Function gens with floating outputs?

[dadler]

I currently have a DG1032z, and its outputs are grounded to mains earth.

I am considering getting another generator that has floating outputs. I have found the following:

- Siglent SDG5162
- Keysight 33500b series (pretty expensive)
- Older 33220a/33210a (poor specs or overpriced?)
- Various Tek models that look straight from the 1980s and have pretty poor specs

Are you guys aware of any others that have floating outputs?

Sometimes when I am using my generator, I find myself needing to get clever whenever I want to use my oscilloscope and/or something else that is mains earthed and not fry something.

For example, I was recently attempting to reverse engineer a Subaru fuel controller. It wants to see a PWM input and pulses the low side of the fuel pump to ground (it's internally an n-channel power mosfet and some intelligent diagnostic circuitry). It seems to be a hybrid pcb/IC, conformally and jelly-goop coated, and I can't really get to any of the internal circuitry without damage.

Since the earthed outputs of my function gen forced me to ground reference the circuit, I had to use a differential probe to do a meaningful evaluation of the device.

Floating FG outputs would also let me more easily add an AC offset to a DC supply, when I have the DC supply earth referenced to avoid common mode/mains noise (or want to float the entire circuit).

Are generators with floating outputs useful in practice, or are there gotchas/reasons many of the manufacturers seem to ground FG outputs?

Thanks in advance.

[Muxr]

My SDG1025 isn't floating, but my cheap Chinese MHS-3225 sig gen is floating.

http://www.ebay.com/itm/181768967145?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

[JoeB83]

I love my 33521A. Expensive, yes, but keep an eye out on Keysights eBay page or other trusted second-hand sources. I also have a 33210A, I like it, but the 33521 is much more versatile.

[dadler]

I love my 33521A. Expensive, yes, but keep an eye out on Keysights eBay page or other trusted second-hand sources. I also have a 33210A, I like it, but the 33521 is much more versatile.

Thanks. I am considering the 33521b or 33522b that are on the used Keysight store. The only thing putting me off is the 30 day warranty. Oh, and either of these would certainly be my most expensive piece of test equipment, which bothers me a little... I do like the specs of the Keysights, though.

[commie]

I currently have a DG1032z, and its outputs are grounded to mains earth.

Why not just disconnect the earth lead in your mains plug?, and stop looking for an excuse to spend your cash.

[dadler]

I currently have a DG1032z, and its outputs are grounded to mains earth.

Why not just disconnect the earth lead in your mains plug?, and stop looking for an excuse to spend your cash.

Not a big fan of floating mains equipment, especially when the equipment was engineered with a expectation of mains earth grounding...

I'm not really happy with the DS1032Z for a few reasons anyways, so I may end up selling it either way.

But thanks for the bizarrely-condescending-without-context tone?

[Jeroen3]

Why not just disconnect the earth lead in your mains plug?

Because floating a device not meant to be floating often introduces up to several mA of leakage current.

[SeanB]

Buy a few small 1:1 transformers, for each frequency range, if a sine wave is what you need, or simply use a digital optocoupler if you need a square wave and use a small isolated supply to drive it.

There might be a market for somebody to make an add on for a FG that has the requisite coupling and power supplies built in to a single smallish case.  You probably can get away with just having an input that is TTL compatible, and an output that can drive either a 50R load or possibly something higher power at a variable voltage that ranges say from 1V to 15V. The transformer would need a few units to have the required frequency range, starting with a regular silicon iron core at the low end and going to some ferrite pot cores at the top end.

[dadler]

Buy a few small 1:1 transformers, for each frequency range, if a sine wave is what you need, or simply use a digital optocoupler if you need a square wave and use a small isolated supply to drive it.

There might be a market for somebody to make an add on for a FG that has the requisite coupling and power supplies built in to a single smallish case.  You probably can get away with just having an input that is TTL compatible, and an output that can drive either a 50R load or possibly something higher power at a variable voltage that ranges say from 1V to 15V. The transformer would need a few units to have the required frequency range, starting with a regular silicon iron core at the low end and going to some ferrite pot cores at the top end.

These are interesting ideas and might make for a fun project. Ensuring the frequency response of the output matched (with a reasonable margin) the generator output could be difficult, but for specific cases as you have mentioned (such as TTL/square wave), it should work. In this particular case, I was only providing a 18.5kHz PWM signal, so it probably would have worked well with this approach.

[Sbampato12]

I've bought a 33522A, never used, but second handed (it got caught by the customs). I've got it for 40% of a new, but without warranties and a greenish display.
The display issue was just a bad flat cable, and the trouble to reassembly everything back again.

A year ago, and no sign of any problem yet. They are superb. I worked with one on my job, and actually we are about to buy another [but only one channel].
They are pretty expensive, but none other problem...

[commie]

Not a big fan of floating mains equipment, especially when the equipment was engineered with a expectation of mains earth grounding...

Okay, when you get ground loop problems remove the earth, replace when finished.Make up a mains cable specific for the job.

[HighVoltage]

I have both, the Agilent 33500B (dual channel) and the older 33220A (single channel) versions and they are really great instruments and no problems at all. You can find them on the used market for reasonable prices. If you are looking for the 33220A, make sure it has the sync input and output board installed, since it is expensive to add later.

[tautech]

I am considering getting another generator that has floating outputs. I have found the following:

- Siglent SDG5162

Are you guys aware of any others that have floating outputs?

Since the earthed outputs of my function gen forced me to ground reference the circuit, I had to use a differential probe to do a meaningful evaluation of the device.

Floating FG outputs would also let me more easily add an AC offset to a DC supply, when I have the DC supply earth referenced to avoid common mode/mains noise (or want to float the entire circuit).

Well I've learnt this today from the forum, thanks.
(doesn't even know his own products.  )

From the SDG5000 datasheet:
Isolation:
Connector shells for channel output(s),Sync,and Mod In are
connected together but isolated from the instrument’s
chassis,Maximum allowable voltage on isolated connector
shells is ±42Vpk

Seems they can supply up to 200 mA on the outputs too.

Sold one recently to a discerning buyer and now I know just why. 

Checked a SDG1000 series for you and all BNCs are common with Mains Earth.

[w2aew]

The Tektronix AFG2000 and AFG3000 series have floating outputs.  (The AFG1000 does not).

[dadler]

Not a big fan of floating mains equipment, especially when the equipment was engineered with a expectation of mains earth grounding...

Okay, when you get ground loop problems remove the earth, replace when finished.Make up a mains cable specific for the job.

The issue is more complicated than this. There are very good safety reasons for not floating equipment, and the internal circuitry may expect that ground *really is* ground. I want the internal power supply/filtering to be grounded, but the outputs floating.

[dadler]

I am considering getting another generator that has floating outputs. I have found the following:

- Siglent SDG5162

Are you guys aware of any others that have floating outputs?

Since the earthed outputs of my function gen forced me to ground reference the circuit, I had to use a differential probe to do a meaningful evaluation of the device.

Floating FG outputs would also let me more easily add an AC offset to a DC supply, when I have the DC supply earth referenced to avoid common mode/mains noise (or want to float the entire circuit).

Well I've learnt this today from the forum, thanks.
(doesn't even know his own products.  )

From the SDG5000 datasheet:
Isolation:
Connector shells for channel output(s),Sync,and Mod In are
connected together but isolated from the instrument’s
chassis,Maximum allowable voltage on isolated connector
shells is ±42Vpk

Seems they can supply up to 200 mA on the outputs too.

Sold one recently to a discerning buyer and now I know just why. 

Checked a SDG1000 series for you and all BNCs are common with Mains Earth.

Yeah, thanks! I just noticed the SDG5000 can do up to 20vpp (high impedance), or 10v into 50 ohms. I don't think I've ever been able to get this high on my Rigol, but perhaps only certain frequencies can go this high. I guess this follows from ohm's law. 10v peak across 50 ohms = 200ma?

The main question I am asking in general is, how many people find the lack of floating inputs on a function gen to be a significant disadvantage?

The higher end generators do seem to have floating outputs (every HP/Aglient/Keysight ever, High-end Teks, Keithley, flagship Siglent). However, most people (at least on this forum) seem to use generators that do not have floating outputs. All of the Rigols have earthed outputs.

I have run into a few, and can think of many cases, where earthed outputs are an issue/disadvantage. Sometimes, these are related to the fact that scopes are earth referenced as well, which can be ameliorated with a diff probe. But some other situations *cannot* be addressed this way, hence the question.

[nctnico]

Not a big fan of floating mains equipment, especially when the equipment was engineered with a expectation of mains earth grounding...

Okay, when you get ground loop problems remove the earth, replace when finished.Make up a mains cable specific for the job.

This is a very bad idea. Besides the safety issues equipment which is supposed to be grounded usually has a filter with 2 caps to ground. Remove the ground and you'll have half the mains voltage at the output and case. It's not enough to kill you but more than enough to destroy a chip.

[ElectroIrradiator]

Are you interested in suggestions for old gear? My HP 3325B from the early stone age has a floating output, and I am quite happy with it. Specs seem fine for what I use it for, though the square waves aren't terribly so once you get up near their frequency limit. Fan is fairly noisy, and it weighs a ton and takes up even more space.

[dom0]

I think there are cheaper options than the 3324/3325 which still catch quite a price used.

[ElectroIrradiator]

I think there are cheaper options than the 3324/3325 which still catch quite a price used.

OK. I have had mine for a number of years, and must admit I don't even know how much they go for these days.

[dadler]

I love my 33521A. Expensive, yes, but keep an eye out on Keysights eBay page or other trusted second-hand sources. I also have a 33210A, I like it, but the 33521 is much more versatile.

Well I went back and forth with the Keysight guy on the Ebay page.

They wanted ~$2436 for the 33522b (dual channel, 30mhz, arb) with 16MB memory, IQ player, and security features. Looked brand new, calibrated a couple of months ago. Still had the plastic on the screen.

They refused to go lower than $2300. He sent a final offer (after the previous offer of $2300) with a message "This is the lowest I can go. Oliver"

My wife is going to kill me

I guess I will have a DG1032Z up for sale soon.

Edit: They have already put up another auction of the same model with same specs and same price:

http://www.ebay.com/itm/Keysight-Used-33522B-Waveform-Generator-30-MHz-2-Ch-Arb-Agilent-33522B-/181804433463?pt=LH_DefaultDomain_0&hash=item2a54638037

The absolute lowest they will take is $2300. Which is still a good deal. The IQ player option is cool, not sure I will ever *need* it but it is more than just an IQ player. It's basically general dual channel arb.

[plesa]

It is really good generator, so there is no reason to kill you ;-) I have two 33522B and I will not change them.
I do not think that plastic on display is the original, it does not looks like that, it is more to protect during transport to new customer since inspection.
I will only upgrade it to HS OCXO ( external Rb OCXO is cheapest solution).
Do not forget to upgrade to latest FW 3.03 after you receive unit.

[dadler]

It is really good generator, so there is no reason to kill you ;-) I have two 33522B and I will not change them.

My clairvoyance ....

"You spent how much on an auction for an item shipped from Malaysia, with only a 30 day warranty?"

"How is this beige box different from all of the other similar looking beige boxes on your shelf?"

("But this one is better than the one I have...")

"Wait, you already have one of these? This is your second one???"

("Yes. This one has floating outputs and a 16bit DAC...It's got TrueForm technology!")

"And how will this make our lives better?"

("Umm. More bits? Can float? ..... Err I love you, beautiful! Want to go out to a nice dinner?")

[dadler]

It is really good generator, so there is no reason to kill you ;-) I have two 33522B and I will not change them.
I do not think that plastic on display is the original, it does not looks like that, it is more to protect during transport to new customer since inspection.
I will only upgrade it to HS OCXO ( external Rb OCXO is cheapest solution)

The single channel model they still have for sale has the factory internal OCXO option. I  almost went for that one, but wanted dual channel so I could sell my Rigol.

I do have a calibrated FEI 5650A (I am currently in the process of putting it in a nice heatsinked enclosure with some support circuitry), and I have an OCXO in my 53132A (bought from Poland on eBay). My old Philips counter also has an OCXO. And... I have a Lucent RFTGm-II-XO GPSDO that has an internal OCXO, although I can't seem to get very good GPS signal here. Might need to put an antenna on a pole outside my lab window.

Ever since I joined this forum I think I spend more time playing with my equipment than designing things, but I have been better lately and have been actually working on my projects. EEVBlog, you ruin me.

[dadler]

Well I've learnt this today from the forum, thanks.
(doesn't even know his own products.  )

From the SDG5000 datasheet:
Isolation:
Connector shells for channel output(s),Sync,and Mod In are
connected together but isolated from the instrument’s
chassis,Maximum allowable voltage on isolated connector
shells is ±42Vpk

Seems they can supply up to 200 mA on the outputs too.

Sold one recently to a discerning buyer and now I know just why. 

Checked a SDG1000 series for you and all BNCs are common with Mains Earth.

Semi-Off-topic:

I was seriously considering the Siglent, since it has good specs, floating outputs, and the build quality seems to have improved since Dave's review (Shahriar reviewed the Siglent again this past fall).

However, in the end I was troubled by the lack of (1) LAN, (2) True GPIB (which I do use myself occasionally), and (3) low arb memory. For a flagship product, I would think they would have at least included LAN and more arb memory. Oh well! I do like how the Siglents have actual glass in front of the screen-nice touch.