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LMH6321: LMH6321MRX/NOPB Schematic Review.

Sudhir D
Prodigy 30 points
Part Number: LMH6321
Other Parts Discussed in Thread: DAC7716, THS3091

Tool/software:

Dear All,

I am Designing a Buffer Circuit using LMH6321MRX/NOPB IC to Amplify the DAC Output Current. The Load requires atleast 150mA Output current for which I was testing the IC with the following connections. 

Pin 1,2&8 are left open. 

+10V & -10V is supplied to the IC pin 7&4 respectively through a .1uF Cap which is given to common Ground.
3.3V is given as input to the PIN 3 of the IC and Pin 6 is connected to the Load Generator. 


As I increase the Load Current from 0 to 100mA the voltage drops gradually from 3.3V to 2.8V at 90mA load but at 100mA Load the voltage drops to 0V. The IC Datasheet specifies that it can give upto 300mA of Buffer. Am I missing something in the connections? 

Also, can I supply +/-15V as I will require Current Amplification for higher voltages as well? And are there any special considerations that I must follow for amplifying negative voltage?

  • 0
    TI__Genius 16565 points

    Hi Sudhir,

    With these supply voltages, this should not be happening. You should expect a reduction in output swing as you pull more current but not at these levels. I am wondering if maybe you have a current limit set at your supplies potentially and it is hitting its limit? You can set this device to +/-15V and should have no issues. The same idea will apply if setting -3.3V at the input for example. The consideration is not hitting the device's output range with either input polarity.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 30 points

    Hi Ignacio

    I had previously set the current limit to 0.1 A, and after increasing it to 1 A, the device is now operating without any shutdown issues.

    However, I’m still observing a voltage drop at the output. To investigate further, I connected a 33Ω resistor at the output and applied an input voltage of 3.3 V. Under this condition, I measured a voltage drop of approximately 0.6 V at the output. Is this drop the usual behaviour of the amplifier?   

    Thanks & Regards
    Sudhir D

  • 0 in reply to Sudhir D
    TI__Genius 16565 points

    Hi Sudhir,

    Just for some clarity, did you now connect a 33-ohm load to ground instead of using a load generator? When setting an input of 3.3V you are measuring a DC signal of 2.7V across that 33-ohm load. Are you still setting the voltage supplies to +/-10V. 

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 30 points

    Hi Ignacio,


    Yes, I connected a 33Ω resistor to ground instead of using the load generator.

    I’ve set the supply voltages to ±15V, which is within the operating range of the IC. I also tested with another IC and observed the same behaviour—when applying a 3.3V input, the output across the 33Ω load drops to around 2.6V to 2.7V.

    Let me know if you think there’s anything else I should check.

    Thanks & Regards
    Sudhir D


  • 0 in reply to Sudhir D
    TI__Genius 16565 points

    Hi Sudhir,

    Thank you for confirming your configuration. I believe this large drop is due to the voltage drop across the internal output impedance of the device. The output impedance of this device is around 4.3ohms, with the load you have you would expect a drop through this internal resistance of around 0.38V. Another reason you would be seeing this which is adding to the output error, is the voltage gain of the device. For this device, in a loaded condition (>200mA) the internal gain reduces to around 0.92V/V. I believe it is the combination of these two things that is causing this drop.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 30 points

    Hi Ignacio,

    Is there a way for me to avoid this drop as I want to implement this IC as amplifier to the DAC7716 output. My requirement is to get at least 150mA gain through the Buffer without any drop. Is there a way to make it work with LMH6321MRX/NOPB?
    If not, can you please suggest an alternate solution.

    Thanks & Regards
    Sudhir D

  • 0 in reply to Sudhir D
    TI__Genius 16565 points

    Hi Sudhir,

    One way to do this would be to implement a composite amplifier circuit to account for this drop as well as other dc errors due to the device being in an open loop. Since there is no feedback, errors such as this are not corrected for like they would in a standard amplifier circuit with negative feedback. We do have the THS3091 that is worth considering for your application. We also sell an EVM for this device if you would like to quickly test its performance.

    Best Regards,

    Ignacio