Planning to measure (varying under switching e-bike load) voltage from a battery.

To step down the voltage panning to use a voltage divider and buffer the output to an ADC via an op-amp.

To completely switch off the device when the e-bike controller is off will need to switch off the current through the voltage divider (maybe this is an overkill considering how small the current is (~2mA), the controller's quiescent current is probably greater).

Planning to switch the divider with a PMOS.

How do I calculate the bandwidth of the divider + PMOS?

I suppose I should use Rds_on of the PMOS and Cin and Cout of the PMOS to get an equivalent circuit?

I first thought I would represent the PMOS as Rds_on in parallel with Cgs but I'm looking at PMOS datasheets and they only give Cin and Cout.


simulate this circuit – Schematic created using CircuitLab

  • 1
    \$\begingroup\$ Why not use \$950k:50k//0.1uF\$ and forget the switch \$\endgroup\$ Commented Apr 4, 2019 at 23:22
  • \$\begingroup\$ Would it be more susceptible to EMI with higher resistor values(lower current) and more error due to the op-amp's finite input impedance? \$\endgroup\$
    – axk
    Commented Apr 4, 2019 at 23:24
  • \$\begingroup\$ The 0.1uF//50k=5ms so as long as wire loop area is minimized with twisted pairs or STP cable, spikes are attenuated so choose T=RC to exceed your ADC conversion time. \$\endgroup\$ Commented Apr 4, 2019 at 23:27
  • 2
    \$\begingroup\$ Check the \$\mathrm{V_{GS}}\$ rating of the PMOS -- most of 'em are only good to 20V. \$\endgroup\$
    – TimWescott
    Commented Apr 5, 2019 at 0:31
  • 1
    \$\begingroup\$ @axk if you're not switching it too fast, you can just use a resistive divider. \$\endgroup\$
    – TimWescott
    Commented Apr 6, 2019 at 0:27

1 Answer 1


This image should help you place the capacitances where they belong in the MOSFET model. You need all three and they should all be listed in the datasheet.

Personally though, I probably wouldn't bother with the MOSFET parasitic capacitances in this case.

enter image description here

https://micro.rohm.com/en/techweb/knowledge/si/s-si/03-s-si/4873 https://micro.rohm.com/en/techweb/upload/2017/03/si_2-2_capf.gif

  • \$\begingroup\$ I saw this article. Need to figure out what would the equivalent capacitance be in this case expressed via Ciss and Coss that are given in the datasheet. \$\endgroup\$
    – axk
    Commented Apr 4, 2019 at 23:48
  • \$\begingroup\$ You need Crss too. which is also given in the datasheet. Note that they are given under a specific set of test conditions. \$\endgroup\$
    – DKNguyen
    Commented Apr 4, 2019 at 23:57
  • \$\begingroup\$ I've also found this: electronics.stackexchange.com/a/244894/69374 which follows from the above model I suppose \$\endgroup\$
    – axk
    Commented Apr 5, 2019 at 23:22

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