provided by the AD8275 also attenuates the noise level of the in-amp. You may find that you need to reduce the noise bandwidth of your amplifier as well as your sampling rate by quite a bit in order to get true 18-bit performance. Increasing the resistor too much increases the distortion of the ADC, but increasing the capacitor too much can be a heavy load on your amplifier at your maximum frequency of interest.

    Regards,

    Scott

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