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ACF2101: APPLICATION BULLETIN SBOA034 table II: How is the feedback capacitor C2=1,5pF implemented

marcel rossewij
Prodigy 30 points
Part Number: ACF2101

As I'm new to this switched capacitor device, I have a question on the APPLICATION BULLETIN SBOA034 (Comparison of Noise Performance of FET Transimpedence Amp/Switched Integrator from Bonnie Baker). 

For the bandwidth optimised configiration on page 7 and TABLE II, C2 is set to 1,5pF. The ACF2101 internal capacitor is already CINT=100pF. Is for this case the internal capacitor not used and only a 1,5 pF external capacitor connected between OUTx and INx?

I somehow expect that external capacitors are used for larger capacitance, not smaller. 

In addition, page 6 states "The fastest feasible sampling frequency for the ACF2101 is 42.55kHz. This assumes that the internal capacitor (100pF) is used." which I interpret that the 100pF should be used for the fastest sample frequencies (so with optimum bandwidth performance I suppose) 

  • 0
    Guru 140200 points

    Hi Marcel,

    marcel rossewij said:
    For the bandwidth optimised configiration on page 7 and TABLE II, C2 is set to 1,5pF. The ACF2101 internal capacitor is already CINT=100pF. Is for this case the internal capacitor not used and only a 1,5 pF external capacitor connected between OUTx and INx?

    The formation of table II is explained in detail on page 7. A feedback resistor of 100M and an external feedback capacitor of 1.5p is connected to the ACF2101, instead of using the internal 100p capacitance.

    marcel rossewij said:
    I somehow expect that external capacitors are used for larger capacitance, not smaller. 

    Why? Whenever the desired capacitance value differs from the internal 100p you would need to connect an external one. This is valid for either a smaller or a bigger capacitance Relaxed

    Kai