OPA320: OPA320

Hi Attila,

Often the common-mode and differential input resistance of a CMOS op amp such as the OPA320 aren't listed because they are so extremely high they are of no consequence in a real, functioning op amp circuit. The op amp input resistances are determined from ∆Vin/∆Ib. If you view datasheet Figure 8, Input Bias Current vs Common-Mode Voltage, it is evident that Ib is flat, having nearly zero slope over the Vcm input voltage range. Nearly zero change in the denominator of ∆Vcm/∆Ib results in a common-mode input resistance that is extremely high.

In reality, both the differential and common mode input resistance for the OPA320 are very likely in the range of teraohms (10¹² Ω). An earlier CMOS op amp to the OPA320, the OPA340 having similar input bias current and input capacitance, lists the typical differential and common-mode input resistances of 10¹³ Ω. Certainly a resistance this high is exceedingly difficult to measure accurately.

Regards, Thomas

Precision Amplifiers Applications Engineering