This op amp converts input voltage to output current. Huh?

[king.oslo]

This is taken form a data-sheet. The bit that confuses me is in bold font:

The LT6106 monitors current via the voltage across an external sense resistor (shunt resistor). Internal circuitry converts input voltage to output current, allowing for a small sense signal on a high common mode voltage to be translated into a ground referenced signal. The low DC offset allows for monitoring very small sense voltages. As a result, a small valued shunt resistor can be used, which minimizes the power loss in the shunt.


I thought opamps could scale a voltage, say from 1V to 5V with a gain of 5. Not 1V to xA at the gain of x.

What is going on?

Thanks.M

[amspire]

Well first, the LT6106 is described as an Amplifier - not an op-amp.  Internally there is an opamp and the internal opamp functions 100% as an opamp.

But the output of the opamp connects to an internal  transistor connected to provide a current source. That is the purpose of the IC, so I don't really understand what is the issue. The internal opamp is not acting as a current source - it is a voltage amplifier that is regulating the current source transistor.

Now the current source is not a very good current source as its accuracy is dependent on the gain of the output transistor, but it is good enough for current limiting circuits in things like power supplies.

Richard.

 

[Psi]

It's not your typical opamp, it's got other components in the chip too.
From the block diagram at the very least it has some sort of output transistor.
(Note: you cant trust IC block diagrams, they are just a simplified representation showing how something works and not how it's really constructed. They're often more complex inside than the diagram suggests)

In this case the opamp output voltage feeds the transistor which varies the current.

[king.oslo]

What I want is to amplify the voltage of a shunt resistor, scaling it to say 0-5V or there about. That way I can interface it with an ADC LED display driver.

What are your thoughts?

Thank you for your time.

Kind regards,
Marius

[amspire]

If you are happy with about 1% accuracy, and the current sense resistor has to be in the positive rail, then it is OK.

Otherwise you can find circuits using conventional opamps that can give much more precision.  Does the sense resistor have to be in the positive rail or can it be in the negative rail?

There are lots of solutions.

Richard

[king.oslo]

The resistor can be in the positive rail. That is no problem.

Which op amp circuit combination will give decent accuracy? The display will have show amperes with two decimal places. So an accuracy of circa 1% would be great.

This is the LED display driver I am using: http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010503

Thanks!M

[amspire]

Can the sense resistor be in the negative rail? It makes it much easier if you want the meter connected to the negative output rail too.

Or can the meter's positive lead be connected to the positive output rail?

Richard

[king.oslo]

Dear Richard,

I don't see why it couldn't be in the negative rail. What are the possible implications of this?

Thanks.M

[alm]

Sense resistor in negative rail is fine as long as the supply is floating. If it's grounded, the negative rail will be I*R above ground, and any alternative ground path (eg. USB or RS-232 cable) will bypass the shunt.

[king.oslo]

Then I think it is best to avoid resistor in the negative rail.

Thanks.M

[Thomas]

But why can't you use the LT6106 you mentioned in the first post?
You know the output current can easily be converted to voltage with a resistor, right?

[DrGeoff]

There are different types of amplifiers. Transconductance (I/V), transresistance (V/I), voltage (V/V) and current (I/I). I have used transresistance amplifiers to generate negative output resistance in loudspeaker driver testing. The amplifier you describe is a transconductance amplifier.

[slateraptor]

input voltage to output current

a.k.a. transconductance amp. In theory, any general-purpose op amp can be configured for transconductance operation...it all depends on how you interpret it, that is to say for every thevenin equivalent circuit, there exists a norton equivalent. In the case of the LT6106, there's a built-in transconductance stage.

[amspire]

What I would probably do is follow exactly the same schematic that you see in the LT6106 device.

You need to find an op-amp whos inputs can operate at the + supply rail, and an output that can operate up near the + rail voltage.  There are a number from companies like TI, NS, Linear Technology, Maxim.  Rail-to-rail opamps are fine.

Replace the transistor with a P channel small signal mosfet with a 100 ohm series resistor to the gate.

It is really important that the voltage on either input cannot under any circumstances get more than 0.3V above the positive rail, so if it is possible, you can add some 1K series resistors to the opamp inputs, and reverse biased schottky diode from the inputs to the + supply rail.

Now choosing the value of the series resistor  is totally dependent on the offset voltage of the opamp.  If you go for a low offset opamp with a 10uV offset, and you have a 3 1/2 digit meter, then you can quite happily work with 5mV drop across the sense resistor at full scale.  If the maximum current is 5A, that would mean a 1mOhm sense resistor. You have to make sure that there is a 4 wire connection to the sense resistor - ie the opamp connection to the resistor does not use any of the PC tracks carrying the full current.

If your opamp has a 100uV maximum offset, then you want to go for something like 50mV voltage drop on the sense resistor at maximum current.

Calibration is easy - just add a pot in series with the resistor across the voltmeter input.

You will not use that many extra parts, you can choose a mainstream opamp instead of a single source device, and the accuracy and linearity will be much higher then the LT6101.
 
May even be cheaper.

Richard.

[amspire]

Is this supply likely to go into current limit? If so you will want to power the opamp from a supply that wont go to 0V.

[king.oslo]

Thanks guys!M