Help setting gain on instrumentation amp, LT1167.

[rwgast_lowlevellogicdesin]

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.

[fcb]

The gain is independent of power supply rails (with the exception of clipping etc...).

Read the DS and lookup "instrumentation amplifier.

[Paul Price]

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.

[rwgast_lowlevellogicdesin]

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??

[Paul Price]

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.

[fcb]

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??

Check your maths.

Gain=(49.4Kr/Rg)+1
So 4.9 ohms would be a gain of 10,083

If you need to get to an awkward value, use two resistors in series.

[rwgast_lowlevellogicdesin]

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?

[Paul Price]

A single turn trimpot would do.

[fcb]

A single turn trimpot would do.

You'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.