OPA170 failures

Hi Tim,

It sounds like your application circuit may be exposing the OPA170 to electrical over-stress (EOS) events and the internal ESD protection circuits are being damaged.

The OPA170 internal ESD protection consists of current steering diodes from each input and output to the supply rails. Then, there is an ESD cell between the two supply pins. An important thing to remember is ESD cells are only intended to protect against out-of-circuit ESD events associated with handling. They are never intended to be turned on once the amplifier is installed in a powered circuit. 

For example, if an input is subjected to an electrical over-voltage a steering diode can be turned on allowing current to flow to a supply rail. If the voltage is exceedingly high, the ESD cell between the supplies can inadvertently turn on and conduct heavy current. The steering diode and that cell can be damaged because the cell doesn't turn off until the supply voltage is removed. usually by then, the cell has over heated, melted and shorted. 

EOS can occur on the power supply line which would directly turn on the ESD cell connected between the internal supply pins.

Regards, Thomas

PA - Linear Applications Engineering 

Hello Thomas

Many thanks for your reply.

I did it again. I shorted pins 2 and 3 together with a scope probe and it became a short between them. The circuit is powered by + - 15V.

When I shorted the two pins together I destroyed the opamp. Pin 3 is the + input, pin 2 is -15V. An AD8226 drives pin 3 without a series resistor. It can deliver 13mA.

I do not understand the failure mechanism.

Should I put a 10k resistor in between the two parts?

Best regards,

 

Tim Orr

Hi Tim,

Sorry to hear you are having problems. It is difficult to know the exact conditions at the moment the scope probe slips and the pins are shorted. Certainly it is a transient event and it may be that the V- supply reacts by momentarily creating an over voltage condition. I don't think the OPA170 would be damaged if the non-inverting input was tied to the V- rail and the supply were brought up normally. The non-inverting input pin goes to the gate of an input FET and an ESD cell both of which appear as a high impedance under normal conditions. However, in a transient situation where an event happens very quickly capacitances must be charged and the transient current can be much higher.

Adding a small amount of resistance in series with the non-inverting input, a few hundred ohms to a kilohm, can go a long way in limiting the input current under steady-state and transient conditions. It is a remedy I often recommend when there is the possibility of an input EOS situation. You don't want to make the resistor too large, or it starts to degrade the circuit's noise performance. The inverting input usually doesn't need the resistor added because it already has a series input resistor included in circuits where the amplifier must deliver a voltage gain.

I hope this helps resolve the issue.

Regards, Thomas

PA - Linear Applications Engineering

Hello Thomas,

I think that it might be a power supply line transient. The power supply is a +-15 DC/DC switching module. To clean up supply noise, I have used LC low pass filters. If the supply is shorted and then released, maybe you can get a big ringing voltage. However, the supply is not actually shorted. It is given a 13mA jolt which might cause ringing. Also, there are 7 opamps in the same area and you would expect them to suffer the same fate.

In the test notes I have written DO NOT USE THE SCOPE ON THESE TEST  PINS.

Best regards,

Tim Orr

Hi Tim,

You might want to consider adding a transient voltage suppressor (TVS) diode from the V+ pin of the OPA170, to ground. It is cheap insurance.

If the various op amps used in your circuit are different from each other they are likely manufactured on different processes, by different companies. Some may be more robust from an EOS standpoint than another. Like most everything, processes have trade-offs.

Regards, Thomas

PA - Linear Applications Engineering 

Hi Tim,
Just wondering if you solved this EOS problem? Did you add protection in your circuit, or discover what was inducing the EOS and corrected it?
~Leonard