# Measure very low dc voltage using oscilloscope help!

Discussion in 'The Projects Forum' started by Kih, Mar 7, 2012.

1. ### Kih Thread Starter New Member

Nov 3, 2011
10
0
Hi everyone !
I have a circuit which only has two 1ohm resistors in series. And I use a dc power supply to apply a 1mv voltage on them.
and then I measure the voltages across the two resistors using the 2 leads of an oscilloscope. The results displayed in the screen is noisy.
(with the lead connected to the hv side and the black clip connect to the lv side )
And then I apply 50mv to the circuit. Voltage measured Channel 1 is about 30mv and the voltage from channel 2 is quite low(only around 400uV and is not stable). However when I inverse the connection of the lead of the channel 2 (lead---lv, clip----hv),the voltage apperas on the screen are about 25mv of CH1 and -25mV of CH2.....

I cannot figure out what happen. I think the way i do the measurement is wrong. and maybe I need to amplify the samll dc voltage ?

2. ### #12 Expert

Nov 30, 2010
16,665
7,310
You have a bad scope probe. They tend to get loose at the connector that plugs into the scope.

3. ### praondevou AAC Fanatic!

Jul 9, 2011
2,936
489
You can measure 400uV with an oscilloscope?

What probe (10:1, 1:1 and model) and oscilloscope are you using and what's the input divider set to?

Mrdouble likes this.
4. ### Kih Thread Starter New Member

Nov 3, 2011
10
0
yeah i changed a probe and the results are much better.
And I think my breadboard is not good

5. ### Kih Thread Starter New Member

Nov 3, 2011
10
0
the 400uV is the number calculated by the oscilloscope....and the number jumps between different values....

now I want to use my dc power supply to apply a small voltage to my measurement circuit and then apply a voltage to activate my op-amp to amplify the small dc voltage across it. The dc power supply has outputs as +6, -6, GND, +25, COM, -25.
what is the proper way to connect the power supply and the oscilloscope ?
I did this: (+6)----circuit------(-6), (-6)---GND----Com,
(+25)---Op-amp---(-25),
Op-amp output-----probe of oscilloscope
COM----Clip in the probe

Nov 3, 2011
10
0