# Newbie op amp gain question

#### Involute

Joined Mar 23, 2008
106
I'm going to be doing some tests with a load cell that outputs 800 uV/V. I'm going to excite it with 5V, so my max. output will only be 4 mV, and the load I'm interested in will only produce 600 uV. I'd like to monitor this with a 5V Arduino that will have a 10 bit AD. I figure I should to try to scale my 600 uV signal to 2.5V to put it in the middle of the AD's range. That calls for a gain of 4166. If I use an op amp (like the MCP6004, because I happen to have some of those sitting around; datasheet here, http://ww1.microchip.com/downloads/en/DeviceDoc/21733j.pdf), the gain is R1/R2 + 1. Can I then just use, say, 42K and 10 ohm resistors (or others providing the same ratio)? I've never used op amps before, and I'm reading/watching various tutorials here and elsewhere, but that's what it's starting to look like. I realize there will be issues like common mode gain, drift, etc., but are there any major, "You've totally misunderstood and you can't do that, you pathetic wanker" issues? Is it better to use two op amps in series, each with a gain of about 64 (square root of 4166)? Any tips at all would be appreciated.

#### absf

Joined Dec 29, 2010
1,949
But the input offset voltage of MCP6004 is from -4.5mV to +4.5mV which I think is too high. Get one with the Vos in the uV range (like LT1051).

Allen

#### wayneh

Joined Sep 9, 2010
17,153
A gain of 4000 is not a problem for a DC signal. It could be, for a high frequency, but not for DC. You might want to read this article, about op-amp input offset voltage and ways to deal with it.

#### Involute

Joined Mar 23, 2008
106
But the input offset voltage of MCP6004 is from -4.5mV to +4.5mV which I think is too high. Get one with the Vos in the uV range (like LT1051).

Allen
Why does input offset matter? I'm mobile and haven't had a chance yet to read the article recommended by wayneh. If the issue's too complicated, I'm happy to wait until I've read the article. Thanks for the pointer to the LT1051.

#### wayneh

Joined Sep 9, 2010
17,153
Why does input offset matter?
Think about what that 4.5mV will look like after a gain of 4000.

In other words, it will swamp your signal.