Forums

slow down an opamp (compensation cap.)

Started by George Herold January 19, 2017
Well first off, I'm afraid this is a lazy post, 'cause I should be able to 
find the answer in my opamp books at work tomorrow.  

Anyway for my 60 Hz pick up problem, I've slowed down the opamp
driving the fet by hanging a bigger compensating cap on it.  
There's a pic of the schematic here.  Standard opamp driving a fet
V->I.  (feedback R is 10 k not 1 k shown.)
https://www.dropbox.com/sh/4xg4fu9gqe1phvj/AADFEzpFFAazbTIrtIuOSf94a?dl=0irst
I increased the 100 pF cap to 1000 pF, and that was close,
2.2 nF more made it OK, a time constant of about 100ms.
(maybe I should have measured (thought in) slew rate?)
Next time it would be nice to figure out what sort of value 
I need before hand.  

Driving home tonight, I could only think
of it as some ratio to whatever cap is inside the opamp, and set's the 
slew rate/ GBW.  This particular opamp is the opa134 
GBW = 8MHz, 20 V/us slew rate.. I need some current 
to turn that into a capacitance.  

Or am I thinking about this wrong?  
TIA

George H. 
On Thu, 19 Jan 2017 16:48:29 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>Well first off, I'm afraid this is a lazy post, 'cause I should be able to >find the answer in my opamp books at work tomorrow. > >Anyway for my 60 Hz pick up problem, I've slowed down the opamp >driving the fet by hanging a bigger compensating cap on it. >There's a pic of the schematic here. Standard opamp driving a fet >V->I. (feedback R is 10 k not 1 k shown.) >https://www.dropbox.com/sh/4xg4fu9gqe1phvj/AADFEzpFFAazbTIrtIuOSf94a?dl=0irst >I increased the 100 pF cap to 1000 pF, and that was close, >2.2 nF more made it OK, a time constant of about 100ms. >(maybe I should have measured (thought in) slew rate?) >Next time it would be nice to figure out what sort of value >I need before hand. > >Driving home tonight, I could only think >of it as some ratio to whatever cap is inside the opamp, and set's the >slew rate/ GBW. This particular opamp is the opa134 >GBW = 8MHz, 20 V/us slew rate.. I need some current >to turn that into a capacitance. > >Or am I thinking about this wrong? >TIA > >George H.
Do the math. Write your loop equations. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website. Thinking outside the box... producing elegant solutions.
On Thursday, January 19, 2017 at 8:00:38 PM UTC-5, Jim Thompson wrote:
> On Thu, 19 Jan 2017 16:48:29 -0800 (PST), George Herold > <gherold@teachspin.com> wrote: > > >Well first off, I'm afraid this is a lazy post, 'cause I should be able to > >find the answer in my opamp books at work tomorrow. > > > >Anyway for my 60 Hz pick up problem, I've slowed down the opamp > >driving the fet by hanging a bigger compensating cap on it. > >There's a pic of the schematic here. Standard opamp driving a fet > >V->I. (feedback R is 10 k not 1 k shown.) > >https://www.dropbox.com/sh/4xg4fu9gqe1phvj/AADFEzpFFAazbTIrtIuOSf94a?dl=0irst > >I increased the 100 pF cap to 1000 pF, and that was close, > >2.2 nF more made it OK, a time constant of about 100ms. > >(maybe I should have measured (thought in) slew rate?) > >Next time it would be nice to figure out what sort of value > >I need before hand. > > > >Driving home tonight, I could only think > >of it as some ratio to whatever cap is inside the opamp, and set's the > >slew rate/ GBW. This particular opamp is the opa134 > >GBW = 8MHz, 20 V/us slew rate.. I need some current > >to turn that into a capacitance. > > > >Or am I thinking about this wrong? > >TIA > > > >George H. > > Do the math. Write your loop equations. > > ...Jim Thompson > -- > | James E.Thompson | mens | > | Analog Innovations | et | > | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | > | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | > | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | > | E-mail Icon at http://www.analog-innovations.com | 1962 | > > I'm looking for work... see my website. > > Thinking outside the box... producing elegant solutions.
Grin... OK. GH
On Thursday, 19 January 2017 19:48:33 UTC-5, George Herold  wrote:
> Well first off, I'm afraid this is a lazy post, 'cause I should be able to > find the answer in my opamp books at work tomorrow. > > Anyway for my 60 Hz pick up problem, I've slowed down the opamp > driving the fet by hanging a bigger compensating cap on it. > There's a pic of the schematic here. Standard opamp driving a fet > V->I. (feedback R is 10 k not 1 k shown.) > https://www.dropbox.com/sh/4xg4fu9gqe1phvj/AADFEzpFFAazbTIrtIuOSf94a?dl=0irst > I increased the 100 pF cap to 1000 pF, and that was close, > 2.2 nF more made it OK, a time constant of about 100ms. > (maybe I should have measured (thought in) slew rate?) > Next time it would be nice to figure out what sort of value > I need before hand. > > Driving home tonight, I could only think > of it as some ratio to whatever cap is inside the opamp, and set's the > slew rate/ GBW. This particular opamp is the opa134 > GBW = 8MHz, 20 V/us slew rate.. I need some current > to turn that into a capacitance. > > Or am I thinking about this wrong? > TIA > > George H.
Design an amplifier to avoid AC hum pickup, one has to avoid radiated common mode input turning into a differential input. This means this input impedance must be matched and balanced. Any references to ground must not be inductive loops with all signal connections using matched impedances coupling to stray hum E fields and ground.
On Friday, January 20, 2017 at 11:48:33 AM UTC+11, George Herold wrote:
> Well first off, I'm afraid this is a lazy post, 'cause I should be able to > find the answer in my opamp books at work tomorrow. > > Anyway for my 60 Hz pick up problem, I've slowed down the opamp > driving the fet by hanging a bigger compensating cap on it. > There's a pic of the schematic here. Standard opamp driving a fet > V->I. (feedback R is 10 k not 1 k shown.) > https://www.dropbox.com/sh/4xg4fu9gqe1phvj/AADFEzpFFAazbTIrtIuOSf94a?dl=0irst > I increased the 100 pF cap to 1000 pF, and that was close, > 2.2 nF more made it OK, a time constant of about 100ms. > (maybe I should have measured (thought in) slew rate?) > Next time it would be nice to figure out what sort of value > I need before hand. > > Driving home tonight, I could only think > of it as some ratio to whatever cap is inside the opamp, and set's the > slew rate/ GBW. This particular opamp is the opa134 > GBW = 8MHz, 20 V/us slew rate.. I need some current > to turn that into a capacitance. > > Or am I thinking about this wrong? > TIA
You might be. One the ways that high frequency interference gets into slow op amps is when the input pair spends time outside it's linear region - roughly +/-50mV for a bipolar pair. FETs have less transconductance, and can tolerate an order of magnitude bigger excursions. Slowing down the feedback loop allows lower-frequency interference to do this. -- Bill Sloman, Sydney
On Thu, 19 Jan 2017 16:48:29 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>Well first off, I'm afraid this is a lazy post, 'cause I should be able to >find the answer in my opamp books at work tomorrow. > >Anyway for my 60 Hz pick up problem, I've slowed down the opamp >driving the fet by hanging a bigger compensating cap on it. >There's a pic of the schematic here. Standard opamp driving a fet >V->I. (feedback R is 10 k not 1 k shown.) >https://www.dropbox.com/sh/4xg4fu9gqe1phvj/AADFEzpFFAazbTIrtIuOSf94a?dl=0irst >I increased the 100 pF cap to 1000 pF, and that was close, >2.2 nF more made it OK, a time constant of about 100ms. >(maybe I should have measured (thought in) slew rate?) >Next time it would be nice to figure out what sort of value >I need before hand. > >Driving home tonight, I could only think >of it as some ratio to whatever cap is inside the opamp, and set's the >slew rate/ GBW. This particular opamp is the opa134 >GBW = 8MHz, 20 V/us slew rate.. I need some current >to turn that into a capacitance. > >Or am I thinking about this wrong? >TIA > >George H.
Why not hang a cap across the TIP32? If there's hum there, it gets rectified into a DC error, and you can't fix that downstream. But why run 10 uA? That makes the transistor/diode impedance high, easier to pick up hum. This isn't a Lakeshore diode! -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Thursday, January 19, 2017 at 8:40:27 PM UTC-5, John Larkin wrote:
> On Thu, 19 Jan 2017 16:48:29 -0800 (PST), George Herold > <gherold@teachspin.com> wrote: > > >Well first off, I'm afraid this is a lazy post, 'cause I should be able to > >find the answer in my opamp books at work tomorrow. > > > >Anyway for my 60 Hz pick up problem, I've slowed down the opamp > >driving the fet by hanging a bigger compensating cap on it. > >There's a pic of the schematic here. Standard opamp driving a fet > >V->I. (feedback R is 10 k not 1 k shown.) > >https://www.dropbox.com/sh/4xg4fu9gqe1phvj/AADFEzpFFAazbTIrtIuOSf94a?dl=0irst > >I increased the 100 pF cap to 1000 pF, and that was close, > >2.2 nF more made it OK, a time constant of about 100ms. > >(maybe I should have measured (thought in) slew rate?) > >Next time it would be nice to figure out what sort of value > >I need before hand. > > > >Driving home tonight, I could only think > >of it as some ratio to whatever cap is inside the opamp, and set's the > >slew rate/ GBW. This particular opamp is the opa134 > >GBW = 8MHz, 20 V/us slew rate.. I need some current > >to turn that into a capacitance. > > > >Or am I thinking about this wrong? > >TIA > > > >George H. > > Why not hang a cap across the TIP32? If there's hum there, it gets > rectified into a DC error, and you can't fix that downstream.
Right, I do that... mostly to get rid of the room lights.
> > But why run 10 uA? That makes the transistor/diode impedance high, > easier to pick up hum. This isn't a Lakeshore diode!
Yeah 100 uA is fine, we could add a switch... as long as it's a transistor connected diode you can get the temperature to ~1% by measuring the voltage at decades of current. (1-100 uA is the sweet spot.) George H.
> > > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
On Thursday, January 19, 2017 at 8:05:22 PM UTC-5, Anthony Stewart wrote:
> On Thursday, 19 January 2017 19:48:33 UTC-5, George Herold wrote: > > Well first off, I'm afraid this is a lazy post, 'cause I should be able to > > find the answer in my opamp books at work tomorrow. > > > > Anyway for my 60 Hz pick up problem, I've slowed down the opamp > > driving the fet by hanging a bigger compensating cap on it. > > There's a pic of the schematic here. Standard opamp driving a fet > > V->I. (feedback R is 10 k not 1 k shown.) > > https://www.dropbox.com/sh/4xg4fu9gqe1phvj/AADFEzpFFAazbTIrtIuOSf94a?dl=0irst > > I increased the 100 pF cap to 1000 pF, and that was close, > > 2.2 nF more made it OK, a time constant of about 100ms. > > (maybe I should have measured (thought in) slew rate?) > > Next time it would be nice to figure out what sort of value > > I need before hand. > > > > Driving home tonight, I could only think > > of it as some ratio to whatever cap is inside the opamp, and set's the > > slew rate/ GBW. This particular opamp is the opa134 > > GBW = 8MHz, 20 V/us slew rate.. I need some current > > to turn that into a capacitance. > > > > Or am I thinking about this wrong? > > TIA > > > > George H. > > Design an amplifier to avoid AC hum pickup, one has to avoid radiated common mode input turning into a differential input. This means this input impedance must be matched and balanced. Any references to ground must not be inductive loops with all signal connections using matched impedances coupling to stray hum E fields and ground.
I've got a four foot antenna (wire) leading to a diode with ~2k ohm of impedance. At the moment the pickup problem has very little to do with the amp... (It's an instrument amp, which should have fairly balanced input impedance.) My (perhaps silly) philosophy, is that if I can get it to work and look nice in my lab bench environs then once it goes into a can with a shielded cable it will be fine. George H.
On Thursday, January 19, 2017 at 7:48:33 PM UTC-5, George Herold wrote:
> Well first off, I'm afraid this is a lazy post, 'cause I should be able to > find the answer in my opamp books at work tomorrow. > > Anyway for my 60 Hz pick up problem, I've slowed down the opamp > driving the fet by hanging a bigger compensating cap on it. > There's a pic of the schematic here. Standard opamp driving a fet > V->I. (feedback R is 10 k not 1 k shown.) > https://www.dropbox.com/sh/4xg4fu9gqe1phvj/AADFEzpFFAazbTIrtIuOSf94a?dl=0irst > I increased the 100 pF cap to 1000 pF, and that was close, > 2.2 nF more made it OK, a time constant of about 100ms. > (maybe I should have measured (thought in) slew rate?)
10k x 2.2nF = 22uS, right? You could make it a LOT slower than that!
> Next time it would be nice to figure out what sort of value > I need before hand. > > Driving home tonight, I could only think > of it as some ratio to whatever cap is inside the opamp, and set's the > slew rate/ GBW. This particular opamp is the opa134 > GBW = 8MHz, 20 V/us slew rate.. I need some current > to turn that into a capacitance. > > Or am I thinking about this wrong? > TIA > > George H.
I'm with John--1mA through your sensor "diode" and a capacitor! Slowing the heater driver seems like a good track. If that doesn't help, it's time to hunt for feedback from wiring drops, e.g. from the heater current getting into in your sensor reading. Cheers, James Arthur
On Thursday, January 19, 2017 at 9:30:25 PM UTC-5, George Herold wrote:
> On Thursday, January 19, 2017 at 8:05:22 PM UTC-5, Anthony Stewart wrote: > > On Thursday, 19 January 2017 19:48:33 UTC-5, George Herold wrote: > > > Well first off, I'm afraid this is a lazy post, 'cause I should be able to > > > find the answer in my opamp books at work tomorrow. > > > > > > Anyway for my 60 Hz pick up problem, I've slowed down the opamp > > > driving the fet by hanging a bigger compensating cap on it. > > > There's a pic of the schematic here. Standard opamp driving a fet > > > V->I. (feedback R is 10 k not 1 k shown.) > > > https://www.dropbox.com/sh/4xg4fu9gqe1phvj/AADFEzpFFAazbTIrtIuOSf94a?dl=0irst > > > I increased the 100 pF cap to 1000 pF, and that was close, > > > 2.2 nF more made it OK, a time constant of about 100ms. > > > (maybe I should have measured (thought in) slew rate?) > > > Next time it would be nice to figure out what sort of value > > > I need before hand. > > > > > > Driving home tonight, I could only think > > > of it as some ratio to whatever cap is inside the opamp, and set's the > > > slew rate/ GBW. This particular opamp is the opa134 > > > GBW = 8MHz, 20 V/us slew rate.. I need some current > > > to turn that into a capacitance. > > > > > > Or am I thinking about this wrong? > > > TIA > > > > > > George H. > > > > Design an amplifier to avoid AC hum pickup, one has to avoid radiated common mode input turning into a differential input. This means this input impedance must be matched and balanced. Any references to ground must not be inductive loops with all signal connections using matched impedances coupling to stray hum E fields and ground. > > I've got a four foot antenna (wire) leading to a diode with > ~2k ohm of impedance. At the moment the pickup problem has very little > to do with the amp... (It's an instrument amp, which should have fairly > balanced input impedance.) > My (perhaps silly) philosophy, is that if I can get it to work > and look nice in my lab bench environs then once it goes into a > can with a shielded cable it will be fine. > > George H.