biasing this op amp

[AFex54]

You can check that with a DVM.

Chris

There actually isnt a problem after all,
R2, the resistor on R1s output, reduces the voltage range to 0-7.5V anyway,
so all I need to do is sample the voltage after this resistor.

 

[CDRIVE]

There actually isnt a problem after all,
R2, the resistor on R1s output, reduces the voltage range to 0-7.5V anyway,
so all I need to do is sample the voltage after this resistor.

If (by this statement) you're intending to inject your S&H voltage on Pin1 of R2 be aware of the following:

The input voltages to U1C - Pin9 (Inverting Input Pin) is DC coupled to R1, R2, R3, R11, R13, R14, R20 & R21. Injecting your S&H voltage directly to Pin1 of R2 will undoubtedly disrupt the balance of this network.

Chris

 

[AFex54]

If (by this statement) you're intending to inject your S&H voltage on Pin1 of R2 be aware of the following:

The input voltages to U1C - Pin9 (Inverting Input Pin) is DC coupled to R1, R2, R3, R11, R13, R14, R20 & R21. Injecting your S&H voltage directly to Pin1 of R2 will undoubtedly disrupt the balance of this network.

Chris

the sampled voltage is taken from pin1 of R2 though, does this make a difference?
I have no idea what DC coupling is so sorry if it makes no difference to what you just said

 

[CDRIVE]

the sampled voltage is taken from pin1 of R2 though, does this make a difference?
I have no idea what DC coupling is so sorry if it makes no difference to what you just said

All amplifiers have what's understood as quiescent state. This is the state where no external signal (AC or DC) is applied to the input of the amplifier. This state dictates what the amplifier's quiescent (no input signal) output voltage will be. Since there are no coupling capacitors employed on the input of the U1-C stage (DC Coupling) your amplifier's quiescent output voltage U1-C Pin8 is governed by all the aforementioned resistors.

Others may have a different opinion about this but I don't think it's wise to bypass R2. Doing so will Inject your S&H voltage directly to the input pin of U1-C.

R2, R14 & R21 are used as isolation resistors to enable VCO, LFO & ARG inputs to share the input to U1-C Pin9 without any of them unduly effecting each other and the operation of that stage. Bypassing R2 disrupts that happy arrangement and the quiescent voltage intended by the designer.

Chris

 

[AFex54]

All amplifiers have what's understood as quiescent state. This is the state where no external signal (AC or DC) is applied to the input of the amplifier. This state dictates what the amplifier's quiescent (no input signal) output voltage will be. Since there are no coupling capacitors employed on the input of the U1-C stage (DC Coupling) your amplifier's quiescent output voltage U1-C Pin8 is governed by all the aforementioned resistors.

Others may have a different opinion about this but I don't think it's wise to bypass R2. Doing so will Inject your S&H voltage directly to the input pin of U1-C.

R2, R14 & R21 are used as isolation resistors to enable VCO, LFO & ARG inputs to share the input to U1-C Pin9 without any of them unduly effecting each other and the operation of that stage. Bypassing R2 disrupts that happy arrangement and the quiescent voltage intended by the designer.

Chris

my last reply was full of misinformation,
but sorry for the long delay, I was in germany on holidays.
having weird issue .
my rail to rail op amp, regardless of the voltage range ,e.g 0-6V, I apply from the potentiometer voltage divider to the non-inverting input (in the first stage of the sample and hold in OP^) Im reading 0V to an unstable 9V on my multimeter from the output ?? what is up with that?

 

[CDRIVE]

"
sorry ,bit of delay, I was on holiday in germany!
I ordered rail to rail op amps and polystyrene caps, total rip off, but they do work perfectly ,
I noticed that the R2 has a negligible effect on the voltage anyway, seemingly less than when I tested first a couple weeks ago.
what Im gonna do is add a second resistor (probably 75k aswell) at the output of R1 and into my S&H and send my S&H into R2.
so no problems hopefully.
something I'm wondering is does the value of these isolation resistors (R2, R14 & R21) matter? if they were all 75k would they still isolate properly? My synth is semi-modular, so these inputs are instead all identical jacks so my preference would be for them to behave identically rather .
"
If it weren't for this snippet pulled from my mailbox I would have posted an answer to something that doesn't exist any longer. For this reason it's best to not delete ANY of your posts. Instead place an EDIT on something you posted. That's what I do even if I've goofed. I'd rather admit it than create a discontinuity in a thread.

Chris

 

[AFex54]

"
sorry ,bit of delay, I was on holiday in germany!
I ordered rail to rail op amps and polystyrene caps, total rip off, but they do work perfectly ,
I noticed that the R2 has a negligible effect on the voltage anyway, seemingly less than when I tested first a couple weeks ago.
what Im gonna do is add a second resistor (probably 75k aswell) at the output of R1 and into my S&H and send my S&H into R2.
so no problems hopefully.
something I'm wondering is does the value of these isolation resistors (R2, R14 & R21) matter? if they were all 75k would they still isolate properly? My synth is semi-modular, so these inputs are instead all identical jacks so my preference would be for them to behave identically rather .
"
If it weren't for this snippet pulled from my mailbox I would have posted an answer to something that doesn't exist any longer. For this reason it's best to not delete ANY of your posts. Instead place an EDIT on something you posted. That's what I do even if I've goofed. I'd rather admit it than create a discontinuity in a thread.

Chris

woops ha,
another lesson learned for again
Im such a noob...

 

[CDRIVE]

my last reply was full of misinformation,
but sorry for the long delay, I was in germany on holidays.
having weird issue .
my rail to rail op amp, regardless of the voltage range ,e.g 0-6V, I apply from the potentiometer voltage divider to the non-inverting input (in the first stage of the sample and hold in OP^) Im reading 0V to an unstable 9V on my multimeter from the output ?? what is up with that?

Post the schematic.

Chris

 

[AFex54]

Post the schematic.

Chris

right give me a few mins to draw it up

 

[AFex54]

Post the schematic.

Chris

 

[CDRIVE]

You may have killed it!
The TLC2264 Data Sheet specs Vdd(Absolute Max) = +8V & Vss(Absolute Max) = -8V.

Chris

 

[AFex54]

You may have killed it!
The TLC2264 Data Sheet specs Vdd(Absolute Max) = +8V & Vss(Absolute Max) = -8V.

Chris

noooo, so much money wasted on these rail to rails too!
and +/- 4.5V would only work from an actual split supply?

 

[CDRIVE]

noooo, so much money wasted on these rail to rails too!
and +/- 4.5V would only work from an actual split supply?

Many moons ago in another life I worked under the chief engineer as his engineering tech. Joe had a sign on the wall that read "Electronics is an exact science". One day after a lot of things going wrong and much hair pulling, I changed it to read "Electronics is an exact sinus!"

Chris

 

[CDRIVE]

Oops, I forgot to answer your question..
Not all OpAmps work well from single supplies. OpAmps that do are fairly specific about it and boast this feature in the "Operating Features" at the top of the data sheets.

Chris

Edit: A common LM324 works well on single end supplies. Have you considered a tiny boost converter to jack up its Vcc? This way you wouldn't need rail to rail capability.

 

[AFex54]

Many moons ago in another life I worked under the chief engineer as his engineering tech. Joe had a sign on the wall that read "Electronics is an exact science". One day after a lot of things going wrong and much hair pulling, I changed it to read "Electronics is an exact sinus!"

Chris

I was actually reading the datasheet of the TLC2274 initally but of course ended up buying the 2264,
talk about careless.
well at least I can buy the 2274 in singles

 

[CDRIVE]

I was actually reading the datasheet of the TLC2274 initally but of course ended up buying the 2264,
talk about careless.
well at least I can buy the 2274 in singles

The TLC2274 has the same max Vdd of +8V.
Take a look at the TS942. It's single supply with a Vdd max of +12V.

Chris

 

[AFex54]

The TLC2274 has the same max Vdd of +8V.
Take a look at the TS942. It's single supply with a Vdd max of +12V.

Chris

this will be my fourth attempt at getting the right op amp,
I now have TLC2264s, 2274s and LM324s,
0-8V with the 324 would have been fine but there was another problem ...input bias current.
now I can get the TS942 is on Mouser but shipping is €20.
tthe only remotely suitable one on RS is this:
http://ie.rs-online.com/web/p/operational-amplifiers/5343582/
my only worry is 18V... how much headroom does this leave?
because my power supply reads around 9.3V.

 

[CDRIVE]

this will be my fourth attempt at getting the right op amp,
the only remotely suitable one on RS is this:
http://ie.rs-online.com/web/p/operational-amplifiers/5343582/
my only worry is 18V... how much headroom does this leave?
because my power supply reads around 9.3V.

I don't understand your issue there? The DataSheet specs: "Wide supply range 1.8V to 24V"

Chris

 

[AFex54]

I don't understand your issue there? The DataSheet specs: "Wide supply range 1.8V to 24V"

Chris

yeah sorry, my issue is that I pay far too much attention to the specs on RS instead of checking the actual data sheets.
thanks for the help, and I hope this is the last time I post about this in this thread

 

[CDRIVE]

I feel for you! What you pay for shipping is OBSCENE! As for hoping this will be the last time you post in this thread.. I hope not. We like to see how things work out.

Chris