LMC6064: unity gain oscillation

Brian Antheunisse

Part Number: LMC6064
Other Parts Discussed in Thread: ADS7852, OPA4187

Hi guys,

My customer is using LMC6064 and has the following question:

I’m taking the reference of 2.5 from the ADS7852YB, doubling it to 5 using an OpAmp, then a second OpAmp unity gain as a buffer.

The problem that I’m having is the 2^nd OpAmp is starting to oscillate. I’m using the LMC6064AIM.

At first I thought I had too much capacitance on the output, but I removed all the caps and it is worse.

This occurs with or without a load. The load is 8 RTD probes in parallel, one side to the 5 volt reference the other side to the 8 inputs to the ADS7852.

Each has a 3.01K resistor to ground. The 8 RTD probes are installed at MTA54.

This is probably something simple, but I just can’t see it.

Thanks,

Brian

over 5 years ago

0 kai klaas69 over 5 years ago

Guru 140200 points

Hi Brian,

I think 3.01k / 8 = 375R is too much load for the LMC6064.

What is your supply voltage?

Kai

0 Luis Chioye over 5 years ago in reply to kai klaas69

TI__Guru 54556 points

HI Brian,

The LMC6064 is a micropower amplifier. It will not be stable driving the large capacitive load, and as Kai has mentioned, it will also have issues driving a 375ohm load to +5V, as you will be right at the current limit.

A suggestion would be to use an amplifier that can handle the current load. Please let us know what is the voltage supply voltage requirement, and we could suggest an amplifier and compensation to drive the capacitive load.

Best Regards,

Luis

0 Brian Antheunisse over 5 years ago in reply to Luis Chioye

TI__Expert 5265 points

They are using 12V, single supply. Thanks!

-Brian

0 kai klaas69 over 5 years ago in reply to Brian Antheunisse

Guru 140200 points

Hi Brian,

you could split the 8 bridges into two groups of 4 bridges each and power each group by this circuit:

brian1.TSC

Kai

0 Brian Antheunisse over 5 years ago in reply to kai klaas69

TI__Expert 5265 points

I'll ask if this is possible.

Luis, do you have a different op amp suggestion that could handle the whole load?

Thanks,
Brian

0 Raphael Puzio over 5 years ago in reply to Brian Antheunisse

TI__Intellectual 1365 points

Hello Brian,

We have many OpAmps that can handle the load. Can you help us with the Maximum IQ you can live with in your design?

This will help us narrow down what amplifiers to recommend.

I look forward to your reply.

thanks,
Raphael

0 Brian Antheunisse over 5 years ago in reply to Raphael Puzio

TI__Expert 5265 points

I'm not sure that spec is very important in their system.

Thanks!
Brian

0 Raphael Puzio over 5 years ago in reply to Brian Antheunisse

TI__Intellectual 1365 points

Hi Brian,

Typically when a customer chooses a micro power Op-Amp, it's because they want the low power. If IQ really is not a concern, point them to the OPA4187. Check out the comparison below and let me know if you have any questions.

thanks,
Raphael

0 kai klaas69 over 5 years ago in reply to Brian Antheunisse

Guru 140200 points

Hi Brian,

it's a bit strange to use a 16µA micropower OPAmp when at the same time the load current is as high as 5V / 375R = 13.3mA anyway...

Kai

0 Brian Antheunisse over 5 years ago in reply to kai klaas69

TI__Expert 5265 points

it was already on their bom and they tried to reuse. We are going with OPA4187 for the time being.

Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

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LMC6064: unity gain oscillation