AD7176-2 sampling rate mismatch in Cont. Read Mode

Dear all,

I analysed the data coming from 2 AD7176-2 EVBs and have questions regarding the stated sampling rate.

In cont. read mode, 4 channels, default filter (sinc5+1), I measure DOUT/RDY timing (with Logic 1.1.15)

Reg setting        Measurements-->

fs/ksps               pulsewidth/us          period/us               f/kHz               ratio f/fs

250                    1.12                         4.00                         250               1.0               (GOOD, as expected)

125                    1.12                         24.12                       41.46               0.322

50                      1.12                         36.1                         27.68               0.554

25                      1.12                         56.12                       17.82               0.713

a) I do not understand why the boards do not deliver correct DOUT/RDY periods (see ratio) apart from 250ksps.

b) Why is there a settling time of 20us stated in the datasheet when one does not see any effect?

unless it means that a period of 4us delivers 17 error free bits at 250ksps, or 24 "error free bits" at 50ksps (1/20us) or

50ksps/Ch# (in my example is this 12.5ksps per channel), or (1/20us) deliver 17 error freee bits at 250ksps (Table 18 of datasheet).

To me it looks as if this converter cannot deliver 250ksps and/or accurate fs below 250ksps. Not sure if or what I confuse.

Would appreciate if anyone could shed light in my observation.


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