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I am working on an old design which opamp is in saturated mode, what I would like to know why some manufacture part number keep staying on a saturating mode such as LT2079IS#PBF but some crash (negative rail voltage goes close to zero and the output is no good) such as LT6012CGN#PBF.

Let take a look at below example: This is transimpedance opamp design which saturates when Vin- be greater than 20uA but how do you know how long it will stay in the saturated mode before crashing(output is not valid), (from Datasheet)

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Please edit your question to add links to the datasheets for your op amps. \$\endgroup\$ – Elliot Alderson Jan 31 at 19:06
  • \$\begingroup\$ Your schematic has the output shorted to ground. \$\endgroup\$ – brhans Jan 31 at 20:09
  • \$\begingroup\$ "(negative rail voltage goes close to zero" Your -5V supply is collapsing? \$\endgroup\$ – sstobbe Jan 31 at 22:18
  • \$\begingroup\$ Yes that's what is happening on LT6012 \$\endgroup\$ – Shahreza Feb 1 at 16:49
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You show no load which Is a fault in your question, yet the specifications are VOL at IOL = 140mV/1mA = 140 Ohms.

When an output is saturated, the gain reduces to zero by effect or math or TIA mV(out)/mA(in), so ask a better question.

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There is no way to predict what will happen when an opamp saturates, and no way to determine what will happen when it recovers (or how long it will take to get to normal). It is due to the internal workings of the amplifiers and their transistor stages, the schematics are not given out by the manufacturers as they are trade secrets. The manufacturers don't usually test opamps or normally list the The best thing to do would be to get a rail to rail opamp.

Some other options are:

  • Increase the supply rails
  • Decrease the gain
  • Use a multi gain approach with two channels.

If you know that the sensor maxes current at 900uA, then size the system accordingly:

5V/900uA = 5555Ω

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