Help setting gain on instrumentation amp, LT1167. - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 24, 2014, 05:34:38 pm
content: I have a few LT1167 (http://www.stanford.edu/class/ee122/Parts_Info/datasheets/LT1167.pdf) instrumentation amps around, I figured a good learning project would be to make a simple amplifier to convert uv/mv up to volts. I get pretty crummy readings on my scope and meter around the 1mv range. I had planned on providing them with a split supply via a TLE2426 rail splitter,http://www.ti.com.cn/cn/lit/ds/symlink/tle2426.pdf. But when I read the data sheet I couldn't understand how to set the gain via a single resistor. Lets say I had a +5, -5 supply how would I chose a resistor for a gain of 10, 100, 1000? I think I just need to see someone clearly work the math.

author: fcb
author_rank: Super Contributor
timestamp: May 24, 2014, 07:10:53 pm
content: The gain is independent of power supply rails (with the exception of clipping etc...).Read the DS and lookup "instrumentation amplifier.

author: Paul Price
author_rank: Super Contributor
timestamp: May 24, 2014, 07:19:17 pm
content: The formula to set the gain and determine the value of the resistor to set the gain is at the bottom of page 11 of the datasheet for this device.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 24, 2014, 08:17:30 pm
content: Ok so after rereading that am would a 4.9 ohm resistor give a gain of ~11? If so 49.4 is kind of a weird resistance to divide in to, what is the best way to set accurate gains, just buy a .1% that is the exact value you need in this instance a 4.94 ohm resistor??

author: Paul Price
author_rank: Super Contributor
timestamp: May 24, 2014, 08:43:33 pm
content: What about the "k" in the formula. Your answer to the gain equation is 1000 times toooooooo small.If you wanted 4.94k you could put in parallel two selected 10k 5% resistors.Or you could put in series a 4.7k and a 220 ohm resistor and a 20 ohm resistor.Or you could use a 4.7k and a 500-ohm trimmerpot configured as a rheostat.Or you use a 5K trimmerpot.Or you could just be satisfied with using a 5k resistor and accepting or otherwise compensating the small gain error.Or if you had a bunch of 5k 5% resistors you might just find one that is 4.94k.Or if you can find a 1% 4.94k resistor for sale, you could just buy it for $.30 pay $25 for shipping and wait for it to arrive.

author: fcb
author_rank: Super Contributor
timestamp: May 24, 2014, 08:46:26 pm
content: Quote from: rwgast_lowlevellogicdesin on May 24, 2014, 08:17:30 pmCheck your maths.Gain=(49.4Kr/Rg)+1So 4.9 ohms would be a gain of 10,083If you need to get to an awkward value, use two resistors in series.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 24, 2014, 08:51:00 pm
content: Hmm i guess a trimpot is a good way to go, its just when I think of instramentation I think things should be fairly accurate up to about 4 digits or so, maybe more depending on the application.What are some good trimpots for stuff like this? 10 turn 20 turn? Trusted brands?

author: Paul Price
author_rank: Super Contributor
timestamp: May 24, 2014, 08:54:43 pm
content: A single turn trimpot would do.

author: fcb
author_rank: Super Contributor
timestamp: May 24, 2014, 09:06:37 pm
content: Quote from: Paul Price on May 24, 2014, 08:54:43 pmYou'll have trouble with a trimpot in instrumentation amp configuration as the TC tends to be huge with them, and you are using them as a variable resistor. TC is less of a problem in potentiometric trimming configurations.The exception would be if you strap the trimpot across a smaller resistor, or a small trimpot in series with a large resistor.