Good day,
I am having problems creating a proper transimpedance amplifer for an ALS-PT243. The expected output range from the sensor in this specific situation was 0 - 5 uA. The amplifier was made to scale this to 0 - 3.3 V. However It currently only works when current flows in the opposite direction. I attempted to simulate the setup before putting it together using Multisim. The following are the results:

When the circuit has an input close to 5 uA:

A negative input close to 0 A.

A negative input close to -5 uA.


What I don't understand is why the op-amp is only working in the negative portion of the cycle. As in the part of the cycle it gives the proper scaling. Also the fact that the signal flips is not Ideal. I any thoughts on how to correct these two problems?

Regards,
~ Sonny120

 

I don't quite understand the problem.
Can you post the output waveforms of your sim?

 

I don't know Multisim, so these might be non-issues, but it looks like you are powering your opamp from undefined nodes. In all simulators I have worked with, a node named "V+_1" would have no relationship to a node named "V+"

Are you sure that your grounds are what you think they are. Your supply center tap appears to just be connected to a node named "GND". Does Multisim treat this particular node name as equivalent to the "0" that most simulators use for the 0 V reference node (and to which the "earth" ground symbol normally refers)?

Is there a reason you are using a 741 opamp?

 

Here is the output waveform from the oscilloscope. It hard clips.

The input is green the output is red. It can be seen that when in the positive part of the input cycle the output waveform clips in its negative portion.

 

You have used different GROUND symbols. Try using the same GROUND.

 

What I don't understand is why the op-amp is only working in the negative portion of the cycle.

Because it is a virtual earth circuit, so the current through R3 is equal and opposite to the input current.

 

I don't know Multisim, so these might be non-issues, but it looks like you are powering your opamp from undefined nodes. In all simulators I have worked with, a node named "V+_1" would have no relationship to a node named "V+"

Are you sure that your grounds are what you think they are. Your supply center tap appears to just be connected to a node named "GND". Does Multisim treat this particular node name as equivalent to the "0" that most simulators use for the 0 V reference node (and to which the "earth" ground symbol normally refers)?

Is there a reason you are using a 741 opamp?

Thank you.
The problem seemed to stem from the nodes. When I replaced all the grounds I got the same output. However After replacing all the supplies with V+ and V- it worked.
Is there a way to make this circuit in non-inverting mode.

 

Your simulation software does not know that a 741 opamp is 56 years old and is VERY noisy.
Bury it and use a modern low noise opamp instead.

 

> Is there a way to make this circuit in non-inverting mode?
No.

Other than adding a unity gain inverter.