# Which op amp should I use?

#### Resckick

Joined Feb 11, 2020
6
Hey guys!

So I'm working on a project. I use CMOS chips and I'm just interested if I can use any op amp with my circuit or are there any restrictions on which op amp should I use with my CMOS circuit(I want to use op amp as a voltage follower). I'm kinda new to this stuff and I would aprecciate if anyone could help me out with my question. Thanks in advance for your answers.

#### ericgibbs

Joined Jan 29, 2010
9,538
hi R,
Welcome to AAC.
Do you have an example of the type of circuits you are working on that you could post.?
E

#### dl324

Joined Mar 30, 2015
10,003
Welcome to AAC!

1. What are the power supply restrictions?
2. What type of CMOS chips are you using?
3. Why do you think you need to buffer the output from the CMOS device (is it a logic chip)?

#### crutschow

Joined Mar 14, 2008
24,330
What are
• Supply voltages
• Signal voltage range

#### Resckick

Joined Feb 11, 2020
6
Thank you all for replying so fast. Ok so I will try to describe my circuit as best as I can. So I am making a frequency counter circuit. I supply my whole circuit with 5V DC. Then I have my 4MHz crystal for my frequency reference and I divide it into 100kHz, 10kHz, 1kHz, 100Hz,10Hz and 1Hz for different accuracies. For frequency dividing I use 4017 chips I know this might not be the best solution for frequency dividing, but those are chips I had laying around. I then use a rotary encoder for selecting between those frequencies and I feed that signal in a flip-flop(CD4013) so I can divide that frequency by 2. So for example if I select a 1Hz accuracy on my rotary encoder I divide that frequency by 2 so I get 0.5Hz and than I and up that 0.5Hz signal and whatever signal I get on my Input so I can count how many cycles repeat in 1 second and I get a signal that I feed into my binary counters where the counting begins. I had a big problem with the signal(it was distorted) that was coming in the binary counter I tought it might be the problem with decoupling, but I tried different value capacitors and it didn't work, so then I just played around and I used a voltage follower and that worked like a charm, I used a LM358 op amp but it isn't the best because when it comes to higher frequencies I don't get a square wave as intended but I get more like a triangle wave(at lower frequencies it works just fine). So I'm interested what op amps would you suggest for my purpose? Oh and I also feed signal from voltage follower to a binary counter - chip 74HC192.

I am also including my shematic, but it isn't done yet(I have my full circuit shematic at home that is drawn on paper). But here is the part of my circuit that is intended for generating signal for binary counters. And I thank you all in advance for your replies.

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#### dl324

Joined Mar 30, 2015
10,003
I had a big problem with the signal(it was distorted) that was coming in the binary counter I tought it might be the problem with decoupling
I don't see a binary counter in your schematic.

#### Resckick

Joined Feb 11, 2020
6
Yes as I said this shematic isn't done yet, I have my full shematic(drawn on paper) at home but currently I'm away because I am going to college. This shematic isn't done yet, I didn't had time to fully finish it. But the basic principle of generating frequency and most important part is drawn. As I said the signal from voltage follower will go to the binary counter(74hc192), but it isn't drawn yet.

#### Resckick

Joined Feb 11, 2020
6
But there weren't any problems with binary counter what so ever...

#### dl324

Joined Mar 30, 2015
10,003
Yes as I said this shematic isn't done yet
At what points were you seeing seeing distortion?

#### Resckick

Joined Feb 11, 2020
6
I was seeing the distortion in the signal that goes into the voltage follower. I basically got a square wave, but there were just some random spikes or how would I put it and the count was off. I can draw a representation of that signal, if that would help?

#### ebeowulf17

Joined Aug 12, 2014
3,153
I see a lot of floating inputs on logic chips. My memory of fuzzy right now, but if I'm remembering right, floating CMOS inputs are prone to oscillating like crazy and causing all sorts of noise and/or heat problems.

Are those inputs really floating, or is that just because you're not drawing the schematic? If they're really floating, you may need to properly terminate them.

#### Resckick

Joined Feb 11, 2020
6
I put a pullup resistor on all of the inputs of inverter chips(74HC4049) and NAND gates(74HC03). I still need to put them in my shematic... But thanks for that anyway. But I don't know what to do with those floating outputs at chip CD4017, or is it okay to just leave them floating? But probably the most sensitive are inputs...

#### ebeowulf17

Joined Aug 12, 2014
3,153
I put a pullup resistor on all of the inputs of inverter chips(74HC4049) and NAND gates(74HC03). I still need to put them in my shematic... But thanks for that anyway. But I don't know what to do with those floating outputs at chip CD4017, or is it okay to just leave them floating? But probably the most sensitive are inputs...
Glad to hear you've got the inputs handled properly. IIRC, floating outputs aren't an issue at all.

#### dl324

Joined Mar 30, 2015
10,003
I can draw a representation of that signal, if that would help?