Voltage-to-Current Converter Circuit: Simulated in OrCAD Spice

In summary: It's possible, but I highly doubt it. Analog legends usually have something to back them up.In summary, the voltage to current converter circuit in OrCAD Spice is not behaving as expected. The current is not constant, but changes with V or R. Adjusting the ohms to make sure the output voltage is less than the supply resolved the issue.
  • #1
Rudinhoob
33
0
Hi,

Here's a voltage-to-current converter circuit I simulated in OrCAD Spice. While the current is supposed to be constant through out the load, it does not seem the case with spice, when altering V or R.

Thanks.
 

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  • #2
What? OrCAD lying? Never.

Haha. Jokes apart. Your voltage to current converter is having the loading effect.
 
  • #3
What's the loading effect?
 
  • #4
Rudinhoob said:
What's the loading effect?

Sorry I was too fast to say it is loading effect. Actually in this case with OPAMP loading effect won't happen. Your output current should not change with R but it should change with V.

Can you show me screenshot of the schematic or netlist?
 
  • #5
i = V/R so it changes with V or R, not R-load.
 
  • #6
Kholdstare said:
Sorry I was too fast to say it is loading effect. Actually in this case with OPAMP loading effect won't happen. Your output current should not change with R but it should change with V.

Can you show me screenshot of the schematic or netlist?

Damnit. I was correct in the first place. It is loading effect.

http://en.wikipedia.org/wiki/Voltage_divider#Loading_effect
 
  • #7
Rudinhoob said:
i = V/R so it changes with V or R, not R-load.

Which resistance are you talking about?
 
  • #8
The grounded R.
 
  • #9
The other resistor R(L) represents the load.
 
  • #10
Rudinhoob said:
The grounded R.

Aha, you can say RL instead of writing R-load. Its easier to understand.

Now, you have one concept wrong here. The RL you are referring to is not the load of the circuit. Its just a resistor connected to the output of the OPAMP to limit the current coming out of it. The actual load is R which is where the current is going to and reaching ground. That is where you connect your next stage of circuits. In other words, your next stage of circuit replaces where the R is.
 
  • #11
Rudinhoob said:
While the current is supposed to be constant through out the load, it does not seem the case with spice, when altering V or R.

Thanks.

I think you misunderstood what the circuit is meant to do (and I'm also confused by the previous post!)

If V and R are constant, it keeps the current through RL constant if RL changes. (The constant current = V/R.)

If you change V or R, you get a different "constant" current through RL.
 
  • #12
When analyzing the circuit, the i = V/R is the current through RL, which is the load, and this current is apparently independent of the RL.
 
  • #13
I think you misunderstood what the circuit is meant to do (and I'm also confused by the previous post!)

If V and R are constant, it keeps the current through RL constant if RL changes. (The constant current = V/R.)

If you change V or R, you get a different "constant" current through RL.

Yes but that's not what SPICE is giving me, when I change RL, it changes the current.
 
  • #14
Did you check if you hit the rails (does the output voltage of the op amp reach the supply voltage)?
 
  • #15
0xDEADBEEF said:
Did you check if you hit the rails (does the output voltage of the op amp reach the supply voltage)?

Thanks for the hint! Adjusting the ohms to make sure the output voltage is less than the supply did the trick.
 
  • #16
Also, Rudinhoob, SPICE almost never lies. If you don't understand the output of a simulation the issue is most likely:

1. You hooked up the circuit wrong. (this is almost always it)
2. You don't understand the circuit.
3. Your models are wrong.
4. You set the accuracy or step-size wrong.

Way, way down the list is SPICE is lying. I'm a professional at this stuff and I've only seen in ONE case where a simulator was making an error. And this was an accelerated matrix-solver, not a vanilla SPICE.

Seriously, "SPICE is wrong" should be the LAST assumption you make.
 
  • #17
Yeah you are right carlgrace, I might be influenced by some analog epic, Bob Pease ;)
 
  • #18
Hahah that famous "What's all this SPICE stuff anyway?" article! :)

That thing is so obsolete it isn't even funny.

Although the photo of Bob throwing his PC off the roof of the National Semiconductor parking lot was pretty funny.

I knew Bob (he passed away a couple of years ago) and he refuted that article years ago. He used SPICE. In fact, in a modern IC, breadboarding is MUCH more likely to lie to you than SPICE.
 
  • #19
carlgrace said:
Hahah that famous "What's all this SPICE stuff anyway?" article! :)

That thing is so obsolete it isn't even funny.

Although the photo of Bob throwing his PC off the roof of the National Semiconductor parking lot was pretty funny.

I knew Bob (he passed away a couple of years ago) and he refuted that article years ago. He used SPICE. In fact, in a modern IC, breadboarding is MUCH more likely to lie to you than SPICE.

That only makes sense. All analog legends might had fixed Tek. 547 using breadboard while eating pizza in MIT building 20 at 3 am. But they can never ignore the convenience and accuracy of SPICE program.
 

1. What is a voltage-to-current converter circuit?

A voltage-to-current converter circuit is an electronic circuit that converts a voltage signal into a corresponding current signal. It is commonly used in applications where a voltage signal needs to be converted to a current signal for accurate measurements or control.

2. How does a voltage-to-current converter circuit work?

A voltage-to-current converter circuit uses a combination of resistors, operational amplifiers, and feedback loops to convert the input voltage signal into a proportional output current signal. The feedback loop ensures that the output current remains constant, regardless of changes in the input voltage.

3. What is the purpose of simulating a voltage-to-current converter circuit in OrCAD Spice?

Simulating a voltage-to-current converter circuit in OrCAD Spice allows for a virtual testing and analysis of the circuit's performance before building it in real life. It helps in identifying any potential issues or improvements that can be made to the circuit design.

4. What are the advantages of using a voltage-to-current converter circuit?

One of the main advantages of using a voltage-to-current converter circuit is its ability to accurately convert a voltage signal to a current signal, which is essential in many electronic applications. It also provides a high level of stability and precision, making it suitable for sensitive measurements and control systems.

5. What are some common applications of voltage-to-current converter circuits?

Voltage-to-current converter circuits are commonly used in applications such as sensor interfacing, current sensing, and signal conditioning. They are also used in power supply circuits, data acquisition systems, and various industrial control systems.

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