ADS131A04: Achieve full scale 0x7fffff to 0 to 0x800000 with single supply

Hi Danilo,

Thank you for your post.

Did you mean to say that you have a 1-uF capacitor between VREFP and AVSS (i.e. a bypass capacitor)?

Generally speaking, the differential analog input voltage is limited to -VREF/Gain to +VREF/Gain and the absolute input voltage on each pin is limited by the analog supplies (AVDD and AVSS). In order to use a 5-V reference voltage to measure a 5-V differential input signal, you would need to use the max 5.5-V AVDD supply and connect AVSS to GND.

Keep in mind that our data is output in binary two's complement format, so 0x7FFFFF represents positive full-scale and 0x800000 represents negative full-scale.

What does your input signal look like? Is it single-ended or differential? Does it have a common-mode voltage or is it centered around ground?

Best regards,

Ryan

Hi Ryan,

Thank you for your response. Here are the customer's feedback.

Did you mean to say that you have a 1-uF capacitor between VREFP and AVSS (i.e. a bypass capacitor)?
- Yes as directed by the datasheet

I am using internal reference 2.442V
- My signal is single ended so i am using pseudo-differential mode, with a constant voltage on AIN-

Keep in mind that our data is output in binary two's complement format, so 0x7FFFFF represents positive full-scale and 0x800000 represents negative full-scale.
- Exactly but i don't get the values for negative full scale, I get (16777216(max) to 8388607(min)) for all values below the common mode voltage.

Any reasons as to why?

Regards,

Danilo

Hi Danilo,

What is the constant voltage applied to AINxN and what is the signal range applied to AINxP during these tests?

With a unipolar 5-V supply, you can maximize the full-scale range by applying 2.5-V to AINxN and centering the signal applied to AINxP around the same 2.5-V common-mode. AINxP could range from +0.058 V to +4.942 V to utilize the full-scale range without exceeding the analog supplies. This, of course, assumes a gain of 1 V/V. Please refer to section 9.3.2.

Danilo A. said:

- Exactly but i don't get the values for negative full scale, I get (16777216(max) to 8388607(min)) for all values below the common mode voltage.

It sounds like the customer is converting the ADC output codes incorrectly. 8,388,607 is a positive value in binary two's complement, yet they describe it as a negative value. Perhaps there is some confusion about the data word size or frame format? Can they capture an example frame on an oscilloscope or logic analyzer showing CSn, SCLK, DIN, and DOUT?

Regards,

Ryan

Hi Ryan,

Here is the customer's response.

I meant 8388608 not 8388607, forgive the typo. I am using 24bit data. I will try to post it but just for asking. How can the ADC make a negative conversion when the REFN is connected to AVSS , while AVSS is connected to ground as it is a single supply as stated in my previous posts.

This has been the basis of my question ?

Like I said it should be two's complement but it shows (16777216(max) to 8388608(min)) for negative values which can be fixed in code though. I just want to confirm if its a wrong doing on my part or its the correct operation.

Regards,

Danilo