LT1013 Differential amplifier problem

[Alex30]

Hi,

I am trying to use an LT1013 as a differential amplifier and having some strange results.

When I simply just use 10K resistors all round I get the voltage difference outputted with a gain of 1x as expected (0.1V difference and just under 0.1V out). But when I substitute the two appropriate resistors for 1K to get a gain of 10x I seem to get a much higher gain than expected.

I am supplying about 6.4 volts with a battery (as I don't yet have a benchtop ps) and I get about 6 volts out when applying a voltage difference of 0.1V. The circuit is on a breadboard btw. What are the possible causes of this? Just to clarify I am talking about this:http://www.electronics-tutorials.ws/opamp/opamp_5.html Thanks for any help!

[Zero999]

Please post a schematic, showing how where you're applying the input voltage.

[LvW]

I am supplying about 6.4 volts with a battery (as I don't yet have a benchtop ps)

So you are using a single supply only, correct?
Do you know how to handle single supply opamp circuits (setting of an operational point)?

[Alex30]

I am supplying about 6.4 volts with a battery (as I don't yet have a benchtop ps)

So you are using a single supply only, correct?
Do you know how to handle single supply opamp circuits (setting of an operational point)?

I am using a single supply. I'm not sure what you mean by your second question? Sorry I have not taken very much formal education in this, just a hobbyist.

I have supplied a basic schematic. Essentially I am building a power supply. This opamp is pretty much a current sense of the supply, I have already measured the load of the supply to be about 100mA using my DMM. So I have simplified my circuit to show this rather than complicate matters further.

[Alex30]

Ohh haha. I think I may have read the wrong datasheet and got mixed up. It appears minimum supply voltage is 13.5 Volts 

[Andy Watson]

It appears minimum supply voltage is 13.5 Volts 

Minimum supply is 3.4V. However, the upper end of the input common mode range is limited to 1.2V below the positive supply. Your 10X circuit is attempting to use a common mode voltage at the input to the opamp of approximately 0.6V below the supply.

[Alex30]

It appears minimum supply voltage is 13.5 Volts 

Minimum supply is 3.4V. However, the upper end of the input common mode range is limited to 1.2V below the positive supply. Your 10X circuit is attempting to use a common mode voltage at the input to the opamp of approximately 0.6V below the supply.

Oh. Just to clarify, do you mean that the input voltage must be at least 1.2V below the supply voltage?

[Zero999]

Oh. Just to clarify, do you mean that the input voltage must be at least 1.2V below the supply voltage?

Both the inverting and non-inverting inputs must be <+V-1.2

[Andy Watson]

Oh. Just to clarify, do you mean that the input voltage must be at least 1.2V below the supply voltage?

Yes, that is my interpretation of the datasheet.

[Alex30]

Thanks for the help. I am just trying to work out where on the datasheet it says this? Does the voltage have to be at least 1.2V too? Also, I intend to use this with a 15V supply but was just using the battery while I source an adapter, so I should be in parameters when I use 15V.

[void_error]

As previously mentioned your maximum input voltage has to be at least 1.2V (datasheet says 1.5V worst case so I'd go with that) below the positive supply.

Also be aware that the four resistors must be very closely matched or your output voltage won't be what you'd expect - get at least a dozen of each value, measure them and make sure R2/R1 is as close as possible to R6/R4.

I am just trying to work out where on the datasheet it says this?

Input Voltage Range

[Alex30]

As previously mentioned your maximum input voltage has to be at least 1.2V (datasheet says 1.5V worst case so I'd go with that) below the positive supply.

Also be aware that the four resistors must be very closely matched or your output voltage won't be what you'd expect - get at least a dozen of each value, measure them and make sure R2/R1 is as close as possible to R6/R4.

I am just trying to work out where on the datasheet it says this?

Input Voltage Range

Thanks for clarifying that and the advice, will certainly do that when I solder everything. Still getting used to all the parameters for opamps, there's quite a difference between different models and I'm only starting to understand the limitations of them all I suppose the 1.2V must be due to the PN junctions etc which makes sense. Is an opamp that has a voltage range up to input voltage possible?

[void_error]

Is an opamp that has a voltage range up to input voltage possible?

Yes there is - rail-to-rail input opamps. However, most of them tend to be rather low supply voltage ones - most of them around 5V, using CMOS technology rather than bipolar. One example would be the LMV321/LMV358/LMV324, a low-voltage rail-to-rail input and output version of the LM358/LM324 with the LMV321 being the single opamp version.

You will get better results if you increase the voltage across your current sense resistor so errors due to the opamp input offset voltage are reduced. If you don't mind getting more specialized parts you might want to take a look at some difference amplifiers - they're basically opamps which have the gain setting resistors built-in and closely matched.

[Alex30]

Thanks for the help everyone. Will try and get the 15V supply working and see how it performs. Cheers