ADS7951: Supply range +VA and +VBD

Hi Alexandru

Sorry for the late reply.

The voltage range for VBD was changed from VBD=1.7 V to 5.25 V to VBD = 1.7 V to +VA to better reflect device recommended operating conditions which are mentioned on page 10 of device datasheet. Both Rev. B and Rev. C has the same recommended operating condition. (VBD = 1.7V min to VA+ max)

We can try to replicate your use case of VA=2.7V and VBD=2.8V at our end and get back to you in couple of days. But we recommend to stay within datasheet recommended operating conditions.

Thanks & Regards
Abhijeet

Hi Abhijeet,

thank you for the reply. I made a mistake, the differences what we have seen are between Rev. A? (January 2010) and Rev. C of the datasheet. 

Best regards,

Alexandru

Hi Alexandru

I was able to replicate your use case on a single device. The device was able to sample and convert, but this cannot be guaranteed across devices/temperature/voltage conditions.

By design, the device performance parameters are guaranteed only for VBD+ <= VA+. It is therefore recommended to stay within the datasheet recommended operating conditions.

Thanks & Regards
Abhijeet

Hi Abhijeet,

could you please be more specific regarding the above mentioned variables.

Our voltages are as follows: VBD+ = 2.85V, VA+=2.7V. These voltages are constant so we do not expect variations in them.
Our temperature range is specified as 5 - 45 °C.

Would we expect to see a problem within this temperature range? What would the behaviour look like?
1. no data coversion e.g. flat lines
2. corrupted data e.g. erratic values
3. no response on SPI bus from device
4. latch-up on power supply resulting in higher power consumption
etc.

If you can provide more information, we would be very grateful.

Regards
Alex

Hi Alex

I checked with the design team and device should be able to sample and convert for your use case (VBD=2.85V and VA=2.7V). For the test conditions mentioned for your application we expect device to work correctly.

Thanks & Regards

Abhijeet

Hi Abhijeet,

thank you for the confirmation that our application will work correctly. During final review, the reviewer popped up the question again as to why TI changed the spec in the first place? Could you tell us what was happening in the field under the original spec conditions? Your previous explaination "to better reflect device recommended operating conditions" is rather vague.

Regards
Alex

Hi Alex

We had changed the recommended operating conditions for VBD from [1.65V to 5.25V] to [1.65V to VA+] in revision B of this device. The changes we have made in Rev C were mainly done to bring consistency across the datasheet.

To answer your question, in this device, the VB supply must be less than VA + 0.3V. When VBD > VA + 0.3V, one may see increased leakage current from VBD supply.

In your use case, as VB (2.85V) < VA (2.7V) + 0. 3V, we expect the device to work correctly.

Thanks & Regards
Abhijeet