Opamp Guro needed Precision rectifier, Absolute value circuits

[Zero Cool]

I am looking for some help from an Opamp guru that has knowledge of Opamp filters, Precision rectifiers and Absolute value circuits. I have been tasked with reverse engineering an existing working circuit that was designed by a company engineer many years ago and all the knowledge of the design is long since gone. I cannot post a public schematic so I am going to have to find someone I can work one on one with through emails. I need a full detailed understanding of how the circuit works. Compensation for your time may be possible.

If you have the bandwidth to help. please PM me.




Zc

 

[jan.didden]

Zc, what does the circuit do?
Do you really need THAT circuit or to reproduce the functionality?

Jan

 

[Zero Cool]

I need to understand THIS circuit.

Absolute value circuits generally use an opamp precision rectifier to turn an AC voltage into a DC voltage like for a metering circuit. this circuit is part of a critical custom built tester.

 

[U230421]

...that was designed by a company engineer many years ago and all the knowledge of the design is long since gone...

Now, how could that ever happen? What will become of the western civilization if all knowledge flow away or vaporise?

Why not follow one of these abundent links?
DuckDuckGo

It's not about what you can find, it's about what you can know.

 

[cbdb]

There are many precision rectifier, RMS/Absolute, and AC to DC ICs available that make the design much easier, and usually better. There data sheets usually explain everything. The problems with opamp circuits is there not acurate at less than 10 mv ac, and there bandwith is limited. If your aplication is audio the RMS detects from THAT are specificaly designed and hard to beat. Dont know much about the RF chips except there more money. What range of voltages and freqs will this meter need to read?

 

[jan.didden]

I need to understand THIS circuit.

Absolute value circuits generally use an opamp precision rectifier to turn an AC voltage into a DC voltage like for a metering circuit. this circuit is part of a critical custom built tester.

Do you know its functionality, what it does, what it is supposed to do?

Jan

 

[Zero Cool]

Do you know its functionality, what it does, what it is supposed to do?

Jan

Yes, I can Pm you.




Zc

 

[Zero Cool]

Now, how could that ever happen?

15 years has past, many changes in employee's management, owners etc

 

[Zero Cool]

There are many precision rectifier, RMS/Absolute, and AC to DC ICs available that make the design much easier,

I first need to understand exactly how the existing circuit works so that we can then explore newer, better options.

 

[stratus46]

Precision rectifier. See page 19 of the AD8036 data sheet. The 8036 can also be an analog 'OR'.
The only requirement is that VH is always more positive than VL. Did I mention that it is a superb
clipping amplifier out to 250 MHz? If you ever tried to build a clamp amp (clipper) from opamps
you will truly appreciate this chip.

https://www.analog.com/media/en/technical-documentation/data-sheets/AD8036_8037.pdf

G²

 

[simon7000]

The normal absolute value circuit used one opamp as a zero loss half wave rectifier that the feeds an adder that gets the full input signal. These then add to the absolute or full wave peak value.

True RMS is quite a different beast. dBx came to the conclusion that they could do the rectification after doing a logarithmic conversion to do the math required to get to an accurate RMS value. There have been other methods the oldest of which is to use the signal under test to heat a sensor and have a matching sensor that is fed DC to make their outputs match.

Filtering usually starts with the Sallen and Key paper on filters. Although it is quite worthwhile to get the book "Rapid Practical Design of Active Filters.) I do forget the author's name though.

But at a 15 year old design the questions would be more of did you do a spice analysis of the design? What is the bandwidth and accuracy? Also what kind of signals are you looking at? Most RMS designs have some peak limit issues.

The modern problem is it is easy to find offered solutions and design guides, not a wide field of instructions in the basics.

Unlike Jan, I can't work for free. (Jan have you finished editing my bit yet?)

 

[steveu]

The usual method is to use an op-amp to diminish the diode curve and sum the resulting ~perfect (-ve) 2x half wave with the input so that the sum is 1x for positive and 1x-2x for negative input.
Precision rectifier - Wikipedia

 

[jan.didden]

The normal absolute value circuit used one opamp as a zero loss half wave rectifier that the feeds an adder that gets the full input signal. These then add to the absolute or full wave peak value.

True RMS is quite a different beast. dBx came to the conclusion that they could do the rectification after doing a logarithmic conversion to do the math required to get to an accurate RMS value. There have been other methods the oldest of which is to use the signal under test to heat a sensor and have a matching sensor that is fed DC to make their outputs match.

Filtering usually starts with the Sallen and Key paper on filters. Although it is quite worthwhile to get the book "Rapid Practical Design of Active Filters.) I do forget the author's name though.

But at a 15 year old design the questions would be more of did you do a spice analysis of the design? What is the bandwidth and accuracy? Also what kind of signals are you looking at? Most RMS designs have some peak limit issues.

The modern problem is it is easy to find offered solutions and design guides, not a wide field of instructions in the basics.

Unlike Jan, I can't work for free. (Jan have you finished editing my bit yet?)

Not so much edited as cut up ;-)

I think it is a waste to try to reverse engineering something 15+ years old, chances are you will find parts that are obsolete anyway.
And after you are done, you still don't know the exact performance. So then when you try to recreate it you may overlook something, because you don't know the specs, and up with something unusable.

The smart engineering thing to do is to measure the performance, list the requirements and then make a replacement, form/fit if needed.

And I don't work for free either, except for you Ed ;-)

Jan

 

[simon7000]

Jan,

I think a good spice simulation would allow exploring more of the circuit quirks that you might miss doing measurements. But of course for the primary purpose measurements are more accurate than a simulation.

I hope you at least puffed up the piece as they pay me by the page!

 

[jan.didden]

That was not necessary Ed, there was enough hot air in them already ;-)

Joking aside, nice set of articles. I basically divided the material up for consecutive installments and also moved a few paragraphs around to make the pieces stand on themselves.

Agree on the value of sims.

Jan

 

[Antoinel]

FYI

Hello Zero Cool.

Please see my thread in the Pass Labs Forum which is entitled "Class aP amplification". The acronym aP stands for analog pulse; essentially generates precision rectified music signals for Class B applications.

Best
Anton

 

[Zero Cool]

I think you guys have missed the point. there is an existing machine. I need to document how it works for the client. therefore designing something new is not the point of this at this time. they first need an understanding of how the existing tester works exactly. so they can document their process and in the future design a new machine based on a newer design.

I found an excellent video that got me 99% of what I needed.
YouTube

 

[Antoinel]

I think you guys have missed the point. there is an existing machine. I need to document how it works for the client. therefore designing something new is not the point of this at this time. they first need an understanding of how the existing tester works exactly. so they can document their process and in the future design a new machine based on a newer design.

I found an excellent video that got me 99% of what I needed.
YouTube

The narrator did a great job on the YouTube above explaining the subject.

I have the book he showed. I bought it at a bookstore in Chicago in 1974. It is entitled "Applications of Operational Amplifiers" Third-Generation Techniques by Jerald G. Graeme, M.S.E.E. The author's work-title was: Manager, Monolithic Engineering at Burr-Brown Research Corporation.

Best
Anton