Oscilloscopes let us measure input voltage in a variety of input signal ranges (either AC/DC, +/- 500V or Gigahertz's of frequency).
I am wondering: how they make it happen?
Say, an oscilloscope has a microprocessor/microcontroller/ADC unit at its heart, which usually works with
0..5 V DC voltage. The input signal could be not DC, but AC also and may have huge peak values, in comparison to the 0..5 V range.
There should be an input cascade, which downscales this voltage to a reasonable range (for example,
+/- 2.5 V) and offsets it to be in the working range of ADC/MPU/MCU (
0..5 V). It is also possible to have a slightly narrower range to allow for error/peak values, say
1.25 .. 3.75 V.
There are three ways I know, which allow for a downscale and offset of an input signal:
- potential transformer (probably won't work for DC)
- resistor-based voltage divider
- operational amplifier configured to have fractional gain (in range
A potential transformer won't work for DC.
Resistor ladder / voltage divider will work, but will be very sensitive to the overall device resistance:
From what I know, the rest of a device may have variable resistance, and thus the calculation of resistor values won't be reliable (provided that
Rdevice may vary).
The opamp input cascade might do the trick, but the IC should have very specific characteristics, since the input voltage offset, temperature drift, slew rate and bandwidth might significantly distort the signal.
And I am not quite sure whether providing high voltages or AC voltage will work fine with opamp.
My question is: is there an effective way of downscaling the input signal of either AC/DC, high frequency (up to GHz range) or high peak values (up to
400 V)? Or how it is implemented in oscilloscopes?