LT1013 Differential amplifier problem - Page 1
Forum section: electronics / beginners

author: Alex30
author_rank: Regular Contributor
timestamp: December 01, 2014, 09:07:36 am
content: 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!

author: Zero999
author_rank: Super Contributor
timestamp: December 01, 2014, 12:40:57 pm
content: Please post a schematic, showing how where you're applying the input voltage.

author: LvW
author_rank: Frequent Contributor
timestamp: December 01, 2014, 12:56:17 pm
content: Quote from: Alex30 on December 01, 2014, 09:07:36 amSo you are using a single supply only, correct?Do you know how to handle single supply opamp circuits (setting of an operational point)?

author: Alex30
author_rank: Regular Contributor
timestamp: December 01, 2014, 01:28:22 pm
content: Quote from: LvW on December 01, 2014, 12:56:17 pmI 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.

author: Alex30
author_rank: Regular Contributor
timestamp: December 01, 2014, 01:41:02 pm
content: Ohh haha. I think I may have read the wrong datasheet and got mixed up. It appears minimum supply voltage is 13.5 Volts

author: Andy Watson
author_rank: Super Contributor
timestamp: December 01, 2014, 02:25:46 pm
content: Quote from: Alex30 on December 01, 2014, 01:41:02 pmMinimum 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.

author: Alex30
author_rank: Regular Contributor
timestamp: December 01, 2014, 03:02:42 pm
content: Quote from: Andy Watson on December 01, 2014, 02:25:46 pmOh. Just to clarify, do you mean that the input voltage must be at least 1.2V below the supply voltage?

author: Zero999
author_rank: Super Contributor
timestamp: December 01, 2014, 03:06:16 pm
content: Quote from: Alex30 on December 01, 2014, 03:02:42 pmBoth the inverting and non-inverting inputs must be <+V-1.2

author: Andy Watson
author_rank: Super Contributor
timestamp: December 01, 2014, 03:07:04 pm
content: Quote from: Alex30 on December 01, 2014, 03:02:42 pmYes, that is my interpretation of the datasheet.

author: Alex30
author_rank: Regular Contributor
timestamp: December 02, 2014, 08:54:10 am
content: 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.

author: void_error
author_rank: Frequent Contributor
timestamp: December 02, 2014, 11:41:22 am
content: 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.Quote from: Alex30 on December 02, 2014, 08:54:10 amInput Voltage Range

author: Alex30
author_rank: Regular Contributor
timestamp: December 02, 2014, 12:10:18 pm
content: Quote from: void_error on December 02, 2014, 11:41:22 amThanks 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 allI 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?

author: void_error
author_rank: Frequent Contributor
timestamp: December 02, 2014, 01:35:55 pm
content: Quote from: Alex30 on December 02, 2014, 12:10:18 pmYes 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 theLMV321/LMV358/LMV324, a low-voltage rail-to-rail input and output version of theLM358/LM324with 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.

author: Alex30
author_rank: Regular Contributor
timestamp: December 02, 2014, 02:34:28 pm
content: Thanks for the help everyone. Will try and get the 15V supply working and see how it performs. Cheers