LP550x maimum external input impedance

[YounesBMA]

Hi 

I can't find the ADC maximum external input impedance of LPC5502 in datasheet  .

Best regards 

[PabloAvalos]

Hi @YounesBMA 

Hoping you are excellent.

After checking internally, the datasheet's input resistance value means RADIN as it is shown on the next diagram:

PabloAvalos_2-1638317250273.png

So, the maximum external ADC input impedance calculation can be obtained with the formula below:

PabloAvalos_3-1638317260293.png

In case that you do not want to calculate it by yourself, use attached excel table that can be helpful to speed the calculate up.

Please let me know if you have more questions.

Best Regards.

Pablo Avalos.

[ianbenton]

I found the datasheet spec of "2k input impedance" confusing, assuming it meant a 2k resistance to ground, but it actually means that there is 2k in series with the S&H capacitor.

The time constant of (2k+your_source_impedance) * 5pF input capacitance should be compared with the sample period (1/sample_rate)

three time constants = sample period will give you 5% accuracy

five time constants = 1% accuracy

11 time constants for full 16 bit accuracy

[frank_m]

I don't know any datasheet that specifies such a value. Only an internal impedance is specified.

It is your task to make sure the driving input can charge/discharge the S&H capacitor over the external and internal impedance during the selected sample time.

A basic calculation as first-order RC element is usually sufficient.