Currently we use AD7323 ADC with;
and moving average over 16 samples.
In case of 5Mhz SPI interface case we got the following results with straight binary coding;
As it is seen above 5V input case all results deviate from the input. If we decrease SPI frequency to 1MHz (our trial frequecny) all results turned back to normal.
Do you have any idea or feedback what could be the reason of this deviation?
I am checking on this and I will get back to you soon.
This has been verified in the lab using the customer conditions on the evaluation board. The device performs as expected. Further information on the setup will be required as this is most likely an application rather than part related issue. Can you please provide schematic for the part and analog input circuitry to the device, including any buffer circuitry.
What reference is in use?
What input buffers are being used, and what supplies have they?
First of all thank you for your reply.
In our case ref voltage is 3V (ADR5043A), Vdd=12V, Vss=gnd and we use ADA4091 as input buffer with 12V supply.
Our analog inputs are slowly changing DC voltage reading position of simple voltage divider.
With fr=5Mhz, when we apply lets say 10V to input buffer we measure 10V at the input pin of ADC but the digitized reading is 9.26. But if we decrease fr to 1MHz reading becomes 9.99V.
Pls keep in touch
You do not have an anti-aliasing filter (AAF). A lot of the SAR apps guys think that everyone knows about
SAR ADCs, so they leave it off the data sheet or use a very beefy op amp, such as an AD8031, to drive
Vin. Also, C76 seems a bit light to me; increase it to 1uF. Finally, resistors such as R78, should match
the impedance seen by the other input, so lower R78 to 3k. Read:
Front-End Amplifier and RC Filter Design for a Precision SAR Analog-to-Digital Converter
Voltage Reference Design for Precision Successive-Approximation ADCs
One remark about test results;
if you look at the ADC reading, after 5V all results deviate from the original inputs.
Additionally i applied DC level for inputs which is the normal working condition.