OPA1612: Power Supply requirement considerations for multiple OPA1612's along with different applications

Hi Bill,

The quiescent current represents the current that the op amp itself consumes, and should remain fairly consistent regardless of configuration. It does not include current output from the op amp, so if you have a load on the output, that load current will need to be considered as well. Assuming a fairly light load of ~10kΩ per op amp, and either a single 15V supply or split ±15V supplies, the maximum output current would be ~1.5mA per op amp (15V into 10kΩ), or 3mA per OPA1612 (since it's a dual op amp). Combining the quiescent current and estimated output current, you'd need around 10mA per OPA1612, or 380mA total. To avoid designing to a minimum, I'd plan on using a voltage regulator that can supply 500mA or more.

Regarding single vs. dual power supplies, there shouldn't be a performance difference for the op amp itself - all it knows is that it's seeing a 15V difference between its supply pins, which is well within its recommended operating conditions.

With a single supply, be careful and ensure you don't violate the input common-mode limitations of the op amp, and also ensure that the op amps' outputs idle at a high enough DC level to provide the output swing you need without clipping. For OPA1612, keep your input common mode at least 2V from either the V+ or V- rails for good performance, and keep your output voltage at least .6V from either rail. Exceeding either of these conditions will cause clipping or increased distortion.  

The drawback to using a single supply is that keeping the circuit biased appropriately will require AC coupling, and introduces some challenges in noninverting gain applications, as the feedback network has to be connected to mid-supply (and a low-impedance mid-supply reference, at that), or AC coupled, which can require fairly large capacitances to preserve low-frequency performance.

Using a dual supply configuration can simplify this, as your op amps can idle at ground (close to the middle of the two supplies) and the ground connection is typically low impedance to begin with, so you won't have to worry about creating an driven mid-supply reference.

I suspect that your power supply filtering is going to be overkill, and such a large inductance and capacitance may have some unintended side effects on power supply turn-on time and inrush current. You may also want to double-check the current rating of your inductor to ensure that it can support the 500mA+ supply current you'll need for the op amps.

If you use a dual supply configuration, you'll benefit not only from LDO's PSRR (I'd expect 40dB or so under load at line frequency), but the PSRR of the op amps as well, which should be 60dB or more across the audio band. At higher frequencies, the PSRR of both the LDO and op amp do fall off, but decoupling capacitors should help to reduce high frequency noise on the power supply lines, making this less of an issue.

Thank you so much, Alex, for taking the time to reply with so much helpful information.

Since I am at the "learning to crawl" stage, I will now change my design from a single 15v rail to dual 15v rails. That change alone should help keep me out of unnecessary trouble. I will also do research in this area to gain a better understanding regarding common mode issues and other issues you mentioned.

Regarding the power supply, I was considering some sort of delay circuit to deal primarily with the inrush current, such as using a simple 555 timer configuration to fire a SCR. But, there again, I must then research to see if there are unintended consequences in doing that. My logic tells me that the simplest design can often be the best design. So, perhaps I will be much farther ahead by taking some steps backwards and designing a power supply much more closely related to the design at hand instead of using one that is overkill for this purpose. Again, more homework to do.

It looks like I have some very good choices regarding voltage regulators rated 1 amp or less. I really like the ones to where you can fix certain pins to ground for the precise voltage output you want. The specs also look very good. I am wondering if there is a "optimal" range for voltage regulators. In other words, is it better to keep the regulator working at, say 70% capacity as opposed to one working at only 30% capacity? This is another area where I must do my homework before choosing one.

BTW, the inductor is one I made, using a Magnetics ZW-46113-TC ferrite toroid core, wrapped with a single layer of 18 ga. wire.

So, now I have a direction to take, along with much homework to do. Once I have gained some better understanding, I will then include some design schematics for "dissection." Now, back to reading about ground planes.

Bill

Hi Bill,

I agree with Alexander, using a dual supply voltage has many advantages. An audio signal is AC signal moving positively and negatively arround a reference voltage. With a single supply voltage this reference voltage must be generated from the supply voltage by the help of voltage dividers. This reference voltage must then be low pass filtered to avoid supply voltage noise from directly being coupled into the signal chain. And this at every OPAmp! This means tons of additional components, huge electrolytic caps and time constants in the seconds range. I would always avoid the use of a single supply voltage whenever possible. It's so much easier to have a dual supply voltage.

Power supply units with huge inductors and capacitors forming a LC-hum-filter is a thing of the past. This was useful at a time when voltage regulator chips weren't available and when the tube circuits and early transistor circuits had miserable power supply rejection ratios (PSRR). This has tremendeously changed today! Modern voltage regulators provide an outstanding hum rejection and ripple rejection and modern OPAmps have an absolutely brilliant power supply rejection. So, a huge mains transformer and a huge storage capacitance or the use of a hum-LC-filter is no longer necessary.

Allthoug being a dino under the voltage regulators I would recommend you to use the LM317. This is such a superb regulator! It's absolutely stable and you can nearly do everything with it. Use a proper heat sink though. To generate a dual supply voltage another LM317 could be used. This can be done when you have two galvanically isolated mains transformer output windings. Otherwise you could take the LM337 for producing the negative supply voltage. Take care, negative voltage regulators are a bit more criticial when it comes to stability. But the LM337 is much more forgiving compared to modern LDOs. Follow the recommendations given in datasheet and you will be fine.

I'm a fan of using RC-filters in the supply lines of OPAmps. Even only very "light" low pass filtering can be enormeously helpful. But for the very most circuits simple 100n Caps from each supply pin to GND are enough.

For your 4-way active crossover I would recommend the use of 4th-order Linkwitz-Riley filters. There is tons of material about this issue in the web.

You might find these threads useful:

e2e.ti.com/.../683879

e2e.ti.com/.../691163

And you know "God" Rod Elliott??

sound.whsites.net/.../vi-regulators.html

sound.whsites.net/articles.htm

Good luck!! :-)

Kai

Hello Kai,

Thank you for chiming in and overloading my brain. Just kidding. I read the thread links you gave me. Good information. I have been buried in so many TI links it isn't funny. I earned BS degrees in physics and applied mathematics back in the 80's. I still remember the burnouts and brain overload miseries. This has begun to make me "reminisce" about the good ole days. Ha!

I will add your regulator recommendations to my list. I will compare them to the ones I am currently interested in, which are the TPS7A33 and the TPS7147. The transformer I have is a Antek, Inc. AS-0518. I had already resigned myself to treat the dual secondaries as a center tapped configuration for making the dual rail power supply. I am simply amazed at how good today's regulators are. Yes, I am now a dinosaur in today's technologically advanced world compared to world I was familiar with while studying for my degrees. Even then, Pluto was still considered a planet! So, it is no wonder why I was thinking of using the LC filter in my power supply. I hope it made Alex and you laugh. If nothing else, I will be good for that!

I am trying to understand why you want to add a resistor in making a RC filter for the supply. Right now, it seems unnecessary when looking at the specs by just using a 100n capacitor. As I recall, capacitors don't add noise, but resistance certainly does. It seems, from a design standpoint, that a given design is better off using the lowest resistance possible (when trying to keep noise to a minimum for the application at hand) if the extra current can be supported. Always trade-offs involved with just the little bit of design considerations I have already begun to make. So, I must evaluate the pros and cons of using a resistor for even a mild RC filter configuration. Interesting.

I will definitely be using the 4th order Linkwitz-Riley filters. Hence, my decision to use the OPA1612's to cascade two Butterworth filters together in series to give me the 4th order Linkwitz-Riley filters. Yes, Rod Elliot is a reliable source I like to use. He has already given me a lot of insight toward the design goals I wish to achieve.

Something that has been bugging me, getting back to figuring out the current usage of each op amp in a circuit. Is the gain vs. power requirement more of a linear function? In other words, knowing the quiescent power demand, is there a mathematical equation for determining (within a fairly good degree of accuracy) of how much power will be needed to support the op amp above the quiescent state? Instinct tells me that the op amp might be fairly linear until heat becomes a greater issue. This is originally why I raised the question in my first thread. Alex gave me a ball park figure to work with, but I wanted to learn the math and physics part of the answer. I am a bit confused when he said, "fairly stable regardless of configuration." I always think in terms of energy. Heat becomes part of the equation when dealing with resistance as the load demand increases.

I hope Alex will clarify what he means by "configuration." Design configuration? Right now, I am equating gain with energy. Is that wrong? Am I to figure out the actual load acting on the output of the op amp? So, if you have 10 op amps in series, each having a gain of 5, how would I determine the load on the first op amp and thus a realistic current requirement for that op amp? Are we looking more at coupling loads? Very confused at this point. Still trying to find links to help me break through the mental barricade. Any known links to point me to would be very helpful.

I don't want to resort to using design tools at this stage, as I am trying to create more of a "understanding" foundation at this early stage.

Thanks again for your help, Kai. I am very grateful.

Bill

Hello again Kai,

Ahhh.   "I'm beginning to see," said the blind man.  What has become obvious to me is my misunderstanding regarding op amps.  I am going to digress and educate myself regarding their various design uses and basic theory.  I have to rewire my brain to focus on the frequency/voltage aspects of op amps.  Many more things should fall into place once I stay focused on building a learning and understanding foundation.  Everything else that Alex and you have said is finally making sense to me now. 

What I am going to do is take a break here so as not to waste anyone's time with such basic issues.  I must do the research on my end in order to lift myself up to a higher level so as to ask more "relevant" questions.  So, I need to figure current things out some before attempting to take another step forward.

I so much appreciate the time you have taken to come down to my level in order to help me see the error of my understanding.  What has always worked for me to learning is the "taking the stairs" approach.  Take a step up, then stay at that level until I am ready to take the next step up, and so on. 

Just want to wish you and Alex a very merry Christmas and happy new year!  I will be looking forward to building my first prototype sometime next year with a good understanding of why I chose each element in the design, including good, fundamental ground plane and circuit techniques.

Bill  

Hi Bill,

If I may chime in, our TI Precision Labs - Op Amps training series is a great (and free) resource for ramping up your op amp expertise. There are several topics of interest to audio designs, such as the sections on bandwidth and low distortion design.

Best regards,

Ian Williams
Applications Engineer/SPICE Model Developer
Precision Amplifiers

Hi Ian,

Yes!  I have been receiving several TI emails to date, encouraging me to check out the various free information from TI.  It's been amazing as to the amount of information I have so far read already!.  That is why I am going to spend a good deal of time going through this information and absorb as much of it as I can.  I have already made one trip to the local library, but will need to go to a local university's library to find the kind of books I need.  I have also found some sites that are also factual and reliable.  This forum has been a real wake-up call for me.  I now have a much better idea of where I am currently at and where I need to get at if I am going to have any success with my design aspirations.  

I'm starting out with some simple op-amp design configurations regarding building a 4 way active crossover, a few buffers and some parametric equalizers.  However, there are so many things hitting me from every direction, even with a simple design such as this.  In other words, there is much to consider and think about, especially when deciding what are my most important design goals.   Again, I must first learn how to crawl.  Just so very thankful to Alex and Kai for taking the time to help me get started down the correct path.

I could simply "mimic" a design someone else has done, but that takes all of the fun out of it.  That's just lazy, boring and cheating.  I want to learn how to design and do my own thing, being happy and proud of what I was able to accomplish, regardless of how many mistakes I make in the process of getting to the finish line.  That's just me.  It's in my blood.

You have a great holiday season as well.  :)

Bill 

Hi Bill,

a merry Christmas and happy new year to you, too!

Kai