AD9851 output amplitude

What clock rate to the AD9851? Are you measuring before or after the filter?

I suspect filter problems, or a clock speed much lower than in that article.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

180MHz, multiplied up from 30MHz crystal. I've probed both before and after the filter.

The waveform looks clean from 1KHz to 30MHz. I've probed it both with the 10x and 1x probes, with the expected attenuation using 1x. It just drops in amplitude. I've seen other reports of that, but no discussion.

I checked the 200R and 100R load resistors. The inductors are continuous at DC. I reckon the filter is working as designed.

Anything else I can probe?

Clifford Heath.

On a sunny day (Mon, 5 Jun 2017 14:09:51 +1000) it happened Clifford Heath wrote in :

I just looked up that AD9851 datasheet. You would not happen to be looking at that DAC output? It has a 120k or so Rout, and in that datasheet is used to drive a comparator via a filter to make a digital clock signal. That comparator does not care so much about decreasing amplitude due to RC time at the DAC output. Figure 2

The slightest capacitance on the 20mA (if it is that much) into a 2k2 load, well you can calculate RC yerself I suppose. Good for audio at -3dB.

I would not want that freaking chip for free, there are much better ones there must be.

MAYBE if you did drive that DAC into a zero Ohm virtual input of an inverting opamp, but WTF. Datasheet seems to use a 1:1 transformer to 50 Ohm. Well, tronix must have changed while I was not looking. But 50 Ohm and 20 mA in my old book is .02 * 50 = 1V, and not 3.5 V

Somebody correct my cynism.

WHATSANDUINO?

From the heath-kits?

You expected a current source to have a small Rout?

There's bugger-all capacitance there.

More expensive definitely. But this one has hordes of happy user, so there must be something wrong with how I'm using it.

That's to derive output from both polarities of the DAC for 3dB more power. Unnecessary unless you need it, and they say so.

I think the output is already loaded to the 200 ohm for which the filter is designed, and a 50ohm load is clearly too much. However, what's the load of a 10x o'scope probe? (200R || probe) * 0.02 is something under

4V, so that looks about right.

It's what people use who grew up from using PICs. Or in my case, home-brewing GNU compile chains for MSP430s and 68HC11's. Sue me, I used C++ on them all. And assembler if I felt inclined, in case you're wondering.

I have a very cheap Chinese copy of the datasheet's reference diagram, including a 7-pole filter.

The 3dB down point varies with load impedance. With a

50ohm load my output is ~500mVpp and the 3dB point is a little under 50MHz. At higher resistance, the 3dB point can be significantly lower.

I also noticed that at certain output frequencies(!?) the comparator switching causes a glitch on the DAC's output. Disconnecting the comparator from the DAC's output removes the glitch.

On a sunny day (Mon, 5 Jun 2017 21:33:40 +1000) it happened Clifford Heath wrote in :

I did not expect anyting, do not even have a duino, nevrr used one either AFAIK.

So have bows and arrows. Mostly in reservations though.

No, it is to short any output capacitance.

I dunno what you use for scope, mine is just after Fred Flinstone, but my probes are 1/10 of 30 pF on X10, so say 3 pF. But you were driving 2k2, for 3 pF probe and 2k2 your 3 dB cutoff is already 24 MHz low pass. Now ads peeseebee and wire capacirance, and whatever any filter you use adds.

Na it is just one of those hobby things that has 'shields' for those who do not know how to design hardware. And crappy C+++++ code that is a crime against humanity. There is a large industry pushing it, the shields, and the bloated objectional crompiler, eh conpiler, eh OK.

I still have a 68000 chip somewhere, Never got around to it, my Z80 did everyting that had not already been invented.

I am not in that crazy US

Now wait, maybe I should move there,!! anyways the International Court Of Justice over here may look into C++ one day, at least once can hope so.

No, was not wondering, just curious why you could not calculate an RC time. No function in sayplusplus for it I take it?

Is there gain after the DDS? The output complance range is limited.

We use the AD9850 and a couple of other ADI dds parts, and a couple of ADI dacs in home-brew DDS applications, and we've never seen anything like that kind of rolloff. Usually we get a bit of filter rolloff and the usual sinc thing. You should be maybe a dB down at Fclk/3.

Can you post the actual schematic?

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I think he's trying to avoid a 50-ohm load.

--
 Thanks, 
    - Win

The AVR has nothing to do with the AD9851.

It's not a 2k2 load. 2k2 just programs the current sources fro the DAC.

That's not what the data sheet and app-notes say.

20MHz Hung Chang 2502. You're right to point out that I don't know much about my probes though.

I think you're right, it just doesn't want to drive a load. I'll add a buffer.

I know how to design hardware, but I couldn't put two AVRs and a USB RS232 chip on a PCB for $4 myself. Could you?

I still have a 68020 somewhere too.

Neither am I, and grateful for that ;).

Probably for the same reason you couldn't spot that the 2k2 is not the R in question.

Clifford Heath.

That's the one. Its cut-off is designed wrong, about 75MHz IIRC. That Chebyshev looks much better, and is designed to transform from 200ohm to 50ohm output too. I need to build that.

That must be what's happening. It looks like a buffer is needed after the DAC load resistors, before the filter, though a buffer afterwards would help. Shame that AD didn't show that on their data sheets and eval boards.

It has a trim-pot to set the comparator threshold, which is turned all the way clockwise so the comparator won't fire.

Clifford Heath.

At 1MHz, 20mA, it's not clipping the sine at 3.5V P-P, though it must be getting close (5V supply) and I'm sure the purity would be better at lower current.

Thanks. I suspect my probes, and perhaps the filter components.

No, but the filter is shown on P21 here: E5-E6 are bridged, and R4/R5 not present. R7/R8 are as shown.

Clifford Heath.

Oh I was going to say, I always have issues with my probes, and forgetting what their load is. Sometimes I'll look at a signal with a x10 'scope probe in series with a 1 pf cap. (or just a turn or two of insulated wire)

George H.

On a sunny day (Tue, 6 Jun 2017 07:14:38 +1000) it happened Clifford Heath wrote in :

So why did you blabber about it?

Well it actually does.

Oh help ;-)

Maybe, just to stop the silly shit, publish a circuit diagram of your whatever it is you talk about?

From your text '2k2' and your lack of a circuit diagram, your writing skills are sub-standard ;-) No wonder it does not work 4 u

On a sunny day (Mon, 5 Jun 2017 16:46:23 -0700 (PDT)) it happened George Herold wrote in :

Sometimes I watch teefee, yesterday I watched a recorded movie the das_turbogeile_gummiboot_1984_german.ts Was original US I think, German voice over, very well done. It reminded me of this discussion. :-)

On a sunny day (Tue, 6 Jun 2017 07:19:11 +1000) it happened Clifford Heath wrote in :

It is just that YOU do not understand the datsheet, as I stated in my fists reply the intention is not that point as output, but they use a comparator (in the same chip did you notice!!) to make the varying filter amplitude into two SQUARE WAVE 50 Ohm outputs. Its a freaking CLOCK generator, not a VFO. you got the WRONG SHIELD :-) :-) :-) :-)

loop_the_loop: hehe LOL LMAF

funny funny funny bra loop_the_loop

stack overflow, cosmic ray, FLASH corruption, loop break, Russian hacker succeeded

:-)

LOL

hahahaha

Jan Panteltje wrote on 6/6/2017 3:42 AM:

The only one blabbering about is you. He simply described his setup. Here you complain about him giving info, later you complain he *doesn't* give enough info. Jeeze, what is up with you?

Pot, meet kettle.

--

Rick C

Jan Panteltje wrote on 6/6/2017 4:00 AM:

You are in one strange mood. You also write a *lot* worse than he does. Your sentences often make no sense... if you even use actual sentences.

--

Rick C

On a sunny day (Tue, 6 Jun 2017 04:29:31 -0400) it happened rickman wrote in :

No he did not,. no circyuit diagram,, nothing, just 'shields' and duinos

Maybe even Dinos come to think of it.

You have no sense wire for humor.

On a sunny day (Tue, 6 Jun 2017 04:32:41 -0400) it happened rickman wrote in :

Apart from the sense-wire for humor you also need in depth No Ledge of the subject to make sense of what I write. Else the humor will not even escape you, it won't even ave been be with you, as your typing exorcism shows.