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OPA188: OPA188 can't replace OPA197 in constant current circuit.

Part Number: OPA188
Other Parts Discussed in Thread: OPA197, OPA333, OPA2387, OPA2191, OPA2197, OPA2192

the following curcuit is a constane current circuit. and the output current range is 200nA to 4uA. we found that the circuit will not output when use OPA188 on U31. and the circuit works OK with OPA197 or OPA333 on U31.

I think OPA188 should be OK for this circuit. Could anybody help point out the root cause of the problem?

Thank you

Frank

  • The OPA188 does not have a rail-to-rail input; it needs 1.5 V headroom to the positive rail (see the VCM specification). So it is not suitable to measure the voltage drop over R195.

  • Hi Frank,

    As Clemens pointed out, OPA333 is RRO (Rail to Rail Output), but not RRI (Rail to Rail Input) op amp. If you multiple R195*4uA = 30kohm*4uA = 0.12V and TP20 input node is measured near AVDD_5Vdc, and OPA333's input Vcm is not applicable for OPA333 op amp (not operating in a linear region). That is why it is not working.   

    You could use OPA2197 (OPA2191 or OPA2192) to implement in this circuit. For low Vos op amps, you may consider OPA2391, OPA2392 or OPA2387 op amps. . 

    If you have additional questions, please let us know. 

    Best,

    Raymond