Current to Voltage and Voltage to Current Converters

[fog37]

Hello Everyone,

I am still trying to get my head around what a current to voltage converter (and vice versa) is. There is a section on my book, in the chapter about op amps, that discusses this topic.

Apparently, a current signal is less susceptible to noise than a voltage signal... But what is a current signal? For instance, a temperature sensor measures variations of temperature over time. That is a signal. A current signal would emulate the behavior the temperature signal. But a current passing through a resistor drop a voltage that is proportionate to the current so we easily get a voltage signal...confused.

Thanks for any clarification.

 

[berkeman]

Apparently, a current signal is less susceptible to noise than a voltage signal...

As a general statement, that is nonsense. Can you post your source? Most likely you are just confused by what you have read in your learning.

Noise and Signal/Noise ratios are important in measurements, but sweeping statements without qualifiers really don't help anybody's learning or understanding.

 

[berkeman]

As a general statement, that is nonsense.

I don't mean that to be an insult to you. It's just frustrating to have isolated statements like that stated as absolute facts.

 

[Windadct]

I think this is referring to industrial instrumentation, for example a 4-20MA analog signal is considered to have very high noise immunity. But then you need to convert the 4-20 to other signal types for various applications.

 

[berkeman]

I think this is referring to industrial instrumentation, for example a 4-20MA analog signal is considered to have very high noise immunity. But then you need to convert the 4-20 to other signal types for various applications.

Maybe that's part of it, thanks. But unless you use twisted pair cabling for the interconnection, the magnetic field noise rejection won't be any better than the common-mode electric field noise rejection, IMO.

I'll try to find some good links for low-noise measurement techniques and post them. There are lots of great tips and techniques to use to maximize the S/N ratio of sensor measurements...

 

[analogdesign]

Noise is really confusing, and often people don't mean the same thing when they say it. A current signal is resistant to power supply fluctuations and IR drop because it is referenced once at the source and re-referenced at the sink (and those references don't have to be the same voltage). As an example, in chip design, you distribute a bias as a voltage locally, but as a current globally. Whether or not it is immune to noise depends a lot on the impedance level.

Anyway, back to the original question, a ideal voltage source has zero output impedance and an ideal current source has infinite output impedance. So, more generally, you can think of voltage-to-current or current-to-voltage converters as impedance converters.

A resistor is sometimes used in practice to convert between current and voltage (usually because it's really fast) but it obviously can't change the impedance the signal sees.

Here is a good writeup on some of the design considerations for low-noise sensor conditioning. The name of the game is to gain up the signal and limit the bandwidth as much as possible.

http://www.analog.com/media/en/technical-documentation/technical-articles/MS-2066.pdf

 

[fog37]

No problem.

The topic of current to voltage converter and voltage-to-current converter is a common application of operational amplifiers. People talk about current signals in instrumentations indeed.

For example:

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/voltage-to-current-signal-conversion/

"In instrumentation circuitry, DC signals are often used as analog representations of physical measurements such as temperature, pressure, flow, weight, and motion. Most commonly, DC current signals are used in preference to DC voltage signals, because current signals are exactly equal in magnitude throughout the series circuit loop carrying current from the source (measuring device) to the load (indicator, recorder, or controller), whereas voltage signals in a parallel circuit may vary from one end to the other due to resistive wire losses. Furthermore, current-sensing instruments typically have low impedances (while voltage-sensing instruments have high impedances), which gives current-sensing instruments greater electrical noise immunity."

http://electronics-course.com/current-voltage-converter

http://www.falstad.com/circuit/e-itov.html

I am just trying to get my head around the idea of current signals and voltage signals.

 

[fog37]

Thanks analogdesign.

I will go carefully read your reply and be back.

 

[CWatters]

Hello Everyone,

I am still trying to get my head around what a current to voltage converter (and vice versa) is. There is a section on my book, in the chapter about op amps, that discusses this topic.

Apparently, a current signal is less susceptible to noise than a voltage signal... But what is a current signal? For instance, a temperature sensor measures variations of temperature over time. That is a signal. A current signal would emulate the behavior the temperature signal. But a current passing through a resistor drop a voltage that is proportionate to the current so we easily get a voltage signal...confused.

Thanks for any clarification.

What is a current signal? Well it's possible to encode 0 to 100C as a variable voltage (0 to 1V) or as a variable current (0 to 10mA). These are just examples.

Transistors are essentially current rather than voltage devices so one way to process a current signal is to feed it into a transistor.

However it's not true to think of op-amps as pure voltage devices and transistor as pure current devices. That's just a model that works in some situations.

 

[fog37]

Again, I was just reading that a current to voltage converter (in the form of an op amp) produces an output voltage that is proportional to an input current (many transducers produce a current). The output voltage is independent of the load connected to it. How can the output be independent of the load. I would think that once is load is connected the circuit operation would changes. And depending on the impedance of the load, the load would draw a different current and drop a different voltage...

That said, there are components like a current source such that no matter what load we connect to it, the current through the load remains the same (but the load voltage changes). This happens because the ideal current source has a dominant impedance that sets the current in the circuit and any other device in series or parallel has a much smaller relative impedance that does it not affect that current.

 

[fog37]

This is the example I found: the current does not enter the inverting terminal of the op-amp which is at 0 volts (virtual ground) and passes completely through the feedback resistor $R_f$ producing a voltage $I_{s} R_{f}$ which ends up being equal to the output voltage $V_0$ no matter the impedance of the load we connect...

 

[Tom.G]

[Edit - added this paragraph] Part of your problem may be that your circuit shows the Positive terminal of I_S going to the Inverting input of the opamp which would give a Negative voltage at the output; the circuit diagram indicates a Positive output though. With the correct polarities, you will find that as the output voltage goes more Negative, it generates a current thru the feedback resistor that cancels the I_S current, yielding zero volts at the input; thus balancing the circuit.

Try looking at this thread to see if it helps any: https://www.physicsforums.com/threa...edback-in-an-op-amp-work-conceptually.584881/

If not, use the Search button in the top right of the window to search for opamp.

 

[jim hardy]

In industry we use DC current to represent an analog measurement.
Two standards are 4 - 20 milliamps and 10 - 50 milliamps represent 0 - 100 % of whatever is being measured.
Most industrial sensors provide that kind of current output.
At the receiving end we turn the current into a voltage, usually 1 to 5 volts DC, by simply passing it through a resistor.

The advantage of DC current is it can be sent a long long way and not get lost . With the low voltages involved you just won't lose much of your signal in any decent insulation. Since most interference is AC it's easily filtered out at receiving end.

https://www.acromag.com/sites/default/files/Acromag_Intro_TwoWire_Transmitters_4_20mA_Current_Loop_904A.pdf

http://www.ti.com/product/XTR117

 

[fog37]

Thank you everyone. I am making progress.

I found this example: the circuit below involves an op-amp with negative feedback. It is said to perform like a current source (the current through the load is the same regardless of the load impedance). The $V_{in}$ battery would be a sensor that produces a variable DC voltage which depends on the measured physical quantity. Both the resistance of the wires and the resistance of the load $R_{load}$ don't affect the current. In general, I would have thought that $R_{load}$ always affects the current in the circuit or the current through it.

So, for different load impedances $R_{load}$, the current $I$ remains the same but the voltage across the load varies depending on $R_{load}$, i.e. $V= I R_{load}$.

There are clearly other configurations that map a current to voltage and maintain the same output load voltage regardless of the load impedance $R_{load}$.

 

[Windadct]

For your entertainment - I found THIS simulator, linked to the OpAmps - but if you go to the main page there are many other circuits.

EDIT -- actually there are a ton of online sims for this, Like CircuitLab, but one I just found was TI's Webench OpAmp designer - looks very useful.

 

[sophiecentaur]

I am just trying to get my head around the idea of current signals and voltage signals.

To me, this is all about Impedances and the way a transmission line is being driven. If you use a high impedance source, that will give a signal that is less affected by induced voltage signals . . . and vice versa for a low impedance source.
In practical terms, this will mean that the choice of which to use would ultimately depend upon the nature of the expected interference - and the noise characteristics of the front end of the receiving equipment. (Plus a lot of other factors, of course.) What really counts in a transmission system is the ratio of signal Power to noise / interference Power and the signal Power can be the same for a 'current' signal or a 'voltage' signal.

 

[Windadct]

Meh - it is a signal; voltage, current, light, pressure, even a shape like a record groove ... it is just a communication channel - transporting information, in which voltage and current are no more related than pressure and movement. Each type of signal has its benefits and drawbacks, and OpAmps are just one way to convert one signal type to another, some are simple some are complex.