Measuring Operational Amplifiers DC Parameters

All op amp DC specifications can be measured with this universal circuit and aquisition card:

• Input offset voltage (Vos)
• Input common-mode voltage range (Vcm)
• CMRR and CMRR linearity (Input offset vs input common-mode voltage)
• PSRR+
• PSRR-
• §Non-inverting input bias current (Ib+) and its linearity (Non-inverting input bias current vs input common-mode voltage)
• §Inverting input bias current (Ib-) and its linearity  (Inverting input bias current vs input common-mode voltage)
• Input offset current (Ios)
• Input common-mode resistance (Rcm)
• Input differential resistance (Rd)
• Noninverting input bias current vs input common-mode voltage
• Inverting input bias current vs input common-mode voltage
• Input offset current vs input common-mode voltage
• DC gain linearity (Input offset vs output common-mode voltage, linear output voltage range) with different loads
• DC gain with different loads (Av)
• DC output resistance (Ro)
• Output voltage swing vs load resistance (Vo)
• Short circuit sourcing current (Isc)
• Short circuit sinking current (Isc)
• Supply current (Is)
• Supply current vs input common-mode voltage

DUT op amp with almost any unity gain frequency and with high open loop output resistance can be measured correctly and without changing loop compensation.

Offset = (OutServo - InputCM) / 101

DC CMRR test:  VEE/VCC=stable. OutputCM=0. S1 – closed, S2 – closed, S3 – open. Sweep InputCM from VEE to VCC and measure offset.

  CMRR(db) = 20*log(ΔInputCM / ΔOffset)

etc