wide range Resistance measurement with opamp

[themaccabee]

Hi,

I need a circuit to measure resistance between 1K to 240K..

I presently use an opamp based circuit in which a noninverting amplifier with feed back resistor is given as the unknown resistor & a reference voltage given at input.. the output voltage is measured by an ADC..But this cant measure wide range as specified above..coz the opamp gets saturated.. any ideas??

 

[AdrianN]

The easiest way is to split it in sub ranges, 2.5k, 25k, 250k.

If this is not acceptable, the solution is more elaborate. You need at least a 12-bit ADC to cover that range. Assuming a reference voltage of 2.5V, the ADC will measure between 10.4mV to 2.5V depending on the resistor to be measured. At 10mV the measurement precision will be 5.7%. To improve the precision you need a higher resolution ADC.

This second method works but you need to manage noise: have good grounds, heavy filtering, decoupling caps for all the active components.