# Question for the Oscilloscope Experts

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by rsfoto, Oct 10, 2015.

1. ### rsfotoMember

Joined:
Jun 14, 2013
Messages:
189
Likes:
9
Location:
Mexico
Hi,

Long time not been here but now I am trying to develop a new device and have a specific question about how to connect an Oscilloscope to a circuit in order to compare reality with theory.

I made a test circuit like in the picture below.

I want to measure the voltage of the voltage divider on the right side as well as the output of the LM358 on the left.

¿ Where do I connect the probes of the Oscilloscope in order to measure the 34 mV of the voltage divider ?

On the output I connect the ground clamp on the output and the clip on the +12V and the reading on the oscilloscope gives me nearly what I calculated, but the big problem is where how do I connect the ground clamp and the clip for reading the 34mV

Another problem I have is that I have no idea how to use two probes on this circuit eg. measure bot voltages at the same time ...

Thanks and regards Rainer

2. ### audioguruWell-Known MemberMost Helpful Member

Joined:
Mar 16, 2004
Messages:
32,575
Likes:
950
Location:
ONLINE
The datasheet for the LM358 says that the Input Common Mode Voltage Range is a maximum of V+ minus 1.5V which means that the inputs DO NOT WORK if they are less than 1.5V from the positive supply voltage. Then your (+) input is at a voltage that is too high.

Another problem is that the input voltage is about +11.96V (NOT 34mV!) and the negative feedback gives the opamp a voltage gain of 22.28 times so the output voltage is trying to be (11.96V x 22.28=) 266.5V which of course is impossible.

Most circuits, especially this one have the 'scope ground wire connected to GROUND!!! Then you use the 'scope to measure the input voltage and output voltage with respect to ground.
Use a 'scope with two channels to measure two voltages at the same time.

• Like x 1
3. ### ronsimpsonWell-Known MemberMost Helpful Member

Joined:
Apr 17, 2007
Messages:
7,361
Likes:
972
Location:
Loveland, CO USA
I agree with AudioGuru. What do you want the circuit to do? Is the input referenced to the top supply? while the output is referenced to ground?

Joined:
Jan 12, 1997
Messages:
-
Likes:
0

5. ### cowboybobWell-Known MemberMost Helpful Member

Joined:
Oct 22, 2011
Messages:
3,050
Likes:
479
Location:
James Island, SC

rsfoto, schematic error, perhaps... ?

If you swap R3 and R4, you'll get the required 34mV (actually 33.51mV, if the resistors were perfect ) at the +input to the LM358.

• Thanks x 1
6. ### ronsimpsonWell-Known MemberMost Helpful Member

Joined:
Apr 17, 2007
Messages:
7,361
Likes:
972
Location:
Loveland, CO USA
Looking at 34mV signals.....the input offset is 2mV typical, 7mV max at room temp and up to 9mV at extreme temp.
On your bench he op-amp could see 34mV as 30mV or 38mV because of input offset.

7. ### rsfotoMember

Joined:
Jun 14, 2013
Messages:
189
Likes:
9
Location:
Mexico
Hi All,

Thanks for the comments. cowboybobs recommendation did the trick.

The above setup is a test. The voltage divider is just provisional in order to see if the volatage amplifier does his job. Sorry, I am not an electronics engineer, I am just a hobbyist who from time to time has some ideas which I want to implement and use in my other hobby which is the Astronomy.

The voltage divider will be taken away and instead of that I will connect a Piezzo sensor for detecting vibrations on my telescope pier. Just for fun

I connected my oscilloscope bout ground clamps on the ground of the circuit and the clips on the point to measure and the calculated values are displayed.

Now I can test the Piezo sensor and see the amplification. http://www.meas-spec.com/product/t_product.aspx?id=2474

Again thanks

regards Rainer

8. ### audioguruWell-Known MemberMost Helpful Member

Joined:
Mar 16, 2004
Messages:
32,575
Likes:
950
Location:
ONLINE
If you connect the piezo sensor parallel with the 280 ohm resistor then the extremely low value of the resistor will kill the output signal from the sensor. A piezo usually feeds an amplifier with a high input resistance.

9. ### rsfotoMember

Joined:
Jun 14, 2013
Messages:
189
Likes:
9
Location:
Mexico
Hi audioguru,

The Voltage divider was only for test purposes and will be taken away ...

If I understand you correctly I need a resistor between the positive and negative pin of the Piezo sensor ? Maybe 1mOhm ?

10. ### rsfotoMember

Joined:
Jun 14, 2013
Messages:
189
Likes:
9
Location:
Mexico
Hi,

The following schematic shows the idea of amplifying the voltage delived by the Piezo sensor.

At the output I will put something like a LM3914 with coloured LEDs to show how big the vibrations are.

I already have such a circuit setup so I just need to find out how much is the max voltage I will be getting with a minimal vibration of the MiniSense 100 and adjust the input values accordingly.

regards Rainer

11. ### rsfotoMember

Joined:
Jun 14, 2013
Messages:
189
Likes:
9
Location:
Mexico
hi,

Well after a search in Internet I think I am reinventing the wheel ...

I hve been trying to connect the piezo to the input of my first circuit but it does not work.

Searching in the net I found a circuit which connects directly the sensor to the inputs. Will try this ...

12. ### rsfotoMember

Joined:
Jun 14, 2013
Messages:
189
Likes:
9
Location:
Mexico
Well I did setup the below circuit but I do not like

Will keep trying how to get the connection of the sensor into the LM358 input in my circuit

13. ### Nigel GoodwinSuper ModeratorMost Helpful Member

Joined:
Nov 17, 2003
Messages:
39,324
Likes:
653
Location:
Derbyshire, UK
Sorry, but that is utter and total CRAP!! - I don't think there's hardly a single part of it that's correct?.

• Agree x 1
14. ### rsfotoMember

Joined:
Jun 14, 2013
Messages:
189
Likes:
9
Location:
Mexico
You mean the circuit I found ?

Or do you refer to my trials with my posted circuits ?

regards Rainer

15. ### Nigel GoodwinSuper ModeratorMost Helpful Member

Joined:
Nov 17, 2003
Messages:
39,324
Likes:
653
Location:
Derbyshire, UK
The one I quoted from post #10

16. ### cowboybobWell-Known MemberMost Helpful Member

Joined:
Oct 22, 2011
Messages:
3,050
Likes:
479
Location:
James Island, SC
rsfoto,

Is this what your trying to do?:

The amplified sig is "bellied" (and ugly) since the Piezo sig is dropping below the minimum signal level for the OpAmp . The belly is from the cap discharging/charging. The belly "effect" starts at about the zero point of the sim'ed voltage level of 0VAC.

Last edited: Oct 11, 2015
• Like x 1
17. ### cowboybobWell-Known MemberMost Helpful Member

Joined:
Oct 22, 2011
Messages:
3,050
Likes:
479
Location:
James Island, SC
If we up the sim'ed signal to 68mV P-P, the 34mV sig you first envisioned gives slightly more than the 723mV you mentioned in your first post.

Notice the flatter "belly".

• Like x 1
18. ### rsfotoMember

Joined:
Jun 14, 2013
Messages:
189
Likes:
9
Location:
Mexico
Hi cowboybob,

Great. That is exactly what I have been looking. The signal form of the output does not matter. After amplifying the AC signal from the piezo sensor I want to activate some leds using a LM3419.

I guess I can flatten the output with a cap.

May I ask what program is that for simulating the circuit and where can I find it ?

Again thanks and regards Rainer

PD I will test that circuit right now

19. ### cowboybobWell-Known MemberMost Helpful Member

Joined:
Oct 22, 2011
Messages:
3,050
Likes:
479
Location:
James Island, SC
Its TINA TI, Ver 9.3, the NOT free version from Texas Intstruments: . I've used the free version (http://tina-ti.software.informer.com/9.2/) but it lacks the bells and whistles. Scroll until you see the gray "Regular Version" button to download the free version. The "Premium version" is not free.

I like TINA because it has motors that turn, LEDS that light up, relays that move, etc.. Just suits me better when I run a sim.

Most members here, though, prefer LTSpice (http://www.linear.com/designtools/software/), also free and very powerful but, IMHO, with a considerably longer learning curve.

20. ### rsfotoMember

Joined:
Jun 14, 2013
Messages:
189
Likes:
9
Location:
Mexico
Thank a lot. Will take a look into it

regards Rainer

Joined:
Mar 16, 2004
Messages:
32,575
Likes:
950
Location: