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output impedance of LPV521 follower

Other Parts Discussed in Thread: MSP430F5438A, LPV521

Hello,

I have a question about LPC521 follower, we are using it with MSP430F5438A, the input is 3.8V and after a resistor divider, 1.9V feed to LPC521 voltage follower. and output to a analog input pin ( p6.0 )of MSP430F5438A, I notice the reading from multimeter at the p6.0 and the ADC value have a 80mV to 100mV difference( ADC reading are higher than multimeter) . we have other ADC reading from p6.1(1.5V DC) and p6.2 which are very close to the multimeter readings, that prove the reference should be OK. it turns out MSP430F5438A ADC require the output side impedance smaller than 10K(see MSP430F5438A errata, ADC 27 work around). I think the voltage follower should have low output impedance. but I am not sure about this LPC521.

I also notice if add a 0.1uF cap bypass LPC521 follower output to ground, will make the ADC reading close to multimeter. could anyone explain why this cap can make this happen. it is a DC voltage, and from the scope the ripple is very low. less than 20mV.

Sincerely,

David Sun

  • Hello David,

    While it seems like it is a "DC" circuit, it is not.

    The input to an ADC is a "dynamic" load. The load is constantly changing as the ADC's internal sampling cap is switched in and out at the ADC's sampling rate.

    This creates a changing load on the amplifier. This changing load affects the amplifiers stability, which can cause the output to "ring" on the transitions. His ringing can cause the reading to read high or low, depending on where the sample point falls on the ring.

    The LPV521 is a nano power (300nA) op-amp, so it has a fairly high output impedance and is slow (6KHz GBW). So it cannot react quickly to fast changing loads. THe effective output impedance rises as the frequency goes up, so the output impedance will be a few Kohms at 6KHz.

    Do you have a series resistor and storage cap? Placing a small capacitor (~30-50pF) at the ADC input pin provides a low impedance charge "reservoir" to provide the initial current required to quickly charge the internal ADC sample cap. See page 22 in the LPV521 datasheet.

    It is not good to place a capacitor directly on the op-amp output. There should always be a small value resistor between the output and the cap.

    Regards,

  • thank you for your fast response and clear explain.