ADC noise goes away when UART connected

[RogerFL]

I have a Kinetis MCU design that senses temperature from a PTD using the internal 16-bit ADC. It also has a UART that I use as a console port for development. When the UART is plugged in (to a FTDI cable), the ADC input is very stable with all samples within a 3 unit range. When I unplug the UART, the samples are much noisier with a range of about 30 units.

Any ideas on what would cause this?

 

[D.A.(Tony)Stewart]

Lack of CMMR on unbalanced lines causes differential noise. UART has ground which lowers CM impedance to suppress stray cap. Coupled RF or E fields at line f. Thus you need a large ferrite CM choke and small RF filter cap (NP0) to common across inputs.

- - - Updated - - -

Lack of CMMR on unbalanced lines causes differential noise. UART has ground which lowers CM impedance to suppress stray cap. Coupled RF or E fields at line f. Thus you need a large ferrite CM choke and small RF filter cap (NP0) to common across inputs.

Added..
NP zero or oh as often said are not only zero tempco. But lower k dielectric and lower ESR at RF f.