I have a WiFi chip that requires a input impedance of > 100 kohms and < 5 pF as seen in the requirements table found in the datasheet below.

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I'm confused if this 5 pF requirement correlates to the output load of the clock I'm using.

enter image description here

Am I confusing things here? I feel like 15 pF output load violates the 5 pF input impedance requirement.

Another question is, how do I know if my output impedance is greater than 100 kohms, do I just place a 100 kohm resistor to do that job?

Any help is appreciated. Very new to this sort of stuff.

Link to Clock datasheet

  • 1
    \$\begingroup\$ As I read it, the <5pF and the >100k input impedance are not requirements, but givens. You known that the input of the WiFi chip will not load the oscillator with more than 5pF or more than 33uA (100k@3.3V). The 5 pF is clearly within the requirement of the oscillator that allows a load of up to 15pF. As far as the resistive load is concerned, the extract provided here does not indicate how much the oscillator allows. As 33uA is no more than 2% of the total current consumption of the oscillator, it is safe to assume that the oscillator can handle it. \$\endgroup\$
    – le_top
    Jun 9 '16 at 0:39
  • \$\begingroup\$ It does say when power is applied or switched off. \$\endgroup\$
    – user110971
    Jun 9 '16 at 0:41
  • \$\begingroup\$ What devices are these? Links to the full manufacturer datasheets? \$\endgroup\$ Jun 9 '16 at 0:42
  • \$\begingroup\$ @le_top thanks for the help, this means, that I can directly connect this oscillator without any passives correct? \$\endgroup\$ Jun 9 '16 at 0:49
  • \$\begingroup\$ @user2943160 updated the question with the links \$\endgroup\$ Jun 9 '16 at 1:19

I have a wifi chip that requires a input impedence of >100kohms and <5pF

You are misunderstanding the datasheet. That means that the input impedance of the clock input will be greater than 100 kΩ, and have a load capacitance of less than 5 pF.

This means that you need a clock source with a significantly lower output impedance than 100 kΩ, and can tolerate at least a 5 pF capacitive load. Luckily, just about every clock source, including your clock source, is able to drive this input.


However, note the input signal amplitude specified: 200-1800 mV. The 3.3 V output from the oscillator exceeds this. A voltage divider should be fine, the 32 kHz signal is slow enough that a voltage divider is not an issue.

  • \$\begingroup\$ so the wifi chip datasheet says this. "Whenever possible, the preferred approach for WLAN is to use a precision external 32.768 kHz clock that meets the requirementsas below table." Are you still sure about your response? Not doubting you, just want to double check. \$\endgroup\$ Jun 9 '16 at 0:48
  • \$\begingroup\$ Please add both datasheet links to your question. \$\endgroup\$
    – uint128_t
    Jun 9 '16 at 1:08
  • \$\begingroup\$ updated the original question with the datasheet, had to delete the pics to be able to post it though. \$\endgroup\$ Jun 9 '16 at 1:18
  • \$\begingroup\$ Yes, it appears that your selected oscillator is an appropriate clock source for the WiFi module. However, in the future, it's often more appropriate to hold off on accepting an answer, as someone else may provide a better answer. \$\endgroup\$
    – uint128_t
    Jun 9 '16 at 1:23
  • \$\begingroup\$ thanks for the help, will remeber. Also I assume this means I don't need any passives in between right? \$\endgroup\$ Jun 9 '16 at 1:31

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