LPC4357 ADC reference: bandgap vs VDDA

[lpcware]

Content originally posted in LPCWare by rmcaleer on Tue Feb 24 15:57:06 MST 2015
I am hoping someone can explain the relationship between the internal ADC bandgap reference and VDDA on the LPC4357

The user guide (UM10503.pdf) documentation for the LPC4357 refers to an ADC bandgap reference: this can be output on PF_7
It also mentions VDDA as being the "Analog power supply and ADC reference voltage"
The datasheet LPC435X_3X_2X_1X.pdf describes the operation of the ADC being "full rail" with respect to VDDA.

I can see that using BGAP_SWITCH (on the POWER_CONTROL I can turn off the band gap)

In order to understand how these interact I have the following questions:
- how/where is the bandgap reference used?
- what is the voltage of the bandgap?
- what is used for ADC reference if I turn the bandgap off?
- if I connect the bandgap to PF_7 and turn it off could I provide an external reference?

Thanks for any help you can provide.

Best regards,
Richard

[lpcware]

Content originally posted in LPCWare by starblue on Mon Apr 13 01:54:08 MST 2015
It seems NXP removed the info in the latest data sheet. I was referring to an older version dated 2012-12-06.

Interestingly, the other data sheets for parts in the LPC43xx family document the band gap voltage in a section "BOD and band gap static characteristics", but its value depends on the subfamily (generally it seems to be higher for flash parts). The value for LPC43Sxx at 745mV is higher than the 673mV+-2% in the old data sheet.

Maybe the info in the old data sheet was inaccurate, it would be nice if NXP could clarify.

Also, in several of the data sheets there is a mixup between mV and V (0.6425mV is way too low).

Finally, I missed the updated data sheet because its date was not updated on http://www.lpcware.com/content/device/lpc43xx .

[lpcware]

Content originally posted in LPCWare by rmcaleer on Tue Apr 07 15:31:00 MST 2015
(Apologies for the delay in replying
- all my attempted posts since my original have until now been spam trapped
- this has only just been fixed)

Thanks for that

Normally when I have used Band Gap references with ADCs they have been to provide the reference for the operating region of the ADC rather than just provide an example output voltage
- what you say would explain why the only description regarding it in the user manual is around how to put it out on a pin

As to the info being in the datasheet I am confused
I have the datasheet from http://www.nxp.com/documents/data_sheet/LPC435X_3X_2X_1X.pdf (Rev. 4 — 19 August 2014 )

There are only two mentions of band gap in the document
- one explaining that it can be output on PF_7
- the other is in the change list and referring to Table 14: this looks to be wrong as Table 14 is the flash characteristics

If you have a different (newer/older/more accurate) datasheet I would be grateful for a link.

Thanks again
Richard

[lpcware]

Content originally posted in LPCWare by starblue on Wed Feb 25 02:15:48 MST 2015
You should look into the data sheet, most of your questions are answered there.

As I understand it, the band gap is not used by the ADC except that you can read it to check VDDA.