# Make high-sensitivity voltage measurements in just 4 steps

Blog Post created by jchang on Jan 2, 2017

Many oscilloscope users measure current flowing through a circuit by measuring the voltage drop across the current sense resistor. But if you’re lucky enough to have a Keysight N2820A high-sensitivity current probe, it is even more than that. While the N2820A (and N2821A) is designed to measure low level current signals with high sensitivity, accuracy and dynamic range, the underlying architecture of the probe is actually a voltage probe. It essentially is an ultra-sensitive voltage probe with extra-low probe loading measuring the voltage drop across the sense resistor in the current flowing path and dividing the voltage by the sense resistor value to derive the current flow. Or simply put, I = V/R.

You can use the N2820A current probe as a voltage probe in just a few steps:

1. Don’t use the current sense resistor. Use the user-defined head (Keysight N2825A) with the probe and connect the +, - input of the probe head directly to the DUT like you would to measure voltage with a voltage probe.

2. Keep the common-mode voltages of the signal input to less than ±12V, and differential input should be less than ±1.2V. If you have a signal with some DC component that exceeds the differential input range of the probe, you will rather AC couple the signal on the oscilloscope.

3. Choose 1Ω Rsense in the oscilloscope menu. That way, you can get the correct values on your measurement. E.g., 1 mA = 1mV.

4. If you have a Keysight Infiniium oscilloscope, you can go a step further and add external scaling to turn the units back into volts (with 1:1 attenuation ratio). Check the External Scaling under the channel’s probe menu and select ‘volt’ as the units. If you are using a Keysight InfiniiVision oscilloscope, you would interpret the voltage reading in ‘Amphere’ as ‘Voltage’.

When using the N2820A as a voltage probe, the key characteristics are:

• Bandwidth (-3dB) : 500 kHz (zoom-in), 3 MHz (zoom-out)
• Measurable input range: 3uV - 1.2V (differential), 12V (common mode)
• Input impedance : 3 Gohm (giga ohm) differential, 1.5 Gohm single-ended
• DC amplitude accuracy : 3% or 10 uV whichever is greater
• Probe gain factor: 300X (zoom-in), 2X (zoom-out)
• Offset : scope vertical positioning only (no probe offset)