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16C773 Analog Questions

 
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djpark



Joined: 02 Mar 2004
Posts: 49

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16C773 Analog Questions
PostPosted: Mon Jun 28, 2004 4:24 am     Reply with quote
I am studying 16C773 12 bit ADC and get many questions. I hope someone can clear for me. 16C773 is OTP, so I wish to get as much homework done before I start working on.

1) Is internal Vref absolute 4.096V? It seems so from the datasheet, then will it be still 4.096V if 16LC773 is used with Vdd lower than that?

2) ADC measurement voltage range is from VrefL to VrefH depending on whish configuration I select? That means 0 count for AI=VrefL and 4096 count for AI=VrefH?

3) If both internal VrefH (4.096V) and internal VrefL (2.048V) are used, will it measure in the resolution of 0.5mV? A part of datasheet minimum resolution of ADC is 1mV.

4) Can AVdd and external VrefH set to 12V so that I can measure 0-12V instead of 0-4.096V? Likewise can VrefL set higher than Vdd?

I am pretty new to the PIC world and 16C773 is first time. Thanks in advance.

-- dj
Ttelmah
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Re: 16C773 Analog Questions
PostPosted: Mon Jun 28, 2004 7:39 am     Reply with quote
djpark wrote:
I am studying 16C773 12 bit ADC and get many questions. I hope someone can clear for me. 16C773 is OTP, so I wish to get as much homework done before I start working on.

1) Is internal Vref absolute 4.096V? It seems so from the datasheet, then will it be still 4.096V if 16LC773 is used with Vdd lower than that?

2) ADC measurement voltage range is from VrefL to VrefH depending on whish configuration I select? That means 0 count for AI=VrefL and 4096 count for AI=VrefH?

3) If both internal VrefH (4.096V) and internal VrefL (2.048V) are used, will it measure in the resolution of 0.5mV? A part of datasheet minimum resolution of ADC is 1mV.

4) Can AVdd and external VrefH set to 12V so that I can measure 0-12V instead of 0-4.096V? Likewise can VrefL set higher than Vdd?

I am pretty new to the PIC world and 16C773 is first time. Thanks in advance.

-- dj

1) Look again at the data sheet.
Parameter no D400, gives the Internal Vref range, as VRL (2.048v), available for supplies >=2.5v, and VRH, only available when the supply is above 4.5v.
2) Yes. Though you never get 4096 counts (4095 max).
3) The 'step size' will be 0.5mV, but the resolution will only be 1mV.
4) No.
Again read the data sheet. 'Maximum voltage on any pin with respect to Vss = 0.3v to Vdd+0.3v'
There is an exception for pin RA4, which is allowed to go to 8.5v, but this is not an analog input.

Best Wishes
djpark



Joined: 02 Mar 2004
Posts: 49

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Re: 16C773 Analog Questions
PostPosted: Mon Jun 28, 2004 9:30 am     Reply with quote
Ttelmah wrote:
1) Look again at the data sheet.
Parameter no D400, gives the Internal Vref range, as VRL (2.048v), available for supplies >=2.5v, and VRH, only available when the supply is above 4.5v.


I missed that though I was finding for something like this. Thanks.

Quote:
2) Yes. Though you never get 4096 counts (4095 max).


Yes, 4095.

Quote:
3) The 'step size' will be 0.5mV, but the resolution will only be 1mV.


I don't understand, would you enlighten me? If VRefL=2.048 and VRefH=4.096, each ADC count is 0.5mV step but it is only half of the resolution. Does that mean the count can be effectively only 11 bit accurate?

Quote:
4) No.
Again read the data sheet. 'Maximum voltage on any pin with respect to Vss = 0.3v to Vdd+0.3v'
There is an exception for pin RA4, which is allowed to go to 8.5v, but this is not an analog input.


Found A20 4.096 <= VRef <= Vdd + 0.3V and VRefL <= VRefH <=AVdd, but I couldn't find anywhere stating maximum AVdd though it doesn't make a difference now.

So if I want to measure 12V, I guess I wll have to use a register voltage divider.

Thanks for guiding me.

-- dj
Ttelmah
Guest
PostPosted: Mon Jun 28, 2004 10:43 am     Reply with quote
Yes. The point is that the last bit of the value becomes unreliable. So the ADC, only gives (at best) in this condition 11bit accuracy.
There is a generic spec for all the pins (excluding ones used for programming), at one diode drop above Vdd, so this sets the limit range. Yes, you'll have to use a divider.

Best Wishes
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