OPA2156: Expected operating temps under bias

Hi Douglas,

I need to ask a couple of questions to help debug the issue.

Can you please clarify, what is the common-mode voltage of the 0.01Ω of the shunt resistor with respect to the OPA2156 local ground?  In other words, is this a high-side current sense resistor sitting at a high-common-mode voltage? or is the shunt resistor referred to the same local GND of the OPA2156?  When you apply a 1V voltage to the shunt, does this mean a common-mode voltage to the shunt referred to the same ground of the OPA2156? 

Please draw in the schematic/diagram above the shunt resistor connections, showing both the common-mode voltage supply with respect to the OPA2156 local ground, and the differential voltage across the shunt to ensure we understand the test conditions.

You have explained that the OPA2156 difference amplifier circuit is driving an ADC (A2D).  Is there an RC filter between the A2D and the OPA2156?  If yes, what are the R C values? 

How many boards or devices show this issue?  Is this an engineering prototype, soldered by hand or a board that went through a standard solder re-flow and clean process?

Thank you and Best regards,

Luis

Hi Luis,  Thanks for the quick reply.    I've updated the schematic image to illustrate the setup that I was measuring under.   This is a high side sense resistor with Vdc being programmable (1V and 6V during my tests) and then just a fixed 1.21K resistor on the other side of the shunt.   The application will have a different load source but right now to simply this issue I'm focusing on the no load condition. I've also added in the voltages I've measured at different nodes.

The output of the op-amp is not driving an RC filter but does have a .01uF and .22uF cap to GND local to the A2D input.  These are prototypes and same behavior is seen on both of them.  Boards went through the standard process of solder and reflow.

Thanks,

Doug

Hi Douglas,

The OPA2156 will not be stable driving directly the relative large capacitance of .01uF and .22uF cap to GND at the ADC input.  Can you please check the output with an oscilloscope when the issue occurs for oscillations?

An open-loop small-signal stability analysis in TINA-TI using the OPA2156 model shows that the circuit is un-stable with an abrupt change in phase while driving the relative large capacitance.  Please see simulation below.  The difference amplifier circuit will need to use an isolation (compensation) resistor between the output of the op-amp and the capacitor at the ADC input to stabilize the circuit, and we would need to select the resistor and capacitor value depending the ADC sampling rate and sample-and-hold settling requirements.  

What ADC device are you driving, is this a SAR or Delta-Sigma? and/or what is the sampling rate (or acquisition time)?

Or what is the resolution of the ADC, and sample-and-hold cap value and full-scale voltage? 

Thank you and Best Regards,

Luis

Hi Luis,

You are correct, I do start getting oscillations when my supply is turned on at both 1V and 6V.     I’ve added some plots below as well as our max voltage condition of 12V below.  I am using a SAR A2D that is integrated in with our uController.    This is 12bit with 5V reference.  The sampling rate is 17us.

1V Condition:

• 1.7V p-p centered around 0V
• 68.5KHz Oscillation
• 71C

6V Condition:

• 836mV p-p centered around 0V
• 270KHz oscillation
• 90C

12V Condition: 

• 956mV p-p centered around 0V
• 286KHz Oscillations
• 93C

Thanks,

Doug

Hi Doug, 

Douglas Grover said:

I do start getting oscillations when my supply is turned on at both 1V and 6V.

You would need to compensate the feedback loop and remove the loop instability from the difference amplifier.

Luis Chioye said:

we would need to select the resistor and capacitor value depending the ADC sampling rate and sample-and-hold settling requirements.  

I tried to compensate the op amp, but it has to lower the BW of the op amp, but the circuit needs to have higher BW at a rate of the 17usec. You may have to lower your capacitive load (230nF) or increase the Rsense and lower the gains. 

If you need to further assistant, please let us know. 

Best,

Raymond