LPC4370 ADCHS Limit Input voltage

[haksookim1]

I'm using ADCHS from LPC4370.
I have a question because the ADC port is damaged during long-term use.

Circuit/Situation Description
  1. Used as Vin neg pin output.
  2. I checked the data sheet. ( 3.3V Max)
  3. Noise is generated in ADCHS(1~3) as mux operation waveform

In the datasheet, the ADC input voltage limit is set to 3.3V and is being used.
The circuit I am using is
  ADCHS0 ( max Input voltage  3.3V) 
  ADCHS1 ( max Input voltage  1.2V)
  ADCHS2 ( max Input voltage  1.2V)
  ADCHS3 ( max Input voltage  1.2V)
and the problem seems to occur when ADCHS1~3 periodically generate noise and ADCHS0 inputs at 3.3V level.

I am asking a question based on the above.
  1. What is the maximum input voltage when using ADCHS (12bit ADC)?
  2. If there is no problem with the input voltage, how can the phenomenon that the Mux operation waveform interferes be eliminated?

[xiangjun_rong]

Hi,

Regarding the absolute voltage of the high speed ADC analog channel ADCHS_x for LPC4370, it is dependent on the ADCHS_NEG pin setting.

When you set the DCINNEG bit in POWER_CONTROL register, the ADCHS_NEG pin is clamped at 0.5V, all analog channel  ADCHS_x pin voltage must be in the range from 100mV to 900mV.

When the clear the DCINNEG bit in POWER_CONTROL register, the ADCHS_NEG pin is an input pin, the voltage of ADCHS_NEG pin can be from 400mV to 800mV, all analog channel  ADCHS_x pin voltage must be fluctuating in the range from 400mV to 900mV  based on the ADCHS_NEG pin voltage at middle point. In the case, the required ADCHS_x pin voltage is in the range:

(voltage of ADCHS_NEG pin)-400mV< ADCHS_x pin voltage <(voltage of ADCHS_NEG pin)+400mV

Assume you set the voltage of ADCHS_NEG pin as 800mV, the ADCHS_x pin voltage must be greater than 400mV, less than 1200mV.

Assume you set the voltage of ADCHS_NEG pin as 400mV, the ADCHS_x pin voltage must be greater than 0mV, less than 800mV.

For detailed information, pls refer to section 13.9 12-bit ADCHS input selection in data sheet of LPC4370.

In any case, the 3.3V is NOT allowed to impress to the analog pin.

Hope it can help you

BR

XiangJun Rong

[haksookim1]

Thanks for your kind reply.

Summarizing the answers

1. What you mean is that 3.3V input is possible.
2. ADCHS output result is converted based on ADCHS_NEG pin.

Is this correct?

[xiangjun_rong]

Hi,

Q1. What you mean is that 3.3V input is possible.

>>>Pls read my last reply carefully, I never say it is possible to connect 3.3V voltage to the analog pin ADCHS_x.

Q2. ADCHS output result is converted based on ADCHS_NEG pin.

>>>>The answer is yes, the ADC sample/convert the voltage difference between ADCHS_x pin and the ADCHS_NEG and get the digital result.

Hope it can help you

BR

XiangJun Rong