On 05/06/17 23:32, Tom Gardner wrote:> On 05/06/17 05:09, Clifford Heath wrote: >> I'm building a sweeper/scalar analyser using an Arduino >> with touchscreen LCD, AD9851 and AD8307. This is reasonably >> well-trodden ground (except for the touchscreen). So I'm >> surprised to find very few comments abut the AD9851 output >> amplitude dropping off at higher frequencies. >> >> I have it programmed to 20mA-ish output current, using a 2k2 >> resister. I'm using a module that has the same output filter >> and topology as the evaluation board (7th order elliptical) >> even though a Chebyshev is much better: >> <http://www.edn.com/design/test-and-measurement/4441389/Optimizing-Arduino-and-the-AD9851-DDS-signal-generator> >> >> >> >> At 1MHz, I'm seeing 3.5V P-P output, but that steadily falls >> to 300mV at 20MHzand continues falling. >> >> Is this a feature of this kind of DDS/DAC output? >> I can always calibrate it out for the sweeper, but for >> general sig-gen use I'd like a flat output over frequency. >> Without adding a PGA, how can I achieve that? >> >> Arduino code is here: <https://github.com/cjheath/AD9851LCD> >> >> Clifford Heath. > > I have a very cheap Chinese copy of the datasheet's > reference diagram, including a 7-pole filter.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.> 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.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.> 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.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.
AD9851 output amplitude
Started by ●June 5, 2017
Reply by ●June 5, 20172017-06-05
Reply by ●June 5, 20172017-06-05
On 06/06/17 00:01, John Larkin wrote:> On Mon, 5 Jun 2017 14:09:51 +1000, Clifford Heath <no.spam@please.net> > wrote: > >> I'm building a sweeper/scalar analyser using an Arduino >> with touchscreen LCD, AD9851 and AD8307. This is reasonably >> well-trodden ground (except for the touchscreen). So I'm >> surprised to find very few comments abut the AD9851 output >> amplitude dropping off at higher frequencies. >> >> I have it programmed to 20mA-ish output current, using a 2k2 >> resister. I'm using a module that has the same output filter >> and topology as the evaluation board (7th order elliptical) >> even though a Chebyshev is much better: >> <http://www.edn.com/design/test-and-measurement/4441389/Optimizing-Arduino-and-the-AD9851-DDS-signal-generator> >> >> At 1MHz, I'm seeing 3.5V P-P output, but that steadily falls >> to 300mV at 20MHzand continues falling. > > Is there gain after the DDS? The output complance range is limited.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.> 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.Thanks. I suspect my probes, and perhaps the filter components.> Can you post the actual schematic?No, but the filter is shown on P21 here: <datasheet.octopart.com/AD9851BRSZ-Analog-Devices-datasheet-56122.pdf> E5-E6 are bridged, and R4/R5 not present. R7/R8 are as shown. Clifford Heath.
Reply by ●June 5, 20172017-06-05
On Monday, June 5, 2017 at 5:49:26 PM UTC-4, Clifford Heath wrote:> On 05/06/17 23:48, Jan Panteltje wrote: > > On a sunny day (Mon, 5 Jun 2017 21:33:40 +1000) it happened Clifford Heath > > <no.spam@please.net> wrote in <SQbZA.3011$Ip7.2863@fx14.iad>: > >>>> I'm building a sweeper/scalar analyser using an ************ Arduino ****************** > >>>> with touchscreen LCD, AD9851 and AD8307. > >>>> I have it programmed to 20mA-ish output current, using a 2k2 > >>>> resister. I'm using a module that has the same output filter > >>>> and topology as the evaluation board (7th order elliptical) > >>>> At 1MHz, I'm seeing 3.5V P-P output, but that steadily falls > >>>> to 300mV at 20MHzand continues falling. > >>> 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, > >> You expected a current source to have a small Rout? > > I did not expect anyting, do not even have a duino, nevrr used one either AFAIK. > > The AVR has nothing to do with the AD9851. > > >>> The slightest capacitance on the 20mA (if it is that much) into a 2k2 load, > > It's not a 2k2 load. 2k2 just programs the current sources > fro the DAC. > > >>> Datasheet seems to use a 1:1 transformer to 50 Ohm. > >> That's to derive output from both polarities of the DAC > > No, it is to short any output capacitance. > > That's not what the data sheet and app-notes say. > > > 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. > > 20MHz Hung Chang 2502. You're right to point out that I > don't know much about my probes though.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.> > > 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. > > I think you're right, it just doesn't want to drive a load. > I'll add a buffer. > > > Na it is just one of those hobby things that has 'shields' for those who do not know how to design hardware. > > I know how to design hardware, but I couldn't put two AVRs > and a USB RS232 chip on a PCB for $4 myself. Could you? > > > I still have a 68000 chip somewhere, > > I still have a 68020 somewhere too. > > >> Sue me, > > I am not in that crazy US > > Neither am I, and grateful for that ;). > > > No, was not wondering, just curious why you could not calculate an RC time. > > Probably for the same reason you couldn't spot that the 2k2 > is not the R in question. > > Clifford Heath.
Reply by ●June 6, 20172017-06-06
On a sunny day (Tue, 6 Jun 2017 07:14:38 +1000) it happened Clifford Heath <no.spam@please.net> wrote in <wlkZA.60881$zj3.3944@fx15.iad>:>>>> 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, >>> You expected a current source to have a small Rout? >> I did not expect anyting, do not even have a duino, nevrr used one either AFAIK. > >The AVR has nothing to do with the AD9851.So why did you blabber about it?>>>> The slightest capacitance on the 20mA (if it is that much) into a 2k2 load, > >It's not a 2k2 load. 2k2 just programs the current sources >fro the DAC. > >>>> Datasheet seems to use a 1:1 transformer to 50 Ohm. >>> That's to derive output from both polarities of the DAC >> No, it is to short any output capacitance. > >That's not what the data sheet and app-notes say.Well it actually does.>> 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. > >20MHz Hung Chang 2502. You're right to point out that I >don't know much about my probes though. > >> 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. > >I think you're right, it just doesn't want to drive a load. >I'll add a buffer.Oh help ;-) Maybe, just to stop the silly shit, publish a circuit diagram of your whatever it is you talk about?>Probably for the same reason you couldn't spot that the 2k2 >is not the R in question.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>Clifford HeathKitten
Reply by ●June 6, 20172017-06-06
On a sunny day (Mon, 5 Jun 2017 16:46:23 -0700 (PDT)) it happened George Herold <gherold@teachspin.com> wrote in <f252e421-4702-4d21-a69e-fdee368cb9d7@googlegroups.com>:>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.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. :-) https://en.wikipedia.org/wiki/Up_the_Creek_%281984_film%29 https://www.youtube.com/watch?v=K6S3KuZvxxQ
Reply by ●June 6, 20172017-06-06
On a sunny day (Tue, 6 Jun 2017 07:19:11 +1000) it happened Clifford Heath <no.spam@please.net> wrote in <NpkZA.37030$J83.33061@fx39.iad>:>> The 3dB down point varies with load impedance. With a >> 50ohm load my output is ~500mVpp and the 3dB point is >> a little under 50MHz. At higher resistance, the 3dB >> point can be significantly lower. > >That must be what's happening. It looks like a buffer is >needed after the DAC load resistors, before the filter, >though a buffer afterwards would help. Shame that AD >didn't show that on their data sheets and eval boards.It is just that YOU do not understand the datsheet, as I stated in my fists reply the intention is not that point as output, but they use a comparator (in the same chip did you notice!!) to make the varying filter amplitude into two SQUARE WAVE 50 Ohm outputs. Its a freaking CLOCK generator, not a VFO. you got the WRONG SHIELD :-) :-) :-) :-) loop_the_loop: hehe LOL LMAF funny funny funny bra loop_the_loop stack overflow, cosmic ray, FLASH corruption, loop break, Russian hacker succeeded :-) LOL hahahaha
Reply by ●June 6, 20172017-06-06
Jan Panteltje wrote on 6/6/2017 3:42 AM:> On a sunny day (Tue, 6 Jun 2017 07:14:38 +1000) it happened Clifford Heath > <no.spam@please.net> wrote in <wlkZA.60881$zj3.3944@fx15.iad>: > >>>>> 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, >>>> You expected a current source to have a small Rout? >>> I did not expect anyting, do not even have a duino, nevrr used one either AFAIK. >> >> The AVR has nothing to do with the AD9851. > > So why did you blabber about it?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?>>>>> The slightest capacitance on the 20mA (if it is that much) into a 2k2 load, >> >> It's not a 2k2 load. 2k2 just programs the current sources >> fro the DAC. >> >>>>> Datasheet seems to use a 1:1 transformer to 50 Ohm. >>>> That's to derive output from both polarities of the DAC >>> No, it is to short any output capacitance. >> >> That's not what the data sheet and app-notes say. > > Well it actually does. > > > > >>> 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. >> >> 20MHz Hung Chang 2502. You're right to point out that I >> don't know much about my probes though. >> >>> 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. >> >> I think you're right, it just doesn't want to drive a load. >> I'll add a buffer. > > Oh help > ;-) > > > Maybe, just to stop the silly shit, publish a circuit diagram of your whatever it is you talk about? > >> Probably for the same reason you couldn't spot that the 2k2 >> is not the R in question. > > From your text '2k2' and your lack of a circuit diagram, your writing skills are sub-standard ;-) > No wonder it does not work 4 uPot, meet kettle. -- Rick C
Reply by ●June 6, 20172017-06-06
Jan Panteltje wrote on 6/6/2017 4:00 AM:> On a sunny day (Tue, 6 Jun 2017 07:19:11 +1000) it happened Clifford Heath > <no.spam@please.net> wrote in <NpkZA.37030$J83.33061@fx39.iad>: > >>> The 3dB down point varies with load impedance. With a >>> 50ohm load my output is ~500mVpp and the 3dB point is >>> a little under 50MHz. At higher resistance, the 3dB >>> point can be significantly lower. >> >> That must be what's happening. It looks like a buffer is >> needed after the DAC load resistors, before the filter, >> though a buffer afterwards would help. Shame that AD >> didn't show that on their data sheets and eval boards. > > It is just that YOU do not understand the datsheet, as I stated in my fists reply the intention > is not that point as output, but they use a comparator (in the same chip did you notice!!) > to make the varying filter amplitude into two SQUARE WAVE 50 Ohm outputs. > Its a freaking CLOCK generator, not a VFO. > you got the WRONG SHIELD :-) :-) :-) :-) > > loop_the_loop: > hehe > LOL > LMAF > > funny funny funny > bra loop_the_loop > > stack overflow, cosmic ray, FLASH corruption, loop break, Russian hacker succeeded > > > :-) > > LOL > > hahahahaYou are in one strange mood. You also write a *lot* worse than he does. Your sentences often make no sense... if you even use actual sentences. -- Rick C
Reply by ●June 6, 20172017-06-06
On a sunny day (Tue, 6 Jun 2017 04:29:31 -0400) it happened rickman <gnuarm@gmail.com> wrote in <oh5ove$uro$1@dont-email.me>:>Jan Panteltje wrote on 6/6/2017 3:42 AM: >> On a sunny day (Tue, 6 Jun 2017 07:14:38 +1000) it happened Clifford Heath >> <no.spam@please.net> wrote in <wlkZA.60881$zj3.3944@fx15.iad>: >> >>>>>> 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, >>>>> You expected a current source to have a small Rout? >>>> I did not expect anyting, do not even have a duino, nevrr used one either AFAIK. >>> >>> The AVR has nothing to do with the AD9851. >> >> So why did you blabber about it? > >The only one blabbering about is you. He simply described his setup.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.
Reply by ●June 6, 20172017-06-06
On a sunny day (Tue, 6 Jun 2017 04:32:41 -0400) it happened rickman <gnuarm@gmail.com> wrote in <oh5p4j$uro$2@dont-email.me>:>Jan Panteltje wrote on 6/6/2017 4:00 AM: >> On a sunny day (Tue, 6 Jun 2017 07:19:11 +1000) it happened Clifford Heath >> <no.spam@please.net> wrote in <NpkZA.37030$J83.33061@fx39.iad>: >> >>>> 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. >>> >>> 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 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 > >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.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.
