Request for Review - Analog to Digital Board

[kaevee]

I have designed the attached schematic for digitizing analog outputs less than 5V.

I am using OPA189 as low-pass filter and ADS8866 16bit ADC.

Please review and help me validate the design.

[Slh]

Not many specifications to review it against there. Do you have a minimum voltage that you want to measure? Eg input range 50mV to 5.0V.

An important thing to check is that the common mode input range of the opa189 only goes to within 2.5V of the positive rail so it's only going to work to around 2.5V.

[kaevee]

Not many specifications to review it against there. Do you have a minimum voltage that you want to measure? Eg input range 50mV to 5.0V.

An important thing to check is that the common mode input range of the opa189 only goes to within 2.5V of the positive rail so it's only going to work to around 2.5V.

I want to measure 0.5v to 4.5v.

Updated for comment on common mode input range: The datasheet says the opa189 is rail-to-rail. But, Vcm Max = (V+) – 2.5. I am confused?

[moffy]

The datasheet: https://www.ti.com/lit/ds/symlink/ads8866.pdf
says that AVDD should be less than 3.6V max preferably 3V, that is what they conducted all their measurements at. The input range is 0V to Vref, so 0V to 5V with your 5V reference. Please refer to the driver circuits on page 27 and 28, it is very important to have that 470pF capacitor right next to the input of the ADC, their recommended driver circuit is a good choice, but you could supply the opamp with a low noise 5V supply, compatible with your Vref.

[Benta]

Looks OK.
One comment: don't use Sallen-Key LP filters for this kind of thing, they're "leaky" at high frequencies.
More info here:
https://www.ti.com/lit/an/sloa049d/sloa049d.pdf

[kaevee]

Looks OK.
One comment: don't use Sallen-Key LP filters for this kind of thing, they're "leaky" at high frequencies.
More info here:
https://www.ti.com/lit/an/sloa049d/sloa049d.pdf

Will checkout the document you shared.