dc amplifier

Hi, I'm trying to amplify a weak dc signal (0-10mV) from a load cell using an LM324 operational amplifier. I am deeply ignorant on how to go about this so I tried using the circuit shown in the datasheet to give a non-inverting DC gain. I connected the 5V supply from the Arduino to pin 4 and the ground to pin 11. I connected a k-type thermocouple to pins 2 and 3 as a small dc source (it gave an output of about 0.4mV when I held it between my fingers). I also connected pin 1 (output) to pin 2 (-ve input) via a 1M resistor and then to ground via a 10k resistor. This should give a gain of 101 and amplify the .4mV input to about 40mV. I've also tried replacing the thermocouple with a low voltage from a battery and voltage divider and the Arduino 5V and ground connections with the +ve and -ve leads from a battery with similar results. I then measured the output by putting a meter across the output pin and ground. I measured a voltage of 3.82V rather than the .04V I expected. Obviously I have missed the point somewhere. Can anyone help? Thanks, Kiwi2


Are you wired up like this, or something else? http://masteringelectronicsdesign.com/how-to-derive-the-non-inverting-amplifier-transfer-function/

Hi Shannon, Yes. The circuit I put together was supposed to be the same as that shown in the masteringelectronicsdesign.com link. The pin references I made were the same as those shown in the TI data sheet. Regards, Kiwi2

Clearly is isn't as you never put the input signal across the two inputs (pins 2 & 3) with an opamp (unless its a transconductance amplifier and you have a current source).

What you've done cannot work as the two inputs are a virtually short (this means the feedback network works to keep the two inputs at the exact same voltage). Given the thermocouple is itself an extremely low-impedance source this means the op-amp saturates as it fails to achieve negative-feedback lock.

Your input voltage goes between GND and the non-inverting input (pin 3) and the feedback network is as you describe on the inverting input (pin 2)

You realize the LM423 has an input offset error voltage of 7000uV - this will dwarf the thermocouple output.

Also thermocouple circuits need cold-junction compensation since the voltage depends on the temperatures at both ends of the circuit where dissimilar metals meet.

Hi Mark,

Thankyou. From the data sheet which shows the inputs as +ve and -ve I assumed that that is where the +ve and -ve from the signal go. I assume the output is measured relative to ground as well. I was only using the thermocouple as a handy source of low (mV) voltage to see if I could get the operational amplifier to work. I'll replace it with a battery and a voltage divider. I'll try this tomorrow and let you know how I get on.

Regards, Kiwi2

You need to learn the basics of operational amplifier circuits, they are very powerful circuit elements once understood. Plenty of resources online.

Thanks for your comments Mark. I tried replacing the thermocouple with a load cell and connecting the -ve signal to ground rather than pin 2 but I still get an output voltage which is about 80% of the supply voltage and which doesn't vary with adding weights to the load cell. I'm not getting a lot of enlightenment from online resources. In other forums there are quite a few requests for help in amplifying small dc signals but no specific circuits offered to achieve it. The main problem with trying educate yourself on the net is that the material has been written by experts (obviously) who have forgotten how ignorant newbies are and how simple they need to keep their explanations. I think my best option is probably to see what courses are available at the local technical school and sign up for one.