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OPA1641: OPA1641 single supply through OPA27. OPA27 feeding two loads with 1/2 Vref

Matteo Bonino
Prodigy 100 points

Part Number: OPA1641
Other Parts Discussed in Thread: OPA27

Tool/software:

Dear all, I'm currently designing an op amp buffer, based on the OPA1641 (part of a more general Hi-Fi preamplifier project). This buffer will receive audio signal from the output of a phono preamplifier. Scope: to reduce phono preamp's output impedance, and to be able to drive several loads.

For a number of reasons, I only have single 34V supply available.

Hence, I've designed the solution you see in the picture: an OPA27 op amp 'generates' 17V ref, which will be my virtual ground.

I'd like to hear your thoughts about this solution you see in the picture; my main doubts are the following:

  • As you see, the OPA27 will drive two different loads: a THAT1203 integrated circuit (not subject of this question), and my OPA1641 buffer. Do you see any issue with that ? 
  • And another question, as I've connected the output of the OPA27 to the non inverting input of the OPA1641 through a high value resistor (475Kohm). I need to keep high input impedance for the OPA1641 in fact. Any concern ?

Thank you very much in advance !

Matteo

  • 0
    TI__Guru* 77501 points

    Hi Matteo,

    Matteo Bonino said:
    As you see, the OPA27 will drive two different loads: a THAT1203 integrated circuit (not subject of this question), and my OPA1641 buffer. Do you see any issue with that ? 

    OPA27 is unable to source current as an adequate voltage regulator for OPA1641 or THAT1203. The OPA27's short circuit current is ±25mA at 25C, meaning that the max. operating current is approx. 30-50% of Isc or < ±13mA . In addition, there is quiescent current change over temperature in this part and the actual operating current should be less than ±10mA (max). 

    If OPA27 is able to source < ±3mA over temperature, maybe this is acceptable. But I will find another op amp with higher output current, such as OPA210 or OPA189 (chopper amplifier). as the voltage reference for the application (increase the design margin). 

    Here is chopper amplifier selection. There are some lower cost ones as well (rated up to 36Vdc supply rail).  

    https://www.ti.com/amplifier-circuit/op-amps/precision/products.html#2192=Zero%20Drift&

    Matteo Bonino said:
    And another question, as I've connected the output of the OPA27 to the non inverting input of the OPA1641 through a high value resistor (475Kohm). I need to keep high input impedance for the OPA1641 in fact. Any concern ?

    However, if you do the following, which I think that this is what you have in mind, then OPA27 or similar precision op amps should work. I would recommend to use chopper amplifier for the Vref application (please do not forget to implement bypass capacitor 0.1-1uF next to the op amp's power pin).

    If you have other questions, please let me know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Prodigy 100 points

    Indeed, this is exactly what I meant, sorry maybe I was not so clear. so, the OPA27 will just generate reference 1/2 voltage for both the THAT1203 and the OPA1641. As this is just a bias, needed for the THAT1203 and OPA1641 to operate with single supply, I believe the very little current drawn will be well within the OPA27 specs.

    I confirm that both THAT1203 and OPA27 will receive their Vcc supply (34V each) from a separate regulator. The OPA27 is just needed for the reference 1/2 voltage (= 17 Volt).

    I confirm the bypass capacitors will be installed close to the relevant chips.

    My doubts were: 1) if the OPA27 can 'drive' both outputs without instability / etc issues. And 2) you see the 475Kohm resistor from the OPA27 output to the signal input of the OPA1641. I need to keep high input impedance for the OPA1641 in fact. so my question is: would this work ?

    Thanks so much again

    Cheers

    Matteo 

  • 0 in reply to Matteo Bonino
    TI__Guru* 77501 points

    Hi Matteo, 

    Matteo Bonino said:
    1) if the OPA27 can 'drive' both outputs without instability / etc issues. And 2) you see the 475Kohm resistor from the OPA27 output to the signal input of the OPA1641.

    Both circuit is stable.

    If THAT1203 has up to 1nF parasitic input capacitive load w.r.t GND (AC), it has phase margin of 70 degree. 

     

    OPA27 Loop Analysis 03042025.TSC

    For OPA1641, you do not need use such large resistor in series. Say 100kohm and 1uF capacitive load and it should be stable (assume the similar output condition as above)

    OPA1641 Loop Analysis 03042025.TSC

    If you have other questions, please let us know. 

    Best,

    Raymond