ADS8688: ADS8688 evaluation observations and queries

Danish Shaikh

Part Number: ADS8688

Tool/software:

We are currently evaluating the ADS8688 ADC using the EVM board and have noted the following observations and queries. Could you please review and provide your feedback at your earliest convenience?

Test Condition :The ADC is configured for a 0 - 5.12V input range, but the input voltage applied to channel 3 is 7.7V, which exceeds the configured input range.

Observation: For the initial ~1.6 seconds, we received a full-scale value of 65535 from the ADC. After this period, the ADC output was 57417 counts. Upon further inspection of the ADC configuration, we found that the input range for all channels had changed to -10.24 to +10.24V. from 0 to 5.12V

Queries:

Does the ADC automatically adjust to the maximum input range configuration(-10.24 to 10.24 V) if it receives an input voltage greater than the configured range?
Why does the input range for all channels change when only one channel receives an input voltage exceeding the maximum configured value?
Is there a workaround to prevent this automatic change in input range configuration?

3 months ago

0 Tom Hendrick 3 months ago

TI__Guru**** 181001 points

Hi Danish,

Welcome to our e2e forum! No, the ADS8688 does not 'auto-range'. I have noticed from time to time that the ADS8688EVM-PDK GUI can change the configurations if you switch screens or change the sampling characteristics. If you remain on the voltage domain tab, without making any changes or clicking anything other than the 'Capture' button, do you still see this configuration change?

0 Danish Shaikh 3 months ago in reply to Tom Hendrick

Prodigy 10 points

Hi Tom,

Thank you for your quick reply.

Currently we are using our own MCU board with SPI interfaced with ADS8688.

Please find attached document where observation is captured in detail.

ADC Range Change observation.docx

0 Tom Hendrick 3 months ago in reply to Danish Shaikh

TI__Guru**** 181001 points

Hi Danish,

Can you provide a schematic and screen capture (o'scope preferred) of your control lines?

0 Danish Shaikh 3 months ago in reply to Tom Hendrick

Prodigy 10 points

Hi Tom,

Currently there is no schematic. The setup is pretty much simple. Controller Evaluation board 4 SPI lines are connected with ADC Evaluation board that's it. Due to this observation we have hold our gerber release.

You can also observe the same issue by simply configuring any channel for 0-5.12V range and give voltage more than 5.12V to that particular channel for suppose 5 sec. After that you will find ADC raw value changes and if you read range register at this instant it would be automatically changed to +/-10V.

Do this at your end and see what result you get.

Regards

0 Tom Hendrick 3 months ago in reply to Danish Shaikh

TI__Guru**** 181001 points

Hi Danish,

Sorry - but I don't see that happening on my end. Do you still have our PHI connected while you are doing this test? Have you tried the same thing with our GUI? Here is what I have with 7.7V on CH3 and all input ranges set to 0-1.25*Vref:

Here is the register dump:

All channel input ranges still set to 0x6. This was after 3-4 captures of 2097152 samples which takes a bit of time (~40 seconds).

Here is CH3 with the range set to +/-10V:

0 Danish Shaikh 3 months ago in reply to Tom Hendrick

Prodigy 10 points

Hi Tom,

Thank You for your reply again.

I have two quick questions related to Gui.

1. From where I can see program register dump as you have shown? I am unable to find it.

2. Also, I am not sure how to configure individual channels for different range setting using GUI.

Regards

Danish

0 Tom Hendrick 3 months ago in reply to Danish Shaikh

TI__Guru**** 181001 points

Hi Danish,

The 'Register Map Config' page is where the register dump comes from:

You can set the range bits directly in the program registers from this page or you can change the input range from (for example) the Time Domain Display using the Select Channel drop down and then set the range with the Channel Range drop down: