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OPA2330: Simple unity buffer not working when driving F28377D microcontroller ADC reference.

Tom Evans
Prodigy 10 points
Part Number: OPA2330
Other Parts Discussed in Thread: OPA333, OPA350, OPA320

Hello,

I have a circuit where both channels of the OPA2330 (full part number OPA2330AIDGKT) are configured as a simple unity buffer. A 3.0V input on both channels comes from a precision reference (REF3430MDBVTEP). Each output drives VREFHI for 2 ADC banks on an MCU (TMS320F28377DGWT-EP). The issue is that the opamp outputs are 1.85V, not 3.0V as was expected. The output is not being loaded by any device other than the VREFHI input pins. The issue exists on both channels and across multiple circuit boards that were tested. We are unable to identify any issue with the implementation.

We are operating the ADC banks in 12-bit mode and, per the suggestion on the microcontroller datasheet, there are 1.0uF bypass capacitors near each of the VREFHI pins, totaling 2.0uF in total between the net and GND; however, these are not shown on the circuit snip below.

This circuit is implemented in a similar way on the LAUNCHLX evaluation board which featured a buffered and filtered precision reference voltage driving two ADC banks.

See circuit diagram:

Thank you for any insight.

Tom

  • 0
    TI__Mastermind 25398 points

    Tom,

    Based on your post, I believe that you are connecting the OPA333 output directly to a filter capacitor at your microcontroller ADC input.  In general, op amps cannot directly drive capacitive loads.  When a capacitor is connected at the output of an op amp the amplifier will become "unstable".  Here unstable means that the amplifier output will oscillate.  So, if you look at your amplifier output with an oscilloscope, you will likely see an oscillation (not your expected DC signal).  Almost all amplifiers behave in this manor.  This application note describes the changes you would make to drive the capacitor:  https://www.ti.com/lit/an/sboa558/sboa558.pdf.  Basically, you need to add a resistor in series with the capacitor.  The precision lab video series section on stability covers the theory on stability:  https://www.ti.com/video/series/precision-labs/ti-precision-labs-op-amps.html 

    I will say that the OPA333 has a disadvantage (and also advantage) when being used for this application.  The disadvantage is that the open-loop output impedance is not flat (i.e. it is a complex output impedance).  Also, the output impedance is somewhat large.  These two factors basically mean that the isolation resistor required will need to be large to stabilize your circuit.  The advantage of the OPA333 from a reference buffer perspective is that it has very low offset and offset drift.  Thus, the error added to the reference will be minimized.  Maybe one final disadvantage is that the OPA333 is that it is a micropower device, so it has limited output drive.  The short circuit current of the opa333 is 5uA.  Generally, the reason for using a reference buffer is to increase the output current capability of the voltage reference and also to improve the transient response (faster response).  I think it is likely that your voltage reference (without a buffer) would have about the same drive and transient response of the OPA333, so I don't think the OPA333 is really doing the intended purpose (I may be missing something in your application).  Normally, a device like OPA350 or OPA320 is used for reference buffer applications.  These devices have flat output impedance, good drive current, and fast transient response.  They will be easy to stabilize and will respond well to the ADC reference transients.  

    I hope this helps.  I can help choose the isolation resistor, but I would like you to confirm the following:

    1. Measure the output with a scope and confirm  the amp is oscillating.
    2. Look at other op amp options.  Will OPA350 or OPA320 work for your requirements?
    3. Once you confirm your op amp selection, I can show you simulations that illustrate how the isolation resistor solves the problem.

    Best regards, Art