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Cobra II: Dynamic range of analog inputs


#1

On the tech sheet for the G120, it says all pins are “5V compatible”.

I’m dealing with higher voltage than 5V, so was planning to create a voltage divider circuit to handle the difference. Problem is I don’t know what the internal resistance of the pins are. I don’t want to pick some resistor values that would send all 12V through my new cobra.

Is there a way to get a more detailed electrical spec sheet for those pins?

Also, what is the dynamic range of the analog inputs? If I provide it a signal that varies from 1V to 4.25V will it be able provide accurate readings without clipping?


#2

The pepper easy would be to use an op amp or similar circuitry. Or use voltage divider with a diode or zener for protection.


#3

I already have a diode in my sketch and was toying with a low-pass filter to reduce the noise the chip would incur.

I am hoping to avoid the use of an op-amp as I didn’t want to have a PCB made just for a proof-of-concept. Possibly a bad choice?

I also have EEs (my specialty is software) which will look over my circuit before I fry something… but I wanted to try designing it myself first. :slight_smile: Can’t learn unless I try!


That being said… Is there a way to get more data on the electrical characteristics of the pins?

BTW, what’s a pepper easy?


#4

If you check the datasheet on the LPC1788 it states that the range on the ADC is 0 to VDDA so you’ll need to have GHI comfirm what VDDA is connected to as there are no schematics for the G120. It is most likely that they are 3.3V as the only supply need by the G120 is 3.3V according to the documentation.

http://www.nxp.com/products/microcontrollers/cortex_m3/LPC1788FET180.html#overview


#5

Thanks Dave! You’ve answered almost everything!

@ gus - can you confirm 3.3V?

Also, can you comment on the available measurement precision? E.g. are the pins using a 16bit adc?

Justin


#6

That link I sent you says it is 12 bit.

If you need higher precision you’ll need to use an external ADC.


#7

I meant to say the proper way, not pepper easy! Funny!


#8

Thanks Dave!


#9

I opened up the GHI G120 page and it says 17xx series processor. I went here: http://www.nxp.com/products/microcontrollers/cortex_m3/series/LPC1700.html and downloaded the Data Sheets from the left column and opened the doc that starts with 1759 (the lowest number of the series). Then I search on ADC…

(PDF page 12)

–> The above doesn’t yet specify a limit on ADC input or ADC reference voltage…

Creepy footnote regarding the ADC input pins (but not pin 10):

[quote][2] 5 V tolerant pad providing digital I/O functions (with TTL levels and hysteresis)
and analog input. When configured as a ADC input, digital section of the pad is disabled and the pin is not 5 V tolerant. This pin is pulled up to a voltage level of 2.3 V to 2.6 V [/quote]

However, get to page 37 of the pdf:

[quote]NXP Semiconductors LPC1759/58/56/54/52/51
8. Limiting values

Table 4. Limiting values
In accordance with the Absolute Maximum Rating System (IEC 60134).[1]
Symbol, Parameter, Conditions, Min, Max, Unit
VIA, analog input voltage. on ADC related pins, -0.5, +5.1, V
[/quote]

The EE part of my brain suggests finding a way to convert the usable output of your sensor to 3V and then use the Zener or other tricks to keep it confined to definitely under 5 V.

The above is the [em]reference voltage of the ADC[/em]. In an ideal world, one would be able to apply a greater amount of voltage to the AREF ADC input, but realistically, this is already inside a microcontroller’s IC, so we’re lucky to even be 5V tolerant.

Sounds to me like the assumed 3.3V limit on any input that matters is a valid assumption.