Problem in measuring potential because of oscilloscope's input impedance

Question

I am trying to use a sensor, whose resistance value change from infinity to around 10kΩ when variable load is applied. The load is applied for a small duration and in order to capture that I'm using TDS6000 series oscilloscope. I'm trying to use a potential divider circuit as shown below (Rs- resistance of sensor).
On multi-meter I'm able to get voltage range of 0-1.6V across R1, but on oscilloscope I'm barely able to get a few mV. This is likely because of the input impedance of the oscilloscope (Ro=50Ω), which makes the net resistance of lower part (A-GND) very small as compared to Rs.
I have tried a couple of combinations of resistors but haven't got any success yet. I want the variable voltage in the range of 0-1.5V. Does anyone have any suggestions?

analogsystemsrf · Answer

Borrow an active_probe, with 1Megohm Rin (or even 10Megohm), that will drive a 50 Ohm load (this will a high-speed probe).

Voltage Spike · Answer

Use a unity gain opamp that has the bandwidth
Maybe use an opamp as a buffer, the opamp will need to have more bandwidth than the sensor (and will add in an voltage offset error Vos, so select an opamp with a low voltage offset). The opamp will also need to be able to source more than ~8mA on the output.
Another option would be to use an opamp with gain.
You might also want to know that there are many eval boards such as this to make adding an opamp for test easier. Almost all manufacturers provide eval boards for opamps also.

simulate this circuit &ndash; Schematic created using CircuitLab

Problem in measuring potential because of oscilloscope's input impedance

2 Answers

Add your own answers!

Ask a Question