Re: [funwithtransistors] quiet DC amplifier?


Oct 24, 2011

 


----------------------------

#499 Oct 24, 2011

Hi all,

Working on a project that requires sensing 30 mA DC current flowing through a 10 ohm load.�� The quickie answer is "put an ammeter in series".�� Unfortunately I've found that most have quite a bit of series resistance, enough to disturb the circuit.



My thought is to sense the DC across a 0.1 ohm resistor (since I have one ;-), and amplify that enough to drive a meter movement.�� The one I found on the shelf takes about 2.3 volts to make it read full scale.�� So, starting with about 3mV at the sense resistor, I need about 770x gain (about 58 dB).



So, a tiny DC signal to start with, and lots of gain.�� Sorta sounds like a mic preamp?�� I'm thinking some opamps should do nicely.�� What amplifier topology do you think is easiest to implement here?



Regards,

Mark KB9VKE



----------------------------

#500 Oct 25, 2011

Your greatest problem is not going to be noise, you can filter that out with

a long integration time. Offset drift is going to get you with such a low

input voltage. Is there any way you can use a larger sense resistor and

design for the voltage drop? If not you will have to go to some of the

ultra low offset amplifiers made by specialty op amp manufacturers. They

might cost you 5 dollars as opposed to the 50 cents of common dip op amps.



Regards.



Max. K 4 O D S.



Email: max@...



Transistor site www.funwithtransistors.net

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Music site: www.maxsmusicplace.com



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----- Original Message -----

From: msalbert@...>

To: funwithtransistors@yahoogroups.com>

Sent: Monday, October 24, 2011 1:11 PM

Subject: [funwithtransistors] quiet DC amplifier?





> Hi all,

> Working on a project that requires sensing 30 mA DC current flowing

> through

> a 10 ohm load. The quickie answer is "put an ammeter in series".

> Unfortunately I've found that most have quite a bit of series resistance,

> enough to

> disturb the circuit.

>

> My thought is to sense the DC across a 0.1 ohm resistor (since I have one

> ;-), and amplify that enough to drive a meter movement. The one I found

> on

> the shelf takes about 2.3 volts to make it read full scale. So, starting

> with about 3mV at the sense resistor, I need about 770x gain (about 58

> dB).

>

> So, a tiny DC signal to start with, and lots of gain. Sorta sounds like a

> mic preamp? I'm thinking some opamps should do nicely. What amplifier

> topology do you think is easiest to implement here?

>

> Regards,

> Mark KB9VKE



----------------------------

#501 Oct 25, 2011

Thanks for the heads up. How much larger sense resistor would be needed��from a reasonable amplifier perspective?�� I could try 1��ohm and see what happens to the load.�� That would��give��me 30 mV DC.�� My plan is to use TL071��for the first stage since it has offset comp, and more importantly, I have them ;-)

RegardsMark KB9VKE



-----Original Message-----

From: Max Robinson max@...>

To: funwithtransistors funwithtransistors@yahoogroups.com>

Sent: Mon, Oct 24, 2011 11:50 pm

Subject: Re: [funwithtransistors] quiet DC amplifier?

Your greatest problem is not going to be noise, you can filter that out with a long integration time. Offset drift is going to get you with such a low input voltage. Is there any way you can use a larger sense resistor and design for the voltage drop? If not you will have to go to some of the ultra low offset amplifiers made by specialty op amp manufacturers. They might cost you 5 dollars as opposed to the 50 cents of common dip op amps.

Regards.

Max. K 4 O D S.

Email: max@...

Transistor site www.funwithtransistors.netVacuum tube site: www.funwithtubes.netMusic site: www.maxsmusicplace.com

To subscribe to the fun with transistors group send an email to.funwithtransistors-subscribe@yahoogroups.com

To subscribe to the fun with tubes group send an email to,funwithtubes-subscribe@yahoogroups.com

----- Original Message ----- From: msalbert@...>To: funwithtransistors@yahoogroups.com>Sent: Monday, October 24, 2011 1:11 PMSubject: [funwithtransistors] quiet DC amplifier?

> Hi all,> Working on a project that requires sensing 30 mA DC current flowing > through> a 10 ohm load. The quickie answer is "put an ammeter in series".> Unfortunately I've found that most have quite a bit of series resistance, > enough to> disturb the circuit.>> My thought is to sense the DC across a 0.1 ohm resistor (since I have one> ;-), and amplify that enough to drive a meter movement. The one I found > on> the shelf takes about 2.3 volts to make it read full scale. So, starting> with about 3mV at the sense resistor, I need about 770x gain (about 58 > dB).>> So, a tiny DC signal to start with, and lots of gain. Sorta sounds like a> mic preamp? I'm thinking some opamps should do nicely. What amplifier> topology do you think is easiest to implement here?>> Regards,> Mark KB9VKE



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----------------------------

#502 Oct 25, 2011

--- On Tue, 10/25/11, msalbert@... msalbert@...> wrote:

> From: msalbert@... msalbert@...>

> Subject: Re: [funwithtransistors] quiet DC amplifier?

> To: funwithtransistors@yahoogroups.com

> Date: Tuesday, October 25, 2011, 1:00 PM

>

> Thanks for the heads up. How much larger sense resistor would be needed.

> from a reasonable amplifier perspective?. I could try 1.ohm and see what

> happens to the load.. That would.give.me 30 mV DC.. My plan is to use

> TL071.for the first stage since it has offset comp, and more importantly, I

> have them ;-)



The traditional way of measuring low DC voltages/currents is to use a chopper

stabilized amplifier. You may want to do some research on those.



When measuring low DC values, all sorts of weird effects come into play. For

example, the Seebeck voltage produced by the contact between different metals

at non-absolute-zero temperature. When measuring AC values, or high DC values,

the Seebeck voltage can be ignored. But, when you're measuring millivolts (and

especially when you're measuring microvolts), that Seebeck voltage can be a

problem.

> Regards

> Mark KB9VKE



Dave



----------------------------

#503 Oct 26, 2011

Typical offsets are 1 millivolt and offset drifts are, well I don't remember

exactly. The data are not hard to look up.



Regards.



Max. K 4 O D S.



Email: max@...



Transistor site www.funwithtransistors.net

Vacuum tube site: www.funwithtubes.net

Music site: www.maxsmusicplace.com



To subscribe to the fun with transistors group send an email to.

funwithtransistors-subscribe@yahoogroups.com



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----- Original Message -----

From: msalbert@...>

To: funwithtransistors@yahoogroups.com>

Sent: Tuesday, October 25, 2011 12:00 PM

Subject: Re: [funwithtransistors] quiet DC amplifier?







Thanks for the heads up. How much larger sense resistor would be needed from

a reasonable amplifier perspective? I could try 1 ohm and see what happens

to the load. That would give me 30 mV DC. My plan is to use TL071 for the

first stage since it has offset comp, and more importantly, I have them ;-)

Regards

Mark KB9VKE





-----Original Message-----

From: Max Robinson max@...>

To: funwithtransistors funwithtransistors@yahoogroups.com>

Sent: Mon, Oct 24, 2011 11:50 pm

Subject: Re: [funwithtransistors] quiet DC amplifier?





Your greatest problem is not going to be noise, you can filter that out with

long integration time. Offset drift is going to get you with such a low

nput voltage. Is there any way you can use a larger sense resistor and

esign for the voltage drop? If not you will have to go to some of the

ltra low offset amplifiers made by specialty op amp manufacturers. They

ight cost you 5 dollars as opposed to the 50 cents of common dip op amps.

Regards.

Max. K 4 O D S.

Email: max@...

Transistor site www.funwithtransistors.net

acuum tube site: www.funwithtubes.net

usic site: www.maxsmusicplace.com

To subscribe to the fun with transistors group send an email to.

unwithtransistors-subscribe@yahoogroups.com

To subscribe to the fun with tubes group send an email to,

unwithtubes-subscribe@yahoogroups.com

----- Original Message -----

rom: msalbert@...>

o: funwithtransistors@yahoogroups.com>

ent: Monday, October 24, 2011 1:11 PM

ubject: [funwithtransistors] quiet DC amplifier?



Hi all,

Working on a project that requires sensing 30 mA DC current flowing

through

a 10 ohm load. The quickie answer is "put an ammeter in series".

Unfortunately I've found that most have quite a bit of series resistance,

enough to

disturb the circuit.



My thought is to sense the DC across a 0.1 ohm resistor (since I have one

;-), and amplify that enough to drive a meter movement. The one I found

on

the shelf takes about 2.3 volts to make it read full scale. So, starting

with about 3mV at the sense resistor, I need about 770x gain (about 58

dB).



So, a tiny DC signal to start with, and lots of gain. Sorta sounds like a

mic preamp? I'm thinking some opamps should do nicely. What amplifier

topology do you think is easiest to implement here?



Regards,

Mark KB9VKE



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