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OPA365 in ADC Driver and DAC Buffer Configurations

Jonathan Newman
Prodigy 160 points
Other Parts Discussed in Thread: OPA320, OPA365, OPA209

Hello,

I am designing a 4 channel ADC/DAC frontend which uses the following chips

Both of these chips convert/generate signals in the range of  0 to VRef. However, my device needs to convert/generate bipolar signals from -Vref to Vref. To do this, I am using the OPA365 to level-shift and amplify input and output voltages into the appropriate range. Below I have pasted the circuit designs I am using to do this:
DAC Buffer (non-inverting 2X gain with -Vref level shift. DAC_A+ signal is coming directly from the output pin of the DAC. VREF is 4.096 volts and coming from the REF5040) :
ADC Buffer (inverting 0.5X gain with +Vbias=Vref/2 level shift) :
The input stage for the ADC is pictured below:
My problem is that after the signals have passed through these op-amps (at the ADC input stage or the final DAC output stage), they have both incurred have a ~10 MHz, ~40 mVP2P oscillation. Here are some oscilloscope shots (10X probe, properly compensated) . I have labeled each trace to correspond to the circuit diagrams above.
DAC Output buffer. Yellow: DAC_A, Pink: DAC_A+, Blue: Positive supply rail. Please ignore the large, high frequency transient that appears in all three traces. This is the result of a switching power supply that has since been removed from the circuit. I interested in the periodic signal on the DAC_A line.

ADC Input buffer. Yellow: ADC_A, Pink: ADC_A+, Blue: Positive supply rail. Please ignore the large, high frequency transient that appears in all three traces. This is the result of a switching power supply that has since been removed from the circuit. I interested in the periodic signal on the ADC_A+ line.
I will mention that each chip derives power from a +/- 4.5 Volt LDO regulator pair. 
I will also mention that I have tried the circuit both on a PCB in which i did my best to observe proper layout procedures and on a breadboard, and both situations show this oscillation. My questions are the following:
1. What could be causing this oscillation?
2. Am I using the right op-amps for these applications? I have begun to wonder if an op-amp with better capacitive drive capability would be a better choice for the DAC output driver
3. Any recommendations for different IC's and topologies to accomplish my goal would be much appreciated.
Thanks.
- Jon
  • 0
    TI__Genius 12325 points

    Jon,

    To start, you are violating the absolute maximum power supply rating of the OPA365, this part is designed to operate with an absolute maximum supply voltage of 5.5V (you are applying 9V. Furthermore, you are correct that the capacitive load drive capability of the OPA365 is limited and may be causing the oscillation that you are seeing. I recommend that you replace the OPA365 with the OPA320 which has an improved cap load drive capability and see if this solves your issue (for both ADC buffer and DAC output driver).

  • 0 in reply to John Caldwell
    Prodigy 160 points

    John,

    Oh wow - that was stupid... I just went over to the breadboard, dialed down the supply voltage and there is no more oscillation. Great!

    Before I mark this as resolved, can you possibly extend your recommendation for a replacement to something that has 9 Volt supply range that I am looking for? I note that the OPA320 has a maximal supply of 6 Volts. Keeping the SOT23 package would be nice, but I would rather keep my  voltage rails, so a change in package is OK.

    Thanks.

    - Jon

  • 0 in reply to Jonathan Newman
    TI__Genius 12325 points

    Jon,

    The OPA209 will operate on those supply voltages, and should be able to handle the capacitive load shown in your schematic. However this is part uses bipolar junction transistors in the input stage and will therefore have slightly higher input current noise (lower input voltage noise though) than the OPA365 or OPA320, and it may be necessary to reduce the values of your feedback resistors from 10k to something more like 1k, if the increase in noise presents a problem. 

  • 0 in reply to John Caldwell
    Prodigy 160 points

    Thanks for your help, John.

  • 0 in reply to Jonathan Newman
    TI__Genius 12325 points

    Anytime! Good luck with your project!