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3 years, 3 months ago.
Maximum capacitance on Analog In pins for LPC1768 =15pF?
Am I interpreting the LPC1768 data sheet correctly with regards to its ADC characteristics?
I refer to page 66 of 93 of this document: https://www.nxp.com/docs/en/data-sheet/LPC1769_68_67_66_65_64_63.pdf
It states that the maximum analog input capacitance is 15pF and the maximum voltage source interface resistance is 7.5kohm. I am assuming that these are the maximum values permitted in my circuit external to the mbed. Is that right?
I'm proposing to drive an analog input pin from the output of an op amp. In between there will be a 470 ohm resistor and two BAT85 protection diodes connected to the power supply rails.
The diodes have <4pF capacitance (at 5V reverse voltage). I believe therefore that I'm within the data sheet limits.
However, I'm curious because, to quote Texas Instruments "Generally, most designers place a large capacitor from the ADC pin to ground. This capacitor is used to lower the source impedance of the channel as seen by the ADC so that the internal sample capacitor can be charged quickly." Source: http://www.ti.com/lit/an/spna061/spna061.pdf
The ADC's internal sample capacitor is 2.2 pF. Why such a low external capacitance restriction? It seems to be at odds with the practice of "most designers".
3 years, 3 months ago.
You are reading the datasheet incorrectly.
What they are saying is that the maximum capacitance of the input pin will be 15pF, your circuit should be able to drive at least that much capacitance. The sample capacitor may only be 2.2pF but the physical pin and package will have a capacitance too and that can vary a fair bit between parts.
Put a nice large capacitor as close to the pin as you can. The only upper limit on the size of the external capacitor is that it will form an RC with R1 in your diagram and so reduce the maximum frequency seen on the ADC input. If possible connect the capacitors ground to the analog blocks ground reference not the system ground otherwise you risk coupling digital noise from ground into the analog input. Whether this matters depends on just how accurate and low noise you are trying to be.