LM2903N differential op amp

[dembkomj]

Hi,
I have a project in which I have two analog DC voltages at different levels, let's say 10V and 2V. I want to have a circuit that takes the difference of these two voltages and produce a single DC voltage, in this case 10-2= 8V DC. I'm a mechanical engineer with basic electrical background, however this is literally my first circuit dealing with op amps. I've built many 12 & 24V DC circuits dealing with relays, resistors, switches, etc, but I'm trying to branch out into the IC world a bit.

After browsing for a chip that would do this voltage subtraction, I purchased some LM2903N dual differential op amps from Mouser.com, and tried one of them out on a solderless breadboard. I wired this up per the spec sheet as such:

Pin 1 = output
Pin 2 = IN1(-)
Pin 3 = IN1(+)
Pin 4 = GND
Pin 5 = not used
Pin 6 = not used
Pin 7 = not used
Pin 8 = 12V DC supply from a "wall wart", 1A rating on it

I'm really just trying to bench test this op-amp by itself before I incorporate it into the rest of my design, however my simple bench test proved that it did not give me the result I was looking for.

Using a Fluke 87V multimeter, I measured 11.4V DC between Pin 8 and Pin 4 of the amp (I have a 1N4004 diode downstream of my power supply for reverse polarity protection). This checked out OK. I then measured 10V DC between Pin 2 IN1(-) and Pin 4, and 2V between Pin 3 IN1(+) and Pin 4, so I know my two inputs are what they should be. However, when I checked the output Pin 1 relative to ground Pin 4 I measured 0.00V. I switched the voltages of Pins 2 and 3 relative to each other to see if that made a difference and I measured Pin 1 to Pin 4 again but got about 0.25V to 0.3V, still not the 8V differential I was expecting to see. I varied the 2VDC by a few volts but the measurement at the output did not move around.

I did not put any external resistors attached to any of the pins of this amp, since I don't want any voltage drop going to the two inputs, I want them to be exactly what they are. Plus the spec sheet states a max input voltage of -0.3 to +36V, and I'm within that window.

Did I understand correctly that a differential op amp produces an analog voltage output that's basically voltage 1 minus voltage 2? I'm sure I'm missing something fundamental here. Could it be my Fluke voltmeter is consuming too many amps when I take my measurement of the output pin? What am I doing wrong or what can I do to produce the results from above? Again, I'm about as rookie as it comes to this stuff, I'm just trying to get me feet wet with the knowledge I have already regarding DC circuitry.

Thanks,
Mike

[rob77]

google for diferential amplifier using opamp - it's single op-amp and 4 resistors circuit.


edit: gooled one - attached.

[c4757p]

You can connect it however you want and it might not work - LM2903 isn't an op amp, it's a comparator...

This particular family of comparators can be forced into linear duty. You might be able to make it work by putting a pullup resistor, about 5k-ish or so, from the positive supply to the output. But it's quite likely to be unstable.

[mij59]

Hi,

You need a basic understanding of opamps, a good way to start is looking at Dave's video about opamps, search for " opamps explained " on the EEVblog or search on Youtube for "opamps"