1
\$\begingroup\$

I've been trying to setup this op amp for the past couple of hours, and I can't seem to do it successfully. I have a single supply, so I created a voltage divider with resistors to get +ve and -ve. I made the connections based on the data sheet by Texas Instruments. After that, I tried adjusting the offset null, but no luck.

Then I said okay, I'll try my circuit in LTSpice. The LTSpice model perfectly matches the inverting amplifier circuit. I re-checked my circuit and matched the model. Still the output voltage is off (~0.15V). I also tried other op amps and they give me approximately the same results. What am I doing wrong?

LTSpice schematic

Photo of breadboard prorotype

AFTER TWO DAYS I FINALLY GOT IT TO WORK

So I believe my issue was that the voltage divider wasn't a true voltage divider because whenever I hooked up my circuit the divider was not splitting the voltage evenly. So I decided to use a voltage follower and use its high impedance to my advantage. Needless to say, it worked! I think I also had grounding issues with my signal. Oh, and I also switched my opamps to the TL051IP since the previous didn't seem to provide accurate results. Anyway, this is the schematic for a Inverting Amplifier with a Single Power Supply! I labeled the V+, V-, and VG for convenience since this tripped me up the most for some reason.

enter image description here

\$\endgroup\$
1
  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Dave Tweed
    Aug 5 '18 at 19:10
1
\$\begingroup\$

The only thing you are doing wrong is your choice of a pitiful and ancient op-amp. Even if you bring R3 and R4 down to 100K, this op-amp is so poor quality that you will still have minor but visible offset errors.

Make R3 and R4 10K and the offset errors are less, but still there. As long as you use the LM741 you will be upset with less-than-perfect results.

Suggest you dump the POS 741 and advance way up to a TL081 or TL071 (for low noise). These are low cost JFET op-amps that can tolerate feedback and gain resistors in the low meg-ohms with little offset.

They offer offset null pins if you want offset to be near 0.000. Their bias current is thousands of times less than the 741, so the "true value" of a gain or voltage divider is much closer to the math equations.

ADVICE: If you buy the TL071 or TL081 series do yourself a favor and buy 1% metal film low drift resistors so your readings are stable over time and temperature. Per each given 1/4 W 1% value you can get 100 pcs on cut-tape for maybe $2.50 USD at Digi-key, Mouser, Allied, etc.

\$\endgroup\$
1
  • \$\begingroup\$ Thanks, I did switch the opamps to TL051IP (the LM741 were definitely broken), and I think I had some grounding issues. However, a new schematic is available and the post has been updated. Thanks for the suggestion!. \$\endgroup\$
    – Alex M
    Aug 8 '18 at 2:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.