OPA340 instability at low temperature

John,

In your email you refer to a buffer configuration driving ADS but the circuit you attached (below) is an inverting 1/9 attenuator - so what do you use?  Are you seeing the instability in the actual circuit or just in the simulation shown below?  The macro-model does not simulate over temperature performance.   Are you using a switching regulator or a linear one?  Switching regulator may cause problems in your application especially with a resistor in series with positive supply.

Lowering the values of gain resistors will improve the stability (see below):

Attached please find a newer macro-model of OPA340 which should more accurately simulate the actual circuit.

Hello Marek,

Thank you for your reply.  You are correct in that we are actually using this as an inverting amplifier with a gain of 1/9.  Here is a small section of the actual circuit; showing the A/D converter:

We only saw the instability in the simulation; I just assumed that the model simulated the temperature performance.

As far as the resistor in series with the positive supply, I was thinking that it along with the 0.1uF capacitor would provide additional supply filtering (the 3.3 volt supply is a switching power supply).  I got the idea from the data sheet for the ADS8320:

Wouldn't the RC filter on the OPA340 power supply just provide additional noise filtering?

Finally, I can't read the images in your reply.  Can you send me image files?

Thanks!

John,

Since you have seen the instability problem only in the simulation, this may be caused by shortcomings of the older macro-model.  The new macro-model (attached in this posting) does NOT show any instability problem - use it instead of the older macro-model available on the website.

Supply filtering with a series resistor is all good as long as you do not load the output with a varying current (e.g. settling a charge injection from sample-and-hold of the ADS).   Changing output current will have to come from the power supply which will have the effect of modulating the supply voltage (contrary to your intentions).   Perhaps the current variation has an insignificant effect across the 10 ohm  series resistor but you need to make sure that it is smaller than the ripple coming from the switching power supply.

I cannot attach more than a single file to this posting but you should be able to see my previous simulation waveforms on the forum itself.

Neil,

Nice to hear from you. 

I have been creating simulation models for all our newly introduced op amps using our new Green-Lis macro-model architecture and these have been uploaded before their respective RTM's (see OPAx140, OPAx209, OPA1641-2-4, OPAx141, OPAx320, OPAx171, OPA1602-4, OPAx322).

I have also created macro-models based on our new architecture for OPAx340, OPAx343, OPAx350, and OPAx353 to replace our older macros (that I created some 15 years ago) but since these are not new RTM's their upload has much lower priority - hopefully, they should be posted to TI website later this month.

In meantime, you may use the OPAx340 macro-model I previously attached;  I have also attached here a new macro for OPAx350 in case you need it.

Good luck.