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OPA569 Basic Connection

Jeff Yang
Prodigy 20 points
Other Parts Discussed in Thread: OPA569

I am using the OPA569 in a design and followed the 'basic connections' diagram in the datasheet (image copied below).

V+ is 5V DC from a power brick and V- is grounded.

What is the purpose of the 47uF capacitor that bridges the V+ and V- pins?  When this capacitor was in-place, we measured 1.6V drop across it when we expected the full 5V.  After removing it, our circuit had no problems operating since it was receiving enough voltage to power the device.

  • 0
    TI__Guru* 93105 points

    Hi Jeff,

    I sometimes see this capacitor applied across the supply pins. It does behave as a very low impedance to a range of ac frequencies and does acts as a charge resorviour providing some of the transient current demanded by the OPA569 during such events. However, that is the purpose of the 47 uF and 0.01 uF capacitors connected at each of the supply pins.  

    It sounds like the 47 uF capacitor that you had connected across the supply pins may have been faulty. The capacitor should appear as a near open circuit to dc. If the circuit is working as expected without this additonal capacitor I do not find a good reason to include it.

    Regards, Thomas

    PA - Linear Applications Engineering

     

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points

    Hi Thomas,

    Thank you for the reply.  We are trying to understand why it is not open circuit at DC.  I don't think the capacitor is faulty since we measured 4 of the same boards and saw the same voltage drop.

    Thomas Kuehl said:

    I sometimes see this capacitor applied across the supply pins. It does behave as a very low impedance to a range of ac frequencies and does acts as a charge resorviour providing some of the transient current demanded by the OPA569 during such events. However, that is the purpose of the 47 uF and 0.01 uF capacitors connected at each of the supply pins.  

    So the purpose of this capacitor is a backup charge reservoir to the 47uF and 0.1uF capacitors at the supply pins?

    Thanks,

    Jeff

  • 0 in reply to Jeff Yang
    TI__Guru* 93105 points

    Hi Jeff,

    I overlooked the point in your original message where you said the OPA569 is being powered by a +5 V brick supply with the V- is grounded. Therefore, if you are placing the extra 47 uF capacitor between the device power supply pins it is essentially in parallel with the 47 uF/ 0.01 uF capacitors in place at the V+ pin. Having one, or two, 47 uF capacitors connected to the V+ pin shouldn't affect the dc voltage on that pin. The only way that the supply voltage could sag -1.6 V from the nominal 5 V value is if the amplifier circuit is drawing too much current for the power supply to handle. Is there any way for you to measure the dc current in the power supply line with, and without the extra capacitor? Also, can you measure if any dc current flows through the extra 47 uF capacitor when you connect it into the circuit?

    Also, take a look at your +5 V power supply line, and the OPA569 output, with an oscilloscope. Do this with and without the extra 47 uF capacitor in the circuit. Look for any oscillation on these circuit points. If oscillation is present the capacitor will offer only limited reactance and ac current can flow likely increasing the dc current drawn from the supply.

    Regards, Thomas

    PA - Linear Applications Engineering