Rail to rail, single rail op amp

[JohnRoberts]

> pimped by the op amp company who made an op amp that worked that way... clever in a gee whiz way...

I very seriously considered it to monitor street-power into my house. Drive from a Current Transformer was very ample and 2 phases all fit on a quad opamp.

Then I realized the Chinese would sell me the whole package far cheaper than I could buy CTs and meters. One of two $11 meter-modules had a bum back-light so I got two more as spares; still cheaper.

You can buy "LED VU Meter" modules far cheaper than you can build them. Many are aimed at across-speaker use and have terrible input sections (but one TL072 might overwhelm their flaws). Of course long-term supply rules-out auction sites unless you can capitalize the initial try-them-all then place a Lifetime Buy.

Over decades working in the business I have seen a number of dedicated meter ICs go end of life. I used a couple different ones at Peavey that were already in the system and in high volume use, but I would never bring in a dedicated chip, and designed  numerous meters using more conventional building blocks during my 15 years there.

I didn't get into micros until after my Peavey gig...

JR

 

[warpie]

Hi Ian,

Just wondering whether you came up with a solution?

 

[ruffrecords]

Hi Ian,

Just wondering whether you came up with a solution?

The solution I am going with is using regular op amps with an artificial 0V and referencing the LED comparator resistor string to the same point. Of course, the precision rectification is only part of the story - the other part is modelling VU (or other) meter ballistics. which basically means providing different attack and decay time constant. One a completely unrelated thread I came across the manual for the Midas TR console. This solves both problems in a couple of op amps. I have attached its LED VU meter schematic which I plan to base my version on.

Cheers

Ian

 

[warpie]

Thanks. I was't aware of this circuit.

 

[abbey road d enfer]

Thanks. I was't aware of this circuit.

It's one of the typical implementations of full-wave active rectifier, charging a cap and driving the inputs of the comparators. Quite academic.

 

[PRR]

Well, IC1a is a Full Wave rectifier made of a half-wave and a 2:1 resistive mixer (yes that works). The output is a big resistance (and not constant). We could hang a cap on there and get a quasi-Average. I planned to do that for my power-line meter (I only cared about wire heat, not short peaks).

However in music metering we want to know Peaks. We need to charge the cap fast and then disconnect. IC2b is a precision diode one-way valve to charge the cap.

This form does not completely "disconnect". The 430k loads the cap. Since this is much less than the <100r of the forward diode, we can pick a cap for short attack and slow decay. 1.5uFd suggests 0.6-0.7 second decay. 10mA from the opamp suggests a slew rate similar to a 1mS attack. That's fair values.

 

[JohnRoberts]

This is a well plowed field....

Here is a good collections of building blocks on Wayne's forum.

https://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=6&t=111

JR

 

[ruffrecords]

Thanks for the link JR. About half way down the page is the rail to rail op amp full wave single rail rectifier I started out with but this time using the AD823. Unfortunately these are £8 each which where I started in post 1. There is also a slightly different version of the Midas circuit I posted where the decay resistance is two resistors in parallel rather than the one in the MIdas.

Very handy resource. Thanks for that.

Cheers

Ian

 

[abbey road d enfer]

This is a well plowed field....

Here is a good collections of building blocks on Wayne's forum.

https://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=6&t=111

JR

Several of these "designs" seem like they're not really tested and tried; that's often the case with app notes and cookbooks.
I was intrigued by the 6th implementation "Frequency doubler gives pure sinewave", as it's some kind of unicorn in analog audio. I sim'd it with LTspice, and of course the results are far from in accordance with claims. The range of usable signals is quite restricted and the output signal is far from pure.

 

[abbey road d enfer]

About half way down the page is the rail to rail op amp full wave single rail rectifier I started out with but this time using the AD823.

I would think it works with any decent FET input opamp, such as OPA2132 (not TL07x coz unpredictable when inputs are too close to rails).

 

[JohnRoberts]

Thanks for the link JR. About half way down the page is the rail to rail op amp full wave single rail rectifier I started out with but this time using the AD823. Unfortunately these are £8 each which where I started in post 1. There is also a slightly different version of the Midas circuit I posted where the decay resistance is two resistors in parallel rather than the one in the MIdas.

Very handy resource. Thanks for that.

Cheers

Ian

Design application notes published by chip makers are rarely cost effective designs, but often recycled designs presented to showcase or pimp some new IC.

As I said this is well plowed field so lots of ways to skin those cats.

@Abbey, at least two of those schematics are from production designs of mine so I know they work.  I have seen cookbook/app note schematics that were clearly not the best or most cost effective implementation, but most work.  Wayne posted schematics from up to several decades ago, including some simplified schematics from inside ICs so clearly not practical to copy using common parts.

JR