0-50 mv generation using pot gives noise

Thread Starter

NAGARAJ_JP

Joined Aug 21, 2020
30
Hi I have to generate 0-50mv using the below circuit, the issue is im getting noise, ihave to reduce the noise can anybody give me the filter desing.

1624251222085.png



The below is the noise im getting in the oscilloscope.
1624251141240.png
 
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Ian0

Joined Aug 7, 2020
3,752
Firstly, the resistor value is wrong - it should be 1MΩ not 890Ω, and the wiper should not be connected to the outer terminal of the pot. The circuit you have drawn will give 0 - 4.59V
To fix the noise, put a capacitor across between wiper and ground. A good few microfarads - as big as you like. If you make it too big, the output will be annoyingly slow to adjust.
LM358 is probably not the best choice of op-amp if you want it quiet. Go for a low noise op-amp!
 

Thread Starter

NAGARAJ_JP

Joined Aug 21, 2020
30
Firstly, the resistor value is wrong - it should be 1MΩ not 890Ω, and the wiper should not be connected to the outer terminal of the pot. The circuit you have drawn will give 0 - 4.59V
To fix the noise, put a capacitor across between wiper and ground. A good few microfarads - as big as you like. If you make it too big, the output will be annoyingly slow to adjust.
LM358 is probably not the best choice of op-amp if you want it quiet. Go for a low noise op-amp!
Sorry i didnot mention the unit of the resistor it is 890 kohms not 890 ohms
And I have changed my circuit as you said as below
1624260886132.png
And still ther is noise as below when the probe is kept on the input point of the circuit or the wiper
1624261069384.png
 

Alec_t

Joined Sep 17, 2013
12,309
That could at least partly be noise picked up on the probe lead.
Was the probe hand-held or clipped to the wiper?
Where is the probe ground clip connected?
 

Ian0

Joined Aug 7, 2020
3,752
That noise doesn't look quite random enough. LM358s don't like capacitive loads or driving long cables, especially at unity gain. Turn up the timebase to see if there is any oscillations in the 100kHz to 1MHz region.
 

Alec_t

Joined Sep 17, 2013
12,309
You could also try disconnecting the scope completely from the board and just shorting the probe tip to its ground clip to see if the noise is probe pickup.
 

Thread Starter

NAGARAJ_JP

Joined Aug 21, 2020
30
That noise doesn't look quite random enough. LM358s don't like capacitive loads or driving long cables, especially at unity gain. Turn up the timebase to see if there is any oscillations in the 100kHz to 1MHz region.
"Turn up the timebase to see if there is any oscillations in the 100kHz to 1MHz region."

There4 is no fft in my oscilloscope
 

click_here

Joined Sep 22, 2020
446
Like 1an0 said, your calculation is off

V_out = V_in * R_top / (R_top + R_bottom)
=5*(10e3/(890e3+10e3))
=55.56mV

If the wiper was at 5k (halfway) the 1uF cap would give a -3dB point of 16kHz - You could put a 10k resistor in between the wiper and cap to bring the -3dB point down to 8kHz at halfway

I would be thinking of changing the op amp to a lower noise - I'm assuming that you have put a bypass cap close to its +ve supply pin?
 

Ian0

Joined Aug 7, 2020
3,752
100Ω between the op-amp output and whatever it is driving might calm it down.
Does it have a stable supply with a nearby decoupling capacitor?
 
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