Forums

Apex HV opamp issues.

Started by George Herold March 13, 2015
Hi guys (and dolls),  So if you read in JL's TI opamp thread,
I am having noise issues in our laser diode.
I turns out (I think) that the extra noise is coming 
from my Piezo drive.  
The original circuit used a PA141 from Apex.
(I can't find a schematic on-line, but I have a hard copy.)
Several years ago this HV opamp went away and the 
recommended replacement from Apex was the PA314
https://apexanalog-public.sharepoint.com/Resources/PA341U.pdf

I basically plugged it in, it worked, and I forgot about it.
Well it's not quite the same... a few differences but the big one
is the noise... Some gawd awful 350 uV rms in a 10 k Hz BW...
(~350nV/rt Hz.)  And no spec on the low freq. 1/f noise.  

So I've been trying to beat the noise down in the current circuit,
but with no luck.  I use it as a x10 inverting amp.  Rin= 10 k ohm and
Rf= 100k ohm.  I tried rolling off the gain with 100pf in parallel
with Rf, but this lead to low level oscillations.. (scratch scratch.)  
I then tried adding more compensation capacitance from 10 pF to ~70 pF 
but this did nothing.  

Any ideas for trying to salvage this circuit?
I could live with a bandwidth of 5 kHz or so.  

TIA 
George H.  
On a sunny day (Fri, 13 Mar 2015 10:10:38 -0700 (PDT)) it happened George
Herold <gherold@teachspin.com> wrote in
<ad164713-93a0-48f2-919e-b5f94b53a806@googlegroups.com>:

>Hi guys (and dolls), So if you read in JL's TI opamp thread, >I am having noise issues in our laser diode. >I turns out (I think) that the extra noise is coming >from my Piezo drive. >The original circuit used a PA141 from Apex. >(I can't find a schematic on-line, but I have a hard copy.) >Several years ago this HV opamp went away and the >recommended replacement from Apex was the PA314 >https://apexanalog-public.sharepoint.com/Resources/PA341U.pdf > >I basically plugged it in, it worked, and I forgot about it. >Well it's not quite the same... a few differences but the big one >is the noise... Some gawd awful 350 uV rms in a 10 k Hz BW... >(~350nV/rt Hz.) And no spec on the low freq. 1/f noise. > >So I've been trying to beat the noise down in the current circuit, >but with no luck. I use it as a x10 inverting amp. Rin= 10 k ohm and >Rf= 100k ohm. I tried rolling off the gain with 100pf in parallel >with Rf, but this lead to low level oscillations.. (scratch scratch.) >I then tried adding more compensation capacitance from 10 pF to ~70 pF >but this did nothing. > >Any ideas for trying to salvage this circuit? >I could live with a bandwidth of 5 kHz or so. > >TIA >George H.
Sometimes {tm) ------||---10k ---- | 1n | |----------||------ | | 100p | 10k ----- 100 k----------------- | | --- - | > ----- --- + if its oscillatiing you have some pole somewhere. This has worked in the past (don't ask me) but I don't know your loop bandwidth and whatever is in it. Anyways the extra RC (could be some other place that moves) could help against oscillations. It could work, it could not work, but.. could save time if it does.
On Friday, March 13, 2015 at 1:23:46 PM UTC-4, Jan Panteltje wrote:
> On a sunny day (Fri, 13 Mar 2015 10:10:38 -0700 (PDT)) it happened George > Herold <gherold@teachspin.com> wrote in > <ad164713-93a0-48f2-919e-b5f94b53a806@googlegroups.com>: > > >Hi guys (and dolls), So if you read in JL's TI opamp thread, > >I am having noise issues in our laser diode. > >I turns out (I think) that the extra noise is coming > >from my Piezo drive. > >The original circuit used a PA141 from Apex. > >(I can't find a schematic on-line, but I have a hard copy.) > >Several years ago this HV opamp went away and the > >recommended replacement from Apex was the PA314 > >https://apexanalog-public.sharepoint.com/Resources/PA341U.pdf > > > >I basically plugged it in, it worked, and I forgot about it. > >Well it's not quite the same... a few differences but the big one > >is the noise... Some gawd awful 350 uV rms in a 10 k Hz BW... > >(~350nV/rt Hz.) And no spec on the low freq. 1/f noise. > > > >So I've been trying to beat the noise down in the current circuit, > >but with no luck. I use it as a x10 inverting amp. Rin= 10 k ohm and > >Rf= 100k ohm. I tried rolling off the gain with 100pf in parallel > >with Rf, but this lead to low level oscillations.. (scratch scratch.) > >I then tried adding more compensation capacitance from 10 pF to ~70 pF > >but this did nothing. > > > >Any ideas for trying to salvage this circuit? > >I could live with a bandwidth of 5 kHz or so. > > > >TIA > >George H. > > > Sometimes {tm) > ------||---10k ---- > | 1n | > |----------||------ | > | 100p | > 10k ----- 100 k----------------- > | | > --- - | > > ----- > --- + >
OK, I'll give it a try... I was thinking some resistance in series with the 100pF..(?) (I just ordered some LTC6090's from DK...) I also noticed that the PA341 has been replaced with the PA441.. 30 times less noise!
> if its oscillatiing you have some pole somewhere. > This has worked in the past (don't ask me) > but I don't know your loop bandwidth and whatever is in it. > Anyways the extra RC (could be some other place that moves) could help against oscillations. > > It could work, it could not work, but.. could save time if it does.
On 03/13/2015 01:10 PM, George Herold wrote:
> Hi guys (and dolls), So if you read in JL's TI opamp thread, > I am having noise issues in our laser diode. > I turns out (I think) that the extra noise is coming > from my Piezo drive. > The original circuit used a PA141 from Apex. > (I can't find a schematic on-line, but I have a hard copy.) > Several years ago this HV opamp went away and the > recommended replacement from Apex was the PA314 > https://apexanalog-public.sharepoint.com/Resources/PA341U.pdf > > I basically plugged it in, it worked, and I forgot about it. > Well it's not quite the same... a few differences but the big one > is the noise... Some gawd awful 350 uV rms in a 10 k Hz BW... > (~350nV/rt Hz.) And no spec on the low freq. 1/f noise. > > So I've been trying to beat the noise down in the current circuit, > but with no luck. I use it as a x10 inverting amp. Rin= 10 k ohm and > Rf= 100k ohm. I tried rolling off the gain with 100pf in parallel > with Rf, but this lead to low level oscillations.. (scratch scratch.) > I then tried adding more compensation capacitance from 10 pF to ~70 pF > but this did nothing. > > Any ideas for trying to salvage this circuit? > I could live with a bandwidth of 5 kHz or so. > > TIA > George H. >
How about a resistor in series with the output? Your piezo is probably in the nanofarads. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Fri, 13 Mar 2015 10:49:11 -0700, George Herold wrote:

> On Friday, March 13, 2015 at 1:23:46 PM UTC-4, Jan Panteltje wrote: >> On a sunny day (Fri, 13 Mar 2015 10:10:38 -0700 (PDT)) it happened >> George Herold <gherold@teachspin.com> wrote in >> <ad164713-93a0-48f2-919e-b5f94b53a806@googlegroups.com>: >> >> >Hi guys (and dolls), So if you read in JL's TI opamp thread, >> >I am having noise issues in our laser diode. >> >I turns out (I think) that the extra noise is coming from my Piezo >> >drive. >> >The original circuit used a PA141 from Apex. >> >(I can't find a schematic on-line, but I have a hard copy.) >> >Several years ago this HV opamp went away and the recommended >> >replacement from Apex was the PA314 >> >https://apexanalog-public.sharepoint.com/Resources/PA341U.pdf >> > >> >I basically plugged it in, it worked, and I forgot about it. >> >Well it's not quite the same... a few differences but the big one is >> >the noise... Some gawd awful 350 uV rms in a 10 k Hz BW... (~350nV/rt >> >Hz.) And no spec on the low freq. 1/f noise. >> > >> >So I've been trying to beat the noise down in the current circuit, but >> >with no luck. I use it as a x10 inverting amp. Rin= 10 k ohm and Rf= >> >100k ohm. I tried rolling off the gain with 100pf in parallel with >> >Rf, but this lead to low level oscillations.. (scratch scratch.) I >> >then tried adding more compensation capacitance from 10 pF to ~70 pF >> >but this did nothing. >> > >> >Any ideas for trying to salvage this circuit? >> >I could live with a bandwidth of 5 kHz or so. >> > >> >TIA George H. >> >> >> Sometimes {tm) >> ------||---10k ---- >> | 1n | >> |----------||------ | >> | 100p | >> 10k ----- 100 k----------------- >> | | >> --- - | >> > ----- >> --- + >> >> > OK, I'll give it a try... I was thinking some resistance in series with > the 100pF..(?) > > (I just ordered some LTC6090's from DK...) > > I also noticed that the PA341 has been replaced with the PA441.. 30 > times less noise! >
Maybe they were getting lots of complaints! Is it pin-compatible? -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
> >> > > >> >So I've been trying to beat the noise down in the current circuit, but > >> >with no luck.
try reading this http://www.ti.com/lit/ml/sloa082/sloa082.pdf especially about the different contributions of noise voltage and noise current Mark
On Friday, March 13, 2015 at 2:09:32 PM UTC-4, Phil Hobbs wrote:
> On 03/13/2015 01:10 PM, George Herold wrote: > > Hi guys (and dolls), So if you read in JL's TI opamp thread, > > I am having noise issues in our laser diode. > > I turns out (I think) that the extra noise is coming > > from my Piezo drive. > > The original circuit used a PA141 from Apex. > > (I can't find a schematic on-line, but I have a hard copy.) > > Several years ago this HV opamp went away and the > > recommended replacement from Apex was the PA314 > > https://apexanalog-public.sharepoint.com/Resources/PA341U.pdf > > > > I basically plugged it in, it worked, and I forgot about it. > > Well it's not quite the same... a few differences but the big one > > is the noise... Some gawd awful 350 uV rms in a 10 k Hz BW... > > (~350nV/rt Hz.) And no spec on the low freq. 1/f noise. > > > > So I've been trying to beat the noise down in the current circuit, > > but with no luck. I use it as a x10 inverting amp. Rin= 10 k ohm and > > Rf= 100k ohm. I tried rolling off the gain with 100pf in parallel > > with Rf, but this lead to low level oscillations.. (scratch scratch.) > > I then tried adding more compensation capacitance from 10 pF to ~70 pF > > but this did nothing. > > > > Any ideas for trying to salvage this circuit? > > I could live with a bandwidth of 5 kHz or so. > > > > TIA > > George H. > > > How about a resistor in series with the output? Your piezo is probably > in the nanofarads. >
Yeah... more band-aids. I'm not sure it would help. As is the piezo can only react at ~3kHz, so I've already got a bunch of filtering. I was looking at the noise more closely... turns out it's almost all 1/f noise.. I need DC and well another opamp is looking like my solution. (And then some sort of mini pcb to shoe horn it in the the existing pcb.) George H.
> Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC > Optics, Electro-optics, Photonics, Analog Electronics > > 160 North State Road #203 > Briarcliff Manor NY 10510 > > hobbs at electrooptical dot net > http://electrooptical.net
On Friday, March 13, 2015 at 2:44:51 PM UTC-4, Tim Wescott wrote:
> On Fri, 13 Mar 2015 10:49:11 -0700, George Herold wrote: > > > On Friday, March 13, 2015 at 1:23:46 PM UTC-4, Jan Panteltje wrote: > >> On a sunny day (Fri, 13 Mar 2015 10:10:38 -0700 (PDT)) it happened > >> George Herold <gherold@teachspin.com> wrote in > >> <ad164713-93a0-48f2-919e-b5f94b53a806@googlegroups.com>: > >> > >> >Hi guys (and dolls), So if you read in JL's TI opamp thread, > >> >I am having noise issues in our laser diode. > >> >I turns out (I think) that the extra noise is coming from my Piezo > >> >drive. > >> >The original circuit used a PA141 from Apex. > >> >(I can't find a schematic on-line, but I have a hard copy.) > >> >Several years ago this HV opamp went away and the recommended > >> >replacement from Apex was the PA314 > >> >https://apexanalog-public.sharepoint.com/Resources/PA341U.pdf > >> > > >> >I basically plugged it in, it worked, and I forgot about it. > >> >Well it's not quite the same... a few differences but the big one is > >> >the noise... Some gawd awful 350 uV rms in a 10 k Hz BW... (~350nV/rt > >> >Hz.) And no spec on the low freq. 1/f noise. > >> > > >> >So I've been trying to beat the noise down in the current circuit, but > >> >with no luck. I use it as a x10 inverting amp. Rin= 10 k ohm and Rf= > >> >100k ohm. I tried rolling off the gain with 100pf in parallel with > >> >Rf, but this lead to low level oscillations.. (scratch scratch.) I > >> >then tried adding more compensation capacitance from 10 pF to ~70 pF > >> >but this did nothing. > >> > > >> >Any ideas for trying to salvage this circuit? > >> >I could live with a bandwidth of 5 kHz or so. > >> > > >> >TIA George H. > >> > >> > >> Sometimes {tm) > >> ------||---10k ---- > >> | 1n | > >> |----------||------ | > >> | 100p | > >> 10k ----- 100 k----------------- > >> | | > >> --- - | > >> > ----- > >> --- + > >> > >> > > OK, I'll give it a try... I was thinking some resistance in series with > > the 100pF..(?) > > > > (I just ordered some LTC6090's from DK...) > > > > I also noticed that the PA341 has been replaced with the PA441.. 30 > > times less noise! > > > > Maybe they were getting lots of complaints! Is it pin-compatible?
Yup.. I order one of those too.. but only 1.. ~$50 each! George H.
> > -- > > Tim Wescott > Wescott Design Services > http://www.wescottdesign.com
On Friday, March 13, 2015 at 4:04:28 PM UTC-4, mako...@yahoo.com wrote:
> > >> > > > >> >So I've been trying to beat the noise down in the current circuit, but > > >> >with no luck. > > try reading this > http://www.ti.com/lit/ml/sloa082/sloa082.pdf > > especially about the different contributions of > noise voltage and noise current > > Mark
Thanks Mark, (I've read plenty about noise.) This opamp seems to have a ton of 1/f noise with a 1/f corner freq. that looks to be >10 kHz! George H.
On Fri, 13 Mar 2015 10:10:38 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>Hi guys (and dolls), So if you read in JL's TI opamp thread, >I am having noise issues in our laser diode. >I turns out (I think) that the extra noise is coming >from my Piezo drive. >The original circuit used a PA141 from Apex. >(I can't find a schematic on-line, but I have a hard copy.) >Several years ago this HV opamp went away and the >recommended replacement from Apex was the PA314 >https://apexanalog-public.sharepoint.com/Resources/PA341U.pdf > >I basically plugged it in, it worked, and I forgot about it. >Well it's not quite the same... a few differences but the big one >is the noise... Some gawd awful 350 uV rms in a 10 k Hz BW... >(~350nV/rt Hz.) And no spec on the low freq. 1/f noise. > >So I've been trying to beat the noise down in the current circuit, >but with no luck. I use it as a x10 inverting amp. Rin= 10 k ohm and >Rf= 100k ohm. I tried rolling off the gain with 100pf in parallel >with Rf, but this lead to low level oscillations.. (scratch scratch.) >I then tried adding more compensation capacitance from 10 pF to ~70 pF >but this did nothing. > >Any ideas for trying to salvage this circuit? >I could live with a bandwidth of 5 kHz or so. > >TIA >George H.
Apex used to use an AT&T high-voltage CMOS analog process that was astoundingly noisy, 140 nv/rtHz or something crazy like that. Maybe the PA141 was one of those. There was an appnote on compounding that dog with a good low-voltage opamp. These days, most of their products are regular surfmount parts on PC boards. You could try my cheap and almost-famous optocoupler-based HV amplifier. https://dl.dropboxusercontent.com/u/53724080/Optos/HVamp.JPG -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com