an evening with the ICL7660

[cellularmitosis]

Circuit K:

For circuit K, I wanted to see the performance contribution of the 100uF electrolytic without the capacitor multiplier.


Schematic:




Performance into 4.7k load:

DC-coupled probe indicates an output voltage range of -8.40 to -8.17V.



AC-coupled probe indicates a ripple of 8.80mV.




Performance into 470R load:

DC-coupled probe indicates an output voltage range of -5.68 to -5.36V.



AC-coupled probe indicates a ripple of 41.2mV.




Comments:

Here we see a similar level of performance as the 100uH / 1uF LC filter combined with the capacitor multiplier.  That would imply that the the 100uH / 100uF LC filter contributes about the same level of filtering as the 1k / 1uF capacitor multiplier.

[Kalvin]

Very nice and informative posting! I too found the Faraday cage very intuitive and clever

The input of the ICL7660 can be filtered with the similar kind of LC filter as the output in order to reduce the ripple seen by the power supply feeding the ICL7660.

Looking forward for more measurements from cellularmitosis. Well done!

[cellularmitosis]

The input of the ICL7660 can be filtered with the similar kind of LC filter as the output in order to reduce the ripple seen by the power supply feeding the ICL7660.

Definitely going to give that a try soon!  Thanks for the suggestion

[ignator]

I didn't see this thread last July 2014.
I'm curious why you don't have any small value decoupling capacitors. Something like 27 to 47 (or 270) pico farads that can do something with the fast nano second pulse edges. When I was working, we compiled Vector Impedance measurements for common stocked capacitors from engineering stock. 1 micro farad if I recall self resonated below 3MHz, and was so inductive above this, that it did nothing to decouple noise. You need to keep lead length short as possible, if you put any capacitor of these small sizes in your circuit.
Sorry, I retired a few years ago, and I didn't keep a copy of various ceramic capacitors and their self resonance frequencies. These were leaded parts as my job was to bandaid avionics that failed emission tests, and the fix needed to be  added to existing circuit assemblies, as there was no time to respin the PWB before certification of the product.
Your circuit board ground plane should be connected to the faraday shield with a wide brass/copper/aluminum strip so the inductance of your ground wire connecting the board to your earth reference is a low as possible. My rule of thumb is every 25mm of wire looks like 25 ohms at 100Mhz (22AWG typically) because of its inductance. This impedance allows RF noise to 'float' on the ground.

[cellularmitosis]

When I was working, we compiled Vector Impedance measurements for common stocked capacitors from engineering stock. 1 micro farad if I recall self resonated below 3MHz, and was so inductive above this, that it did nothing to decouple noise.

Sorry, I retired a few years ago, and I didn't keep a copy of various ceramic capacitors and their self resonance frequencies.

Thanks for the reply.  Man, I would love to compile such a set of measurements myself!  What sort of equipment did your department use to make those sort of measurements?

I been contemplating looking up schematics for older / simpler instruments to see if it would be feasible to produce reasonably priced similar circuits with today's components.

Your circuit board ground plane should be connected to the faraday shield with a wide brass/copper/aluminum strip so the inductance of your ground wire connecting the board to your earth reference is a low as possible. My rule of thumb is every 25mm of wire looks like 25 ohms at 100Mhz (22AWG typically) because of its inductance. This impedance allows RF noise to 'float' on the ground.

Thanks for the tip.  Maybe I can tap a short screw directly into the bottom of the dutch oven and mount a copper braid.

[paulie]

I found this thread very interesting from the start because of using the close cousin 7662 quite a lot. 7660 voltage limitation too restrictive for me. Early on I settled on inductor and couple cheap caps to get ripple below 100mv using 1k resistor load. Also playing with the pump caps had some benefit to minimize drop. IIRC one was much larger than the other and contrary to datasheet recommendations.

[ignator]

cellularmitosis:
I looked at ebay, here's a broken one, but it's exactly what we used HP4815A:http://www.ebay.com/itm/Vector-Impedance-Meter-4815A-RF-Hewlett-Packard-/111671742452?pt=LH_DefaultDomain_0&hash=item1a00278ff4

Note the crank handle, this controlled the frequency generator, and the meters, one indicates ohms, the other phase angle. You would crank the frequency up until you find a minima of ohms, and for a cap. any frequency above this was inductive (lead length and internal paths inductance took over). If you get one, make sure it comes with the probe.

The biggest problem in packaging avionics, was to find a way to get the PWB boards ground planes connected with a wide sheet of connection to the box chassis (the infinite sheet conductor in field theory). Screws were sometimes used to multiple standoffs that were pressed into the aluminum chassis. Sometimes we would make clamps from aluminum bars to sandwich a PWB with edge plated, as well EMI gaskets (spring brass with nickel plate). 
As both the radiated emission, conducted emission, and then the radiated susceptibility, conducted susceptibility compliance evolved over the years, the ability to make your PWB grounds low inductance to LRU (line replaceable unit) chassis, and to make the LRU mount to the aircraft mounting tray to have extremely low impedance was the game to pass tests.
Susceptibility became a big deal, as airframes went from 100% aluminum, to composite, so test levels of HIRF (high intensity radio/radiate frequency) went from 5 v/meter to 1000s v/m.
Everything became low phosphorus nickel plating between the chassis and PWB. Original plating was just zinc chromate on the aluminum when I started working in 84. Aluminum grows oxide almost immediately, and this is non conductive, hence some sort of conductive plating on aluminum. PWBs use either Nickel or tin lead still. 

Note the screws needed to be spaced 20mm minimum (1/4 wave length of max frequencies HIRF was up to 18GHz when I retired), the point being a single screw probably will work with just your single switching circuit, but put a whole high speed processor with lots of parallel address and data buffers all switching at the same time, and you need to have a real good ground connection to prevent the PWB from being an antenna and radiating. As well, if you have noise on your ground, instead of decoupling capacitors quieting the noise, you coupled noise from the ground to the circuit signal (very painful lesson when your out of time, and the emission testing gets worse with every part you add to decouple). 
Also the point about a single screw path is an inductor, as all current flows through just this. The larger the diameter, the lower the inductance. So keep this in mind. Multiple in parallel is better.

[Tombs Balsam]

A serious post, thanks for sharing

[cellularmitosis]

cellularmitosis:
I looked at ebay, here's a broken one, but it's exactly what we used HP4815A:http://www.ebay.com/itm/Vector-Impedance-Meter-4815A-RF-Hewlett-Packard-/111671742452?pt=LH_DefaultDomain_0&hash=item1a00278ff4

Note the crank handle, this controlled the frequency generator, and the meters, one indicates ohms, the other phase angle. You would crank the frequency up until you find a minima of ohms, and for a cap. any frequency above this was inductive (lead length and internal paths inductance took over). If you get one, make sure it comes with the probe.

Wow, very interesting!  It didn't even occur to me that you could make such measurements with an instrument which doesn't have a cathode ray tube or LCD screen!  What's even more interesting is that, since you are just looking for the minimum amplitude value, I could potentially cobble together an instrument to do this with a DDS which performs a sweep, and an ADC which looks for a minimum value.  Hmm...

Aluminum grows oxide almost immediately, and this is non conductive, hence some sort of conductive plating on aluminum.

Funny you should mention that, it had been months since the last time I used the aluminum dutch oven, and I had to use a file to rough up the lip where the lid meets the pan in order to get a good noise floor .  I might also consider bolting a copper strap to connect the lid to the pan...

Note the screws needed to be spaced 20mm minimum (1/4 wave length of max frequencies HIRF was up to 18GHz when I retired), the point being a single screw probably will work with just your single switching circuit, but put a whole high speed processor with lots of parallel address and data buffers all switching at the same time, and you need to have a real good ground connection to prevent the PWB from being an antenna and radiating. As well, if you have noise on your ground, instead of decoupling capacitors quieting the noise, you coupled noise from the ground to the circuit signal (very painful lesson when your out of time, and the emission testing gets worse with every part you add to decouple). 
Also the point about a single screw path is an inductor, as all current flows through just this. The larger the diameter, the lower the inductance. So keep this in mind. Multiple in parallel is better.

Thanks again for sharing your tips, you've obviously got a wealth of knowledge here!

[gildasd]

Sorry to re-spawn this topic, but it has been real useful:

The 47puff capacitor is especially marvellous to kill off the noise that gets dumped into the ground by the 7660...
I used a simple RC filter on the output and I have reached the floor of my 15 Mhz oscilloscope. The flat screen 1m away is more noisy...
This is a couple of magnitudes less noise than a 555 negative supply.
I can reach this level of low sound with the 555, but the output is too weak to even drive the -Vcc of a 741 for a -2.5 follower...
Even dumping unfiltered 12V into the 7660 only generates marginally more noisy should the headroom be needed.

A big thanks!

Pictures:

Easy to spot the inspiration, the output filtering is minimal as this is to create a negative voltage reference via an Opamp.


And I'm getting these results on a breadboard


Top: 8.88V rail bottom 7660 output.


And with the holy 47 puffs (with the scope set at 5mV per division, it's max)

[cellularmitosis]

A small update to this thread:

I turned this into a PCB design, which can be used as a "daughter board" (it can be mounted on a standoff):

https://github.com/pepaslabs/NegativeRail

The performance seems to match the prototypes.

Shown below is the noise into a 1k load, and the noise with the power disconnected (i.e. the noise floor).  I'm guessing the ~750kHz spikes are coming from the scope itself.

[ignator]

Cellularmitosis:
It looks like your PWB is just 2 sided. So you don't get any benefits from a ground plane, where if the plane has a .004 inch dielectric spacing from traces to gnd/pwr plane, the capacitance of this provides LC filters (the L being the intrinsic inductance of the circuit traces).
Your measured noise is pretty good considering that you are not using a true earthed ground plane in your PWB that is bonded with low inductance to earth. You have lots of impedance between your circuit card and the shielding of the dutch oven. The red/black wire connecting the output to your external scope is a very long inductor and antenna, have you tried a coax for this?

A photo of the bottom side would be nice to see. That standoff plated hole, is it connected to ground on your board? That would be the path of providing a low inductance ground if you solder staked a solderable standoff in that hole. Keep in mind every wire is an inductor. In aircraft wiring, where twisted shielded pairs are implemented, the ground drain wire that is soldered to the shield, and connected to aircraft ground, represents 25 ohms of impedance per inch at 100MHz. And aircraft installers like to make that wire longer then needed. I've seen them a meter long. The ME and flight line mechanics don't understand the RF issues with this drain wire being excessively long. I've always tried to have my installation drawings show this at 4 inches maximum (and it's ignored by the airframe manufacture). As this is where I did my qualification testing at with the product test wire bundle. The shielding is more for preventing EMI upset from external noise. As if your box leaked RF out, it would fail the emissions test. The game was, to do as you are doing, and filter the noise at it's source.
Keep having fun.

[Zero999]

for Circuit D we get a bit more serious with our output filter, by adding a 100uH inductor and a second pair of capacitors.  Additionally, the ferrite bead is moved in between the two capacitor stages.

Schematic:

Be careful with LRC circuits. If the circuit is underdamped (low ESR capacitors and inductors) then it's possible to excite its resonant frequency, causing oscillation. Adding some resistance in series with the capacitors or parallel with the inductor can help damp any resonance.

Try connecting a load to the output of the filter via a transistor and switching the transistor on and off at a few hundred Hz. Is there any ringing on the output of the filter when the load is connected/disconnected?

[schmitt trigger]

Very interesting thread! The idea of using the dutch oven as a Faraday cage is brilliant!
Two thumbs up:
 

Back in the days (late 1970s) where good audio opamps required mandatory dual supplies, I used an ICL7660 to generate the negative voltage from a single 9V battery, which I used in a small portable project.

I did notice the whistling, but as I did not have a scope at the time, I never knew what was going on.

Thanks for sharing, it has brought fond memories.

[ignator]

Quote from: Hero999 on May 20, 2017, 12:27:10 PM

Be careful with LRC circuits. If the circuit is underdamped (low ESR capacitors and inductors) then it's possible to excite its resonant frequency, causing oscillation. Adding some resistance in series with the capacitors or parallel with the inductor can help damp any resonance.

Try connecting a load to the output of the filter via a transistor and switching the transistor on and off at a few hundred Hz. Is there any ringing on the output of the filter when the load is connected/disconnected?

______
Very good advice. I know back in the early 90s, when we were transitioning from 6MHz 80186 to 25MHz 80486 processors, we would use 3 capacitors on each chip that would drive octal or more width bus interface drivers. That indeed was our worry that we would make a tank circuit amongst the parallel capacitors.
Your test method is a neet method to try to induce ringing. The Radiated emission test curve was tough to pass. And harmonics in the communication band had notches in those bands, that made it tougher to qualify the product. They were modules that slid into a card cage, and only 6 pins on the 130 pin backplane connector provided ground to the board.  We were stuck trying to decouple each part to prevent them from radiating.

[VanitarNordic]

The designed low pass filter (LC) seems that it does not cut below the ripple frequency. the L and C values must be changed to make it more effective.

[dudester]

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Guys, is this design or parts of it still a best practice given the new (sort of) ICL7660S with it's "Boost Pin (Pin 1) for Higher Switching Frequency"?

What are the best parts to keep to preserve maximum output voltage/current while keeping any noise at lowest?

[Fronberry]

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Guys, is this design or parts of it still a best practice given the new (sort of) ICL7660S with it's "Boost Pin (Pin 1) for Higher Switching Frequency"?

What are the best parts to keep to preserve maximum output voltage/current while keeping any noise at lowest?

Looking at the ICL7660S datasheet, it suggests that the boost pin provides about 3.5X increased oscillator frequency.  Seems like this would only lead to having lower allowable capacitance values on the LC filters, with the tradeoff of having lower power efficiencies at the lower end of the frequency band.

Also, does anyone have any info on the mechanism of the boost pin upon the RC oscillator?  Based on my understanding of RC oscillators, the output frequency is independent of supply voltage, so hopefully there are some interesting guts going on here.

[GigaJoe]

why don't use 79LXX on output ? ( ... 08 06 05 )

[Zero999]

why don't use 79LXX on output ? ( ... 08 06 05 )

This was investigated by the original poster and does work, but it's better to use a voltage regulator, with a lower quiescent current.