Hallo Chris,

according to 'Note 4' of datasheet input voltage must not leave 0V-100mV....Vcc+100mV input voltage range, when additional errors shall be avoided. Exceeding this limit will probably not cause damage, because internal protection diodes are connected to Vcc and 0V, provided that input current is limited. Unfortunately datasheet does not tell anything about allowed maximum input current. So, it's not wise to bias these diodes in operation.

Datasheet tells, that inputs are so high ohmic, that resistor dividers may directly be connected to inputs. That's not obvious, because many ADC on the market does not allow any relevant source impedance, but must directly be driven by a very low ohmic buffer output.

But keep in mind, that input leakage current of up to about 2µA can flow, causing a voltage drop across input resistor divider circuit and fabricating some error. So, an equivalent source impedance of less than 10kOhm is recommended.

For designing this resistor divider first find out, what your maximum output voltage of sensoring circuit is. Means the really worst case! Then, calculate divider such that maximum input voltage at ADC0809 is less than Vcc+0.1V. That's not so easy, because all sorts of tolerances and drifts must be taken into account. Perhaps it's the best to use a 10kOhm linear potentiometer (multi turn cermet precision type), connecting wiper with input of ADC0809. First, turn-off potentiometer, so that output voltage is zero. Then, carefully turn it on, so that maximum output voltage of sensoring circuit will result in 5.0V at input of ADC0809. The last 100mV you shoudln't use. Let it untouched for having some headroom.

Is your mimimum output voltage of sensoring circuit really 0V? Or must negative output voltages be expected? If mimimum output voltage is really >=0V, no further problems occur. But if negative voltages can exist, you will leave allowed range and additional measures must be taken.

Don't forget, that additionally wiper of potentiometer should be connected via about 100kOhm to 0V.

Why that?
Input impedance of ADC0809 can be very high and looks like a 'dry load'. Dry loads can make trouble at the contact region, where wiper touches resistor material surface. Additional 100kOhm resistor guarantees a minimum current to flow through this interface and prevents trouble.

Additonally, a capacitor should be connected from input of ADC0809 to 0V, if possible. It works like a first order anti-aliasing filter and helps to keep high frequency noise away from ADC. Also, dynamic input leakage currents resulting from internally switched stray capacitance will see a low ohmic path to 0V.

Kai