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OPA140 Alternative

Hassan Kelley
Prodigy 210 points
Other Parts Discussed in Thread: OPA140, OPA2140, OPA132

Is there an old fashioned non-rail to rail alternative to the OPA140?  I like everything about the OPA140 except the open loop output impedance. 

Background:

The OPA140 generates the error signal in an analog PID that measures mass.  The OPA140 error amp is configured a differential input photodiode amplifier with two bootstrapped anti-parallel photodiodes between the inverting and noninverting inputs.  There is a resistor between the noninverting input and common in addition to the usual resistor between the inverting input and the output.

This OPA140 photodiode error amplifier drives a PID where the I term is 1/2 of an OPA2140 conventional integrator and the D term is the other 1/2 of the OPA2140 bandlimited differentiator (series RC source impedance and parallel RC feedback impedance).  The P term is an inverting op1177 amp.  The I term is practically noiseless;  the P term noise is insignificant; the D term noise is a killer do to the noise from the OPA140 photodiode error amp.

My solution is to use the decoupling phase compensation on the OPA140 photodiode error amp to roll off the output noise.  This is the usual capacitance load drive op amp circuit.  However, to properly tune this circuit I have to match RC to open loop output impedance of the op amp which is huge for the OPA140.

Hence I would love a non rail to rail alternative to the OPA140.

If this not well written or too confusing I can write it up in Word and send it to you.

Thanks in advance

--hnk

  • 0
    TI__Genius 12335 points

    Hassan,

    The OPA132 can be considered the "old-fashioned" version of the OPA140. Although its open loop output impedance is not characterized in the datasheet, the spice macromodel can be used to generate a reasonable approximation:

    While this will be a fairly accurate representation of the open loop output impedance, the designer (Marek Lis) asked me to caution that since this is an older spice macromodel that its does not model many other characteristics as accurately as our new models. I've attached the Tina file used to generate the plot if you would like to simulate this yourself.

    OPA132 Zout Beta.TSC