ADC Reference voltage value in LPC236x series

Started by prab...@yahoo.co.in November 22, 2010
Hi all,
What is maximum ADC reference voltage(3 or 3.3 V)?Because I am getting ADC value 900 - 920 for 3.3 ADC Vin voltage.
But maximum value of adc is 1023 for 3.3 voltage.Is is correct??

Thanks,

An Engineer's Guide to the LPC2100 Series

p...@yahoo.co.in wrote:
> What is maximum ADC reference voltage(3 or 3.3 V)?

I think the data sheet is quite clear on this.

> Because I am getting ADC value 900 - 920 for 3.3 ADC Vin voltage. But
> maximum value of adc is 1023 for 3.3 voltage.Is is correct??

Yes, when Vin is the same as Vref (and Vssa is the same as Vss and all other
relevant signals are on spec) you should get 1023. Have you checked that your
input signal really is 3.3V at the moment the ADC samples it and/or that your
input voltage source resistance complies with the spec?

--

Timo

Thanks for quick response.When i changed processor(replaced 236x with 176x) on
same board,
i am getting ~ 4096(which is max Vin (3.3 V)) on LPC176x.So this issue seems to
be processor specific..I think max voltage or reference voltage is 3V for
lpc236x means,ADC Value 910-920 seems to be correct.So i am suspect ing
reference voltage

Thanks,

________________________________
From: "t...@gmail.com"
To: l...
Sent: Mon, 22 November, 2010 2:14:39 PM
Subject: Re: [lpc2000] ADC Reference voltage value in LPC236x series

p...@yahoo.co.in wrote:
> What is maximum ADC reference voltage(3 or 3.3 V)?

I think the data sheet is quite clear on this.

> Because I am getting ADC value 900 - 920 for 3.3 ADC Vin voltage. But
> maximum value of adc is 1023 for 3.3 voltage.Is is correct??

Yes, when Vin is the same as Vref (and Vssa is the same as Vss and all other
relevant signals are on spec) you should get 1023. Have you checked that your
input signal really is 3.3V at the moment the ADC samples it and/or that your
input voltage source resistance complies with the spec?

--

Timo



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pra bu wrote:
> Thanks for quick response.When i changed processor(replaced 236x with 176x)
> on same board, i am getting ~ 4096(which is max Vin (3.3 V)) on LPC176x.So
> this issue seems to be processor specific..

Notice that there is a voltage source resistance spec for the LPC236x but not
for the LPC176x. Therefore my previous comments still hold.

> I think max voltage or reference voltage is 3V for lpc236x

Where do you read something like this? DS says 3.6V. And it doesn't matter here
what the maximum possible voltage is. All that matters is the ratio of Vin and
Vref and that the voltages are inside operational specifications.

> means,ADC Value 910-920 seems to be correct.

No, it absolutely is not. If you had Vref 3.0V and Vin 3.3V your conversion
value would theoretically be 1126, in other words, you would be overloading the
ADC.

> So i am suspect ing reference voltage

If I was you, I would be suspecting input voltage.

--

Timo