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ADC10080 - Admissible load to VREFB and VCOM pin

Kawai
Guru 19775 points
Other Parts Discussed in Thread: ADC10080, ADC10040, LMH6550

Hi Team,

Please allow me to ask you question about admissible load to VREFB and VCOM pin of ADC10080.

I understand ADC10080 recommends not to connect loads to these pins, however, customer would need the VCOM and VREFB information in their system. They may connect high impedance input buffer to these pins. It may be difficult but could we have estimated drive ability (acceptable drive current ) for these pins ?

Best Regards,
Kawai

  • 0
    TI__Mastermind 44480 points

    Hi Kawai-san

    I have not had success finding the allowed load current for these pins.

    I do know light loading is acceptable, given what is shown in Figure 41 of the ADC10040 product datasheet. Since the Vcm pin of the LMH6550 can be connected to the Vcom terminal we know that a small load is acceptable. In the case of the LMH6550 the bias current typical value is 2uA flowing out of the amplifier Vcm terminal. Based on this I would limit the loading on the Vcom and Vrefb pins to +/-2uA.

    Regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    Guru 19775 points
    Hi Jim-san,

    Thanks for your advice.

    I need to apologize that I had a typo that it was VREF instead of VCOM. Customer is considering to connect light load (High impedance input amplifier) to VREF and VREFB pin to know the 000h level and center level of the ADC dynamic range. This will be used for video application.

    I think VCOM is an input pin and VREF are output pins. Internal circuit seems to be different, however, can you say +/- 2uA would be acceptable to VREF and VREB pins ?

    We are grateful for your support.

    Best Regards,
    Kawai
  • 0 in reply to Kawai
    TI__Mastermind 44480 points

    Hi Kawai-san

    I have done more research into the reference pins of this ADC. The reference voltages are generated using a resistor divider ladder and are unbuffered. The resistor ladder current is approximately 290 micro-amps. To give a maximum DC error of 0.1 lsb at 10 bits I would limit the external current load to a maximum of 28.5 nano-amps. This low loading can be achieved by using a low input bias current amplifier to buffer each of the reference pins.

    In the amplifier parametric sort below I have limited the selection to amplifiers with 25nA (25000pA) maximum input bias current. You can adjust the other parameters as needed for the other application constraints.

    http://www.ti.com/lsds/ti/amplifiers-linear/precision-amplifier-products.page#p10max=0.02;25000

    Best regards,

    Jim B

     

  • 0 in reply to Jim Brinkhurst1
    Guru 19775 points
    Hi Jim-san,
    This information would help us very much. We appreciate for your great support.
    Best Regards,Kawai