Help Choosing an Opamp - Page 1
Forum section: electronics / beginners

author: n1kt0
author_rank: Newbie
timestamp: January 29, 2021, 11:35:53 pm
content: Hey folks, I have built a programmable microcontroller-driven sine wave generator around an AD9835 DDS DAC; I've selected external components so the waveforms it outputs are ~1V peak to peak.I have a rudimentary understanding of opamps, but still find it very overwhelming when it comes to actually choosing one for a particular application. I'd like to be able to buffer my output signal so I can drive lower impedance loads, and maybe add a little gain to get the signals up to 3.3 or 5v peak to peak levels. I'm not planning to drive any transmission lines or anything, so precise output impedance isn't important to me; I'd just like to be able to use it to experiment with analog filters, maybe drive an 8 ohm speaker when I have it set to audible frequencies, but still have the flexibility to play around with higher frequencies, say up to 10 MHz.Also, how should I plan this amplification stage -- should I just AC couple the output from the DDS through a high value capacitor into a single opamp with a virtual ground of half my power supply voltage? Should I run the output directly into a unity gain buffer before going to a higher gain stage? I haven't found that these practical concerns are every really addressed in the opamp tutorials I've read so far, so if anyone could provide some insight or guidance, I'd be grateful.Thanks!

author: Terry Bites
author_rank: Super Contributor
timestamp: January 30, 2021, 12:52:28 pm
content: I'm a fan of EL5111T for up to 10MHz. Works well at 5V. But you will need a separate LV audio amp like an TPA721 that you can switch in.I don't think there's a regular opamp that can can drive 8 ohms and has a bandwidth suitable for 10MHz.Prove me wrong some one, surely.....

author: MathWizard
author_rank: Super Contributor
timestamp: January 30, 2021, 08:05:37 pm
content: For ages I only had salvaged op-amps, and never used them. Then I got a bunch of LM358's, and barely used them, then I a few more types of them, and started learning to use them.IDK, I could have been using them all along, just by following schematics, but I rather know what to expect form calulations, than just hook up stuff and see what happens, and FOR YEARS, I've been too LAZY AND STUPID to solder/wire up a whole bunch of variable resistors, the good kinds, not the pain in the rear cheap THH type that barely fit's my standard breadboards....For breadborading these days, I have LM358 for clunky stuff, TL072 and TL082 for JFET high input impedance stuff. I have a few fast slew rate op-amps, a few low noise high pressicion types, I don't remember their names yet, still learning to use them better.I don't have any of the threshold level type op-amps.Then I have some decent audio op-amps, at least for low/mid end soundcards/ stereo's, like LM4558 I think, I have a few ok ones.I have all kind of cool parts I never used yet, I just went ahead and ordered bits and pieces over the last few years, because I like to have it around for when I want to use it...like now, I'm really getting into op-amps, and still never tried the higher quality ones yet for most breadboard stuff.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: January 30, 2021, 09:15:01 pm
content: Op amps can be a nightmare to specify.Input impedance, input bias current need to be taken into account.A 10Mhz op amp isnt 10Mhz unless the signal is small !So slew rate must be taken into consideration.I have done a bit with USB scopes and found I had to over spec my op amps to get a decent signal bandwidth at full voltage range.Got caught out with input bias current with high impedance signals.Had to offset the bias current with a 10M trimmer or scope AC mode really messed up.

author: David Hess
author_rank: Super Contributor
timestamp: February 11, 2021, 01:58:49 am
content: Higher frequency operational amplifiers that support a higher output current allowing them to drive 50 and 75 ohm transmission lines directly are commonly called "video" operational amplifiers and suitable for your application. No additional buffering is needed if operated as a non-inverting amplifier.I would suggest some parts but you did not give any supply voltage requirements.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: February 14, 2021, 08:34:09 pm
content: Quote from: David Hess on February 11, 2021, 01:58:49 amI just design low sampling rate USB scopes up to about a couple of mega samples per second.The op amps need to be rail to rail input and output.Then +/- 5 volts.Around 5MHz bandwidth with at least 5us/V slew rate.On RS Components site that comes down to about half a dozen.After factoring in low price it comes up with about two. TLV274 and TLV2374.I found a quicker TLV794 but that messed up as input bias current is high and gives a offset in conjunction with 1 meg inptu resistor.It can be counteracted wit ha high megs pot to power rail.

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2021, 09:54:54 pm
content: The TLV274 and TLV2374 are far too slow and I couldn't find the TLV794 using a search engine. A much higher GBWP than 10MHz is required and slew rate of 5V/µs won't cut it. Go for at least 50MHz and 50V/µs.I agree with the suggestion of using a video amplifier, especially if DC characteristics such as offset aren't critical.

author: David Hess
author_rank: Super Contributor
timestamp: February 15, 2021, 12:10:26 pm
content: I do not think any part meets the requirements of rail-to-rail, low input bias current, and speed. (1) But input amplifiers for oscilloscopes almost always operate over a very limited input range with at least one high impedance attenuator before their input so rail-to-rail input operation should not be required. If only the output is rail-to-rail, then there are many good parts which meet those requirements. Analog devices has the AD8033 and some similar but faster parts. Texas Instruments has the OPA810 which actually has a rail-to-rail input.The typical input range for an oscilloscope amplifier is less than +/-1 volt. (2) Classic oscilloscopes have more input attenuator sections so get by with less than an input range of +/-25 millivolts. The wider input range for the input amplifier of modern oscilloscopes is responsible for some undesirable characteristics. For one thing, it requires a much higher slew rate that some cannot support.(1) After I wrote that I found the Texas Instruments OPA810, but it is not as fast the alternatives.(2) The exception are oscilloscopes which use bootstrapped input stages like the Tektronix 7A13 and 7A22 but this is done for other reasons and they are limited to 100 MHz and slower.

author: Zero999
author_rank: Super Contributor
timestamp: February 15, 2021, 07:08:45 pm
content: The original post didn't mention anything about rail-to-rail, or oscilloscope input amplifiers.If there's only 5V available, then make a higher voltage power supply, with a charge pump, or boost converter. It's the only sane way to do this.