topic Re: Vadc absolute measurement input voltage in Power Management

Vadc absolute measurement input voltage

kshitij_dadheka — Mon, 09 Mar 2020 09:50:05 GMT

Does anyone know the maximum Vadc input voltage for absolute measurements? The MC33771 datasheet only specifies the maximum input voltage for ratiometric measurements (AGND..Vcom) #MC33771 #Vadc #absolute

Re: Vadc absolute measurement input voltage

noel_lanier — Mon, 09 Mar 2020 23:09:04 GMT

I believe the max range for ADC1 for a CTx input can be inferred from the specifications for VCT_RNG. (0 <= VCT <= 4.85)V

Re: Vadc absolute measurement input voltage

guoweisun — Tue, 10 Mar 2020 06:46:32 GMT

Do you mean the CTx pin specification?

Re: Vadc absolute measurement input voltage

kshitij_dadheka — Tue, 10 Mar 2020 09:07:03 GMT

No, I mean the specification for absolute analog measurements on the GPIOx inputs. For ratiometric it is for Vadc specified in the 'General purpose input/output GPIOx' section of the datasheet as between AGND and Vcom, but for absolute measurements no range is specified.

Re: Vadc absolute measurement input voltage

noel_lanier — Tue, 10 Mar 2020 16:39:30 GMT

VADC(MAX) for a GPIOx input is relative to VCOM. See Fig 17 in the datasheet. The ADC reference is switched from the internal ADCREF to VCOM when using GPIO as analog inputs. VGPIOx(ABS MAX) is (VCOM+0.5)V However, if VCOM is the reference then anything above that will measure as Full-Scale.

Re: Vadc absolute measurement input voltage

kshitij_dadheka — Wed, 11 Mar 2020 08:42:18 GMT

Hello Noel,

thanks for your reply. However, if I look at table 16 (table 12 for the MC33771C) I see 2 operational modes for the analog modes of any GPIOx: absolute and ratiometric. If Vcom is the reference for both I do not see why the settings for the GPIOx should be different (see table 57 (MC33771B)/table 51 (MC33771C): GPIO_CFG1. If Vcom is the reference for both modes, the fact that the measurement is then absolute or ratiometric is determined by the external circuitry: if it is fed by Vcom it will be ratiometric, if it is fed by an independent source is will be absolute. A different setting in GPIO_CFG1 however implies to me that the ADC is fed with different references (VCOM? ADCREF? other?), hence a different maximum voltage range on the GPIOx inputs. It is this voltage range that I am interested in for an absolute measurement. The voltage range for a ratiometric measurement is explicitly mentioned in table 9 (MC33771B)/table 8 (MC33771C) under 'General purpose input/output GPIOx' -> 'Vadc: Amalog ADC input voltage range for ratiometric measurements'

Re: Vadc absolute measurement input voltage

noel_lanier — Thu, 19 Mar 2020 02:43:56 GMT

The voltage range for GPIOx for either measurement method must be AGND to VCOM. The maximum voltage per (MC33771B)/table 7 is as I originally stated and have to be the thresholds for the protection clamp diodes. As I read section (MC33771B)/9.16.4 I interpret it to mean that ratiometric conversion is only used for temperature measurements. If GPIOx is configured as a ratiometric input then the MC33771 will test the generated digital value against the OT and UT thresholds to qualify for fault conditions. If a GPIOx is configured as absolute then no testing is done on the digital result. However, in both measurement modes VCOM is the reference. Why the VADC specification description adds "...for ratiometric measurements" is confusing.

Re: Vadc absolute measurement input voltage

kshitij_dadheka — Fri, 20 Mar 2020 09:26:35 GMT

Hello Noel,

thanks for this clarification, this was indeed the answer I was looking for!

Thanks and regards.