Hi all kindly need some advice regarding this :
am using ad620 amp to work with EMG signal, am using 4 AAA batteries as a source and the circuits as in the attached image.
so as I understand the ground would be the point at the middle of the 4 batteries so I get the +3v / -3v for the AD620, and I connect the refV to it.
so I suppose the output should be close to zero but it is not, the output is 3v or -3v, even if both inputs are floating. or with 1k ohm to ground.

I believe my source ground is somehow wrong, but I could not manage to know what is it or what to do.

thank you in advance.

 

hi,
Where is the common node for the simulation?
E

 

Are you referring to a real circuit or a simulation?
The simulation needs a ground connection shown on the schematic.

 

I removed it just to make it look like the IRL circuit . here is it, it working fine in the simulator though.


I thought it could be damaged, i used another AD620 but same.

 

hi,
Do you have a photo of the project to post?
E

 

hi,
Do you have a photo of the project to post?
E

thanks alot for the fast reply ^^ ... sure I will provide it ASAP.

 

it working fine in the simulator though.

Then there is likely a wiring difference between the simulation schematic, and the real circuit.

 

As the two amplifier inputs are connected together. The amplifier has an input offset voltage so the output will be the gain times that offset voltage but that should only be 500uV maximum.
Try connecting the inverting input to the output instead.

 

Try connecting the inverting input to the output instead.

That is not normally done for an instrumentation amplifier, which this is, as that type of amp already has internal negative feedback to determine its gain.

 

hi,
Do you have a photo of the project to post?
E

please check it :

 

Hi w,
The circuit looks OK.
E

 

am really confused ..... is it damage ? I have 3 other AD620 to test ..... some question please :
- is this a good ground refrance ?
- can I connect the refV to +3v ?

 

can I connect the refV to +3v ?

If you mean the pin 5 Reference, no.
That should be connected to circuit common (0V).

 

Hi w,
As post #13 states, pin #5, Vref to 0v, which should be at the mid-connection of the two battery pairs.

Use your voltmeter to measure these voltages.
1. the +3v at the battery.
2. the -3v at the battery.
3. All the pins on the AD620
Post the voltages you measure.

E

 

Hi w,
As post #13 states, pin #5, Vref to 0v, which should be at the mid-connection of the two battery pairs.

Use your voltmeter to measure these voltages.
1. the +3v at the battery.
2. the -3v at the battery.
3. All the pins on the AD620
Post the voltages you measure.

E

Hi Ericgibbs
thanks to you and to crutschow for the replies.

I measured all the pins and the point between the 2 batteries pairs as a reference to all =0v :
pin1= 0v , pin2= 0v , pin3 =0v , pin4= -3v
pin5= 0v , pin6= +3 or-3v, pin7= +3, pin8= 0v

all as expected but the pin6 with the 3v output !

I test all other 4 AD620 ... same thing , do you recommend buying new AD620 or there is something I can try?

 

Hi w,
The print on the OPA looks like AD520.???
Please double check.
E

 

Hi w,
The print on the OPA looks like AD520.???
Please double check.
E

you scared me a bit XD ... it is correct AD620 :

 

hi w,
I would suggest a quick test circuit.
Assume they are a standard OPA, say like the LM741, can you build a simple amplifier on your bread board.
Using the pins as though it is a LM741, then apply test input voltages, see what you get out on

The AD620's you have, maybe fakes, or defective.
E

 

pin6= +3 or-3v

What makes it sometimes +3 and sometimes -3?
Could this be a dodgy contact on the breadboard?

 

hi w,
I would suggest a quick test circuit.
Assume they are a standard OPA, say like the LM741, can you build a simple amplifier on your bread board.
Using the pins as though it is a LM741, then apply test input voltages, see what you get out on

The AD620's you have, maybe fakes, or defective.
E

HI Eric

sadly it seems like you are right ..... fake (-_-)
Using LM741 as an example, I made a circuit where the feedback resistance is 1k and the input 10k so the gain is 0.1

and connecting the 2 offset ground, using another battery as inverter input, non-inverter to ground.
the output is -0.15v.

thanks a lot and sorry for the disturb X)