Hello, I am currently having problems utilizing only one channel of the LMV722 Op Amp. I am wiring the op-Amp as an inverting amplifier with a gain of 100. In order to achieve desired results, I have to use both op-amps in the same circuit. That is, I have to wire both of them as inverting amplifiers with the same source. If I try to use only one and just ground the other one with 1K ohm resistors (with exception to the output), the one under use just put out Vdd and never change. If I utilize both of them, only one channel puts out the desired result while the other one puts out 1/2 of Vdd and never change. Thinking it is probably a bad chip, I tried two more others and got the same results. Am I using this op-amp correctly or is there something that I have missed? Thank you.
It's essential that unused channels are terminated correctly, though the problem is usually "increased power consumption and added noise." You've picked a lesser method. See Maxim AN-1957. Another for your collection, Op-Amps for Everyone.
Thanks a lot Cpemma. I have two other questions. With the current setup of the op-amp, the output is now set a Vdd/2 and appears to change around that value when there is an input. Is this correct op-amp behavior since reference, due to the non-inverting input, is half of the rail? Since I am already putting out Vdd/2, I guess the resolution of the ADC I am feeding into is cut in half since I already have Vdd/2 when there is suppose to be "no signal". Is this correct? I am using a DC blocking capacitor but the Vdd/2 is still present after at the input of the DAC. Thanks for all your help.
With a dual, what goes off in one half shouldn't affect the other, but in practice as they share the same supply pins, if one channel is bouncing from one supply rail to the other it will create noise on that rail. Some 'fast' op-amps are prone to bursting into oscillation if the PCB is not well laid-out with decoupling caps near the IC, maybe that's what you had, triggered by the unused half. "With the current setup of the op-amp, the output is now set a Vdd/2 and appears to change around that value when there is an input. Is this correct op-amp behavior since reference, due to the non-inverting input, is half of the rail?" Yes, AFAIK. That's why it's normal practice to use dual-rail supplies. Another App Note from my collection. Have you thought of using a non-inverting amplifier? With an op-amp that can swing to ground and an always >=0V input it may make life easier.
I appreciate your help Cpemma. I kind of wrote my last post in a hurry because I had to go to work -- I signed up to work 11 hours a day during Spring Break from college Range not resolution for ADC. I had never thought about designing it in non-inverting form. I will investigate that since making it easier does appeal to me a great deal. Thanks for the sharing some pieces of your collection; they answer many other questions that I had.
Gain of 100? That sounds like a lot, for stable opamp operation. It's usually not practical to push the gain past, say, 20. Try two opamps, gain of 10, cascaded.
Too late, production has already started. The gain is at 200 on a single op-amp instead of 100. I love this Op-Amp from TI. There is now much interest in this little thing at the EE department now. Portable ECG, here we come.
A gain of 100 is nothing. We frequently use gains of over 1000000 for photodiode amplifiers. It's all down to your circuit design preventing unstable operation.
A major limiter on gain is the cut-down on bandwidth, so for audio you're limited; with say an 8MHz op-amp if you want 25kHz to come through keep gain well below 32.
