How does a latchup manifest in op amps? We have an LTC1150 that seems to stop working properly. By the latter, I mean it's in a PID loop and the loop just... stops working and sits at a particular voltage. It may well be experiencing input spikes going beyond its rails. How plausible does that sound? What else might I expect, or should I look for?

Here is a circuit diagram. The problem occurs with U9, marked as an OPA227U on the diagram but actually using an LTC1150. The coil sits in a 1T magnetic field and is connected across RED and BLACK. It is part of a PID loop in a microbalance. enter image description here

  • \$\begingroup\$ It may well be experiencing input spikes going beyond its rails. The ESD protection should take care of those, clamp the spikes to the supply rails I mean. As long as the spikes do not contain enough energy to damage the IC, it should survive these spikes. You could try adding series resistors at the inputs to limit the current from those spikes. \$\endgroup\$ May 11 '17 at 15:29
  • \$\begingroup\$ @FakeMoustache The problem is that the spikes are likely coming from a coil, which will continue to push up the voltage until the current it wants to dump actually flows. Adding a bit more input impedance may not help \$\endgroup\$ May 11 '17 at 15:40
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    \$\begingroup\$ Show a schematic, maybe then I can make a better suggestion. \$\endgroup\$ May 11 '17 at 19:22
  • \$\begingroup\$ @FakeMoustache Circuit diagram now attached to question \$\endgroup\$ May 12 '17 at 8:43
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    \$\begingroup\$ From the circuit I deduce that the back EMF spikes from the coil have nowhere else to go but into the opamp so they end up in the opamp's ESD protection which I would try to avoid. I would try adding flyback (schottky) diodes which can take at least 1 A, from the both sides of the coil to the +/- 15 supply lines. Diodes in reverse mode of course, like in an overvoltage protection: radio-electronics.com/images/esd-protection-diodes-circuit.gif maybe also add 1 k resistor in series with inverting input of U9. \$\endgroup\$ May 12 '17 at 9:06

There are two disparate meanings for 'latchup' in op-amps.

  1. Some op-amps experience a phase reversal when the common mode range at the inputs is violated. For example, if you pull a non-inverting input below the negative supply voltage by more than a few hundred mV the output may snap to the positive rail. That can cause the output to latch in that state depending on the nature of the feedback. If the current is limited, this condition typically disappears when power is cycled (unless the conditions are repeated). There's nothing in the datasheet that indicates that happens- but the makers don't always advertise it.

  2. CMOS circuits can experience latchup meaning that the parasitic SCR structure turns on and draws excessive supply current. If enough current is available that can be destructive. Modern circuits are pretty resistant to this and you would have to hit it with tens or hundreds of mA outside the supply rails to get that to happen, most likely, however it will not likely operate properly if you are hitting it regularly with such inputs. No maximum current is given on the datasheet that I can see. Note this:

Note 2: Connecting any terminal to voltages greater than V + or less than V – may cause destructive latch-up. It is recommended that no sources operating from external supplies be applied prior to power-up of the LTC1150.

  1. Not really latchup, but spikes on the pins can can cause internal capacitors to fail, which will generally cause the output to rail. This is permanent damage.

The usual way of dealing with the first two problems is to add some series resistance if necessary and use Schottky diodes to shunt current away from the circuit. Schottky diodes have significant leakage so you may have an issue with them. You can also look at clamping using biased diodes or BJTs, but there still may be a trade-off between leakage and clamp voltage for a given clamp current. Active circuits are more ideal, but may not be fast enough.

Adding resistance after the clamp will help too, however it does increase the noise etc. Dividing down the input voltage after the clamp will definitely help (the absolute maximum input voltage is 300mV beyond the rails) but that op-amp is horribly noisy as it is.

Edit: looking at your added schematic, you could try back to back Schottky diodes from the inverting input to ground (not the supply rails). Something like a BAT54 dual should be adequate and should have minimal effect on the circuit.

Also, make sure that the behavior you are observing is not a high frequency oscillation of some kind - from the limited information this seems like a strong possibility to me.

  • \$\begingroup\$ Thanks - I have now added a circuit diagram to the question \$\endgroup\$ May 12 '17 at 8:44

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