First, since 90V can kill your Arduino I'd recommend a pair of protection diodes* and a series resistor.
(This circuit will knock-down the voltage in a non-linear fasion, so it's just protection against over-voltage... It is not
used to attenuate a 0-90V signal down to 0-5V inearly... i.e. 90V or 45V would both read "5V".)
Then, you'll need to measure the voltage output under your real-world conditions. You can experiment with a parallel resistor (probably in the in the megohm range) to knock-down the signal
(linearly), if necessary. You are unlikely to get 90V and you might end-up getting less
than 5V depending on your pressure & physical configuration. (But, I'd still recommend the protection diodes.)
Or, you can use a regular 'ol voltage divider
(again along with the protection diodes). But, with the high source impedance of piezo will create a 3-way voltage divider, and your signal will be reduced by more than the calculated amount... A lot
more if you use low-value resistors in the voltage divider.
i need a voltage sensor because i want to detect the voltage comming from the piezo from different pressures. so i want the exact number...
That's only gong to work for quick pressure CHANGES. A constant pressure (with no physical movement) is NOT going to generate a constant voltage.** I believe the piezo acts like a small capacitor, so it might
sort-of hold the voltage for a several microseconds as it discharges through the load resistance.
* The Arduino has built-in protection diodes, but they are rated for low current and are only there as a "last resort" in case something unexpected happens (such as static discharge). If you are experimenting with something that puts-out more than 5V in normal operation, you should take steps to reduce the voltage before
it hits your Arduino.
** Conservation of energy... Gravity can generate electricity as water flows
down and through a generator. But, you cannot use the static pressure of water behind a dam to generate electricity.