# Sampling high impedance, higher voltage source

I have a low voltage ADC. How do I sample a high impedance signal that has a voltage higher than my ADC can handle?

I understand the concept of buffering the ADC input to deal with the high impedance aspect. I also understand the concept of using a voltage divider to lower a voltage range. But a voltage divider is a current sink, and my signal is high impedance.

Is there a general strategy? Do people use a voltage divider and just pick resistances that are high enough to meet an error goal?

To throw some numbers at this, my ADC can measure GND to 3.3V (it's in a MCU). The signal of interest is 0-5V, and I'd like to be able to sample signals having an impedance of up to 500 kOhm. My sampling frequency is low (in the order of 1 Hz).

• Buffering will convert the high-impedance signal into low-impedance signal. From there, using voltage dividers becomes less problematic (But still, you should should take the input behaviour of the ADC, though). Plus, they'll be no longer driven by the high-impedance source so shouldn't be a problem from your high-impedance signal generator. One of the most important thing to take into account is the input bias currents of the op amp / buffer as they'll affect the signal seen by the op amp. Oct 27, 2023 at 9:52
• How high frequency is the signal? Have you tried to simulate your divider with the high impedance source modeled? Oct 27, 2023 at 10:07
• @winny Low frequency (in the order of 1 Hz). I'd like to accommodate a wide range of source impedances, so before I settle on an impedance number/spec I'd like to get a better grasp on the theory Oct 27, 2023 at 10:51
• Good, then you are deep into the resistive only domain. What's your source impedance? Oct 27, 2023 at 10:56
• The implementation will depend on what are the actual numbers of the voltages and impedances involved. And also how often you want to take a sample. Please provide actual numbers, othetwise we don't know if your high voltage high impedance is 5V and 100kohm or 5kV and 500Mohm. Oct 27, 2023 at 11:10