OPA445 Op amp question.  - Page 1
Forum section: electronics / beginners

author: Jwillis
author_rank: Super Contributor
timestamp: March 27, 2019, 08:25:37 pm
content: I got these OPA 445 op amps to mess around with because of the higher voltage rating then the more commonly used 741s. Now they should both operate generally the same way aside from the difference of supply voltage correct? The OPA445 needs a minimum of +/- 10 volts to a maximum of +/- 45 V so they should operate at 25 volts which means +12.5 /-12.5 or am I missing something in the data sheet .I've tried higher voltages but still get no reaction from the chip. Is there a different way I can test these other than setting them up as an oscillator because I can't get them to oscillate at any voltage.http://www.ti.com/lit/ds/sbos156b/sbos156b.pdf

author: Audioguru
author_rank: Super Contributor
timestamp: March 27, 2019, 09:06:01 pm
content: The input and output voltage ranges are 5V more positive than the negative supply to 5V more negative than the positive supply so with your -12.5V and +12.5V supplies, the input and output voltage ranges are -7.5V to +7.5V.

author: Jwillis
author_rank: Super Contributor
timestamp: March 27, 2019, 10:13:13 pm
content: Ok .So what does that mean? The input voltage is to high?

author: Bassman59
author_rank: Super Contributor
timestamp: March 27, 2019, 10:20:40 pm
content: Quote from: Jwillis on March 27, 2019, 10:13:13 pmIt means that with power-supply rails of ± 12.5, the output voltage cannot swing beyond ± 7.5 V. Nor can the inputs, although you don't show any circuit.You say "how can I test this other than setting them up as an oscillator" -- why would you do that?A reasonable circuit might be a simple inverter.

author: Jwillis
author_rank: Super Contributor
timestamp: March 27, 2019, 10:45:40 pm
content: I don't a have a real use for them in any particular circuit right now .Because they're used with unknown condition I wanted to set them in a simple circuit to test them so I put them in an flashing LED circuit like this one below . But get no response.Did I burn them out?

author: rstofer
author_rank: Super Contributor
timestamp: March 27, 2019, 10:50:39 pm
content: If you used that 9-12V supply, you have nowhere near enough voltage. The minimum is +-10V so you need 20V across the rails. MINIMUM!

author: rstofer
author_rank: Super Contributor
timestamp: March 27, 2019, 10:55:12 pm
content: Why don't you just build a voltage follower and change the voltage on the + input with a potentiometer. Connect a 100k pot with 10k series resistors on each end across the power supply. Take the center tap to the + input.http://www.learningaboutelectronics.com/Articles/Voltage-followerThe series resistors will prevent you from shorting an input directly to the rails.Or form a voltage divider out of a pair of resistors. Two 100k in series with the center tap headed toward the + input should produce a voltage near 'ground' or half way between the rails.A 56k 47k divider will provide a different voltage.

author: rstofer
author_rank: Super Contributor
timestamp: March 27, 2019, 10:58:22 pm
content: It's handy to have an actual center point in the power supply. This forms 'ground' and would be the reference for measuring the inputs and outputs. If you don't have a center tap, form a symmetric voltage divider with a couple of 22k resistors. Given a sufficiently high impedance of a DMM, this should suffice but it is not ideal. The voltage division will never be exactly half.

author: Jwillis
author_rank: Super Contributor
timestamp: March 27, 2019, 11:02:15 pm
content: Quote from: rstofer on March 27, 2019, 10:50:39 pmI put 25volts across the rails to as high as 30volts with no response .Isn't 25v that the same as +12.5 and -12.5 and 30v the same as -15 and +15 ..This is whats confusing me, there should be enough supplied voltage.

author: Jwillis
author_rank: Super Contributor
timestamp: March 27, 2019, 11:05:23 pm
content: I ordered a couple new ones to check out .These being used may well be no good.

author: rstofer
author_rank: Super Contributor
timestamp: March 27, 2019, 11:09:03 pm
content: Quote from: Jwillis on March 27, 2019, 11:02:15 pmYou still need a valid circuit and the starting point is a simple voltage follower. If that doesn't work, nothing will. That you have two independent sources tied together make things easy. You actually have a 'ground' to measure from.Since you can adjust the + pin input voltage, you should be able to see the output change. Relative to ground, it doesn't need to change very much. You could even use a 1.5V battery. If you put +1.5V at the + input pin, you should get 1.5V on the output. Both measured relative to 'ground'. Turn the battery around and you should get -1.5V at the output. Whatever the voltage is on the + input pin, you should get the same on the output.

author: rstofer
author_rank: Super Contributor
timestamp: March 27, 2019, 11:11:52 pm
content: Have you watched Daves Op Amp videos?How about w2aewThese are excellent introductions to op amps. Maybe you'll see something that helps.

author: Jwillis
author_rank: Super Contributor
timestamp: March 27, 2019, 11:21:14 pm
content: Yes I did watch some but still leaves me scratching my head. But if I use a similar circuit like this but with a 25 supply and inject a maximum of 7 volts into the input I should see a voltage at the output?

author: rstofer
author_rank: Super Contributor
timestamp: March 27, 2019, 11:35:44 pm
content: Assuming I am reading the schematic properly, the input voltage will be 2.5V above ground and the output should be as well. Again, assuming +-12.5V rails and 'ground' is the point where they are tied together.

author: Jwillis
author_rank: Super Contributor
timestamp: March 28, 2019, 07:23:50 pm
content: Ok so I can assume that these are no good and not worth further investigation. That's fine they were used and free so no great loss.