Electronic – Analog values without adc

The current measuring opamp has a low impedance output and won't cause problems. I didn't find an equivalent ADC input circuit in the NXP datasheet, but Atmel shows a circuit consisting of a 14 pF capacitor and a 1..100 k\$\Omega\$ series resistor (p. 259). That's high impedance, and won't disturb a resistive voltage divider.

But note that I/O pins often have a leakage current up to 1 \$\mu\$A, whether you use them as analog inputs or not. So if the voltage divider draws less than 100 \$\mu\$A I would buffer it. Use a low input bias current opamp as buffer, or you might have the same 1\$\mu\$A leakage as the microcontroller has.

The AD7798 is a differential ADC. It has a negative input and a positive input. When configured for bipolar operation:

• When the voltage between in+ and in- is positive (in+ > in-) at full scale the output will be all one.

• When the voltage between in+ and in- is 0 (in+ == in-) the output will be 1000...0000.

• And When the voltage between in+ and in- is negative (in+ < in-) at full scale the output will be all zero.