I need to measure a signal that arrives on the board through a 100K resistor. I am trying to avoid having to add an operational amplifier to buffer it before the ADC input. I could use a bit of help understanding what the data sheet says about the ADC.
I only need 10 bits and the measurements do not have to happen quickly. The max Ts of 239.5 cycles (17.1 us) would be fine. In fact, I can wait longer if necessary.
On page 67, Table 50 I see Rain listed as "External input impedance" and a max value of 50K.
Applying equation 1, on page 68, to a 10 bit resolution with the maximum Ts of 239.5 cycles gives me a maximum external input resistance of 256K. This, of course, is more than my 100K, so it would seem I'm OK.
Table 51 on page 69 has "NA" listed for Ts of 71.5 and 239.5. There's no explanation as to why other than, as I see it, table 50 stating that the maximum external resistance is 50K.
What left me wondering is that this number seems to correspond to a 1/4 LSB maximum error at 12 bits.
Am I correct in assuming I can actually use a 100K resistor if all I want is 10 bits?
Also, on page 70, note 2 under Figure 24, it says the ADC operating frequency can be reduced. I looked in MX and can't seem to find a way to do this.
Thank you.
