ADM7150 Output Impedance vs Frequency

Hi,

Are there any plots or measurements of output impedance vs frequency of ADM7150?

  • 0
    •  Analog Employees 
    on Jun 4, 2014 5:01 AM over 6 years ago

    Bez,

    we do not publish or test output impedance as it varies based on operating conditions. Output impedance will be very low as you can see from the Load Regulation specification in the datasheet (<1%/A or < 10mohm at 1V). Depending on the output capacitance and the effective bandwidth of your design this will be valid up to something around 100Khz and then starts increasing over frequency and will be dominated by the output cap impedance.

    This is valid for most LDOs not just the ADM7150.

    I hope this helps.

    LucaV

  • Many thanks, very helpful!

    I am assuming, translating this would give low frequency Zout for a typical 3.3V output of <30mOhms

  • 0
    •  Analog Employees 
    on Jun 4, 2014 9:18 PM over 6 years ago

    Yes, but typically it will be lower than that at lower current. You can see that on Figure 16. The output impedance is almost negligent from no load to 100mA, and then the output regulation drops mostly when closer to 800mA.

    LucaV

  • Thank you for your continued support

    My application draws less than 100mA so pretty much in the sweet spot of this output impedance characteristics.

    Actually, if I may, I would like to request a bit more further advice:

    The target design is a low phase noise oscillator (50M&100MHz) and I am using the ADM7150 to power it @3,3V, right close to the XO, Design is based on the reference circuit as in Figure 1 of the datasheet. So aside from the 10uf output capacitor there are also bypass caps of 100nf//10nf near the XO to reduce the high frequency impedance. These ceramic caps, as I understand it, have ultra-low ESRs, possibly <1mOhm. Would these affect the stability of the AD7150. Datasheet indicates 1mOhm ESR as the minimum this LDO can tolerate.

    Please check and advise.

  • 0
    •  Analog Employees 
    on Jun 5, 2014 9:09 PM over 6 years ago

    The additional caps 100nF and 10nF will not affect the stability of the circuit as the equivalent circuit is dominated by the 10uF cap. You do not have to worry about it and it is always a good idea to use different types of caps in parallel to improve PSRR at high frequency.

    Oscillators and especially ones that have PLLs integrated will be sensitive to noise on the power supply at every frequency. The ADM7150 has excellent PSRR up to and passed the 1Mhz range. You may need to manage noise at the input of the ADM7150 if you have content above the 10Mhz range from a switching regulator for example (switch node ringing and feedthrough), an additional ferrite bead may be useful.

    Best regards,

    Luca