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'Op Amp recommendatiuon sought'
2000\01\18@013441 by Russell McMahon

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I'm looking for an opamp that is "somewhat better" than the standard LM324
but still as cheap as possible.

The application is a very simple one with a "real" opamp. Using a 324 it is
a little challenging.

The LM324 is very cheap - 4 opamps for about $US0.20 or so in volume but it
has some very nasty problems with linearity when the output transits between
its class A and class B modes. There are many duals and quads but few come
close to the 324 in price - someone may have a favourite recommendation. The
circuit is desirably a single supply one but if a cheap enough dual supply
opamp was available I could consider generating a negative rail.

Application:     Monitor current sense resistor in source of a FET (or IGBT)
and translate the current into a voltage of about 0 to 4 volts.
Current is up to about 5 amps, so to avoid excessive power dissipation
(which is not totally critical) the resistor should be a maximum of about
say 0.01 to 0.1 ohms. (0.1r =  2.5 watt dissipation at peak power). Voltage
then = 0.5 volt.
Therefore a gain of about 4/0.5   = 8 x is required. Not a very demanding
application. There is some AC noise on the current so a filter is required
to produce a smooth DC mean - this could be a 1 pole prefilter or,
preferably, the gain stage would be an active filter.
The current is ground referenced and the output is ground referenced.
The output of the amplifier drives a resistor of a few hunded ohms which has
a variable DC voltage and a significant high frequency AC component. This
point MUST see the amplifier output as a very low impedance point.

Using a 324 horrendous things happen. The cross over with increasing voltage
is "interesting". Driving a load which has its "other" end above the 324
output leads to unexpected behaviour.

SO -

Any favourite amplifiers with the following specs.

- MUST be cheap. MUST be.
- 5 volt single supply operation preferred but not essential.
- Dual or quad preferred (single OK if price appropriate)
- Output must drive to ground (but not +ve rail)
- Input common mode needs to include ground.(but not +ve rail)
- Did I say that it must be cheap?
- Must behave "normally" when driven in a "normal" manner (unlike a 324 :-))
- Essentially only a DC amplifier but must provide low impedance sink to 100
KHz odd signal on load.

AMP

   A              270r say
---AA
   AAA------RRRRR---- DC + 100 KHz
---AA
   A


TIA



     Russell McMahon
_____________________________

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Help the hungry at no cost to yourself!
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(* - or woman, child or internet enabled intelligent entity :-))

2000\01\18@023958 by Dr. Imre Bartfai

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Hi,
try MCP604 ! Far better, below $1, made by Microchip! Rail-to-rail, offset
< 2 mV, etc.

Regards,
Imre


On Tue, 18 Jan 2000, Russell McMahon wrote:

{Quote hidden}

2000\01\18@080025 by Smith, Clay

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Just look for the description "instrumentation amp"
These with be hi gain, hi cmrr, low bias current and low offset voltage.
The old standby is OP07
$.80 from digikey or mouser
both Analog devices and linear devices will send you books on in-amps FREE

CSS

               {Original Message removed}

2000\01\18@093855 by wwl

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On Tue, 18 Jan 2000 07:54:14 -0500, you wrote:

>Just look for the description "instrumentation amp"
>These with be hi gain, hi cmrr, low bias current and low offset voltage.
>The old standby is OP07
>$.80 from digikey or mouser
>both Analog devices and linear devices will send you books on in-amps FREE
No - an instrumantation amp is a different animal - typically a
programmable-gain differential amplifier with buffered inputs and an
output reference connection.  These are never cheap.
You need a 'precision' opamp. op-07 is probably cheapest but may not
be good at a 5V supply.
The main parameter of interest is the input offset voltage, which
effectively determines the accuracy. This needs to be less than your
required input measurement resolution.
Nat Semi and Texas do zillions of opamps, and I'm sure some will be
suitable. SGS-Thomson may also be worth a look - I don't know if they
do any precision parts but I've been pleasantly surprised by pricing
on some of their opamp parts in the past.  
Also check out Analog devices, Burr-Brown, Linear Tech and Maxim,
although all these guys tend to be in the higher precision, higher
cost area.
At the higher-end you don't tend to get multiple devices - there's a
dual op-07 but that's about it. It's obvious why if you think about it
- they need to be selected for input offset, so if you need 2 good
amps on one die, the yield is going to be significantly less, and the
chances of getting 4 good ones are even lower!
If you only need one _precision_ amp and some vanilla ones, it's
usually cheaper to use a single plus a multiple cheaper package for
the others  


>CSS
>
>                {Original Message removed}

2000\01\18@095427 by Terry A. Steen

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Just FYI, the dual OP-07 is costly... It is cheaper to put aside board
space for two OP-07s.


At 02:38 PM 1/18/2000 GMT, Mike Harrison wrote:
{Quote hidden}

>>                {Original Message removed}

2000\01\18@112928 by Dennis Gearon

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First of all, National Semiconductor has the best online selection pages
BAR NONE for their components! Absolutely what you'd design yourself as
a customer.

The link below is for a reverse sorted by offset table of their opamps.
SORRY, nothing under 1.59 for quads.

Other companies to try are motorola, linear technology, and maxim.

http://www.national.com/parametric/0,1850,703-c13-1,00.html

--
________________________________________________________________
Dennis K. Gearon (Kegley)
Scientific Instrument Technician, School of EIT
Oregon Institute of Technology - One of USA's 100 Best College Buys
3201 Campus Drive
Klamath Falls, OR 97601
Voice   1-541-885-1563
FAX     1-541-885-1689
email   spam_OUTgearondTakeThisOuTspamoit.edu
I am here only to be truly helpful
________________________________________________________________

2000\01\18@114418 by Dennis Gearon

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The first one below lists several competitors versions: <heh heh>

products.analog.com/products/info.asp?product=OP400
http://products.analog.com/products/info.asp?product=OP421

--
________________________________________________________________
Dennis K. Gearon (Kegley)
Scientific Instrument Technician, School of EIT
Oregon Institute of Technology - One of USA's 100 Best College Buys
3201 Campus Drive
Klamath Falls, OR 97601
Voice   1-541-885-1563
FAX     1-541-885-1689
email   .....gearondKILLspamspam@spam@oit.edu
I am here only to be truly helpful
________________________________________________________________

2000\01\18@161802 by p.cousens

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Russell McMahon wrote:
>
> I'm looking for an opamp that is "somewhat better" than the standard LM324
> but still as cheap as possible.

Snip

> The current is ground referenced and the output is ground referenced.

Are any of you inputs at or close to 0 Volt opamp supply ?
If they are less than a volt or so above ground you will have problems
with most opamps

If you are approching ground you definitly will need an opamp
with inputs that include ground.
I have used CA3130 in this sort of apps without problems
You could also try CA3140 or the Dual CA3240 but I have not used
them in this way, but from the specs they should work

--
Peter Cousens
email: p.cousensspamKILLspamcwcom.net  or  .....p.cousensKILLspamspam.....virgin.net
smail: 48, Yarmouth Cresent, London, N179PQ, England.

2000\01\19@111557 by Dennis Gearon

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I reread the requirements, AFAIK there are no op amps that will drive a
100 ohm load.

--
________________________________________________________________
Real friends are those who, when you feel you've made a fool of
yourself, don't feel you've done a permanent job.
________________________________________________________________
Dennis K. Gearon (Kegley)
Scientific Instrument Technician, School of EIT
Oregon Institute of Technology - One of USA's 100 Best College Buys
3201 Campus Drive
Klamath Falls, OR 97601
Voice   1-541-885-1563
FAX     1-541-885-1689
email   EraseMEgearondspam_OUTspamTakeThisOuToit.edu
________________________________________________________________

2000\01\20@035741 by Roland Andrag

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I don't know what requirements etc. you were referring to, but how about the
National Semiconductor LM12 - 80 W Op Amp - "The LM12 is a power op amp
capable of driving +-25 V at +-10 A while operating from +- 30 V supplies.
The monolithic IC can deliver 80 W of sine wave power into a 4 Ohm load with
0.01 % distortion." etc. etc.

Last price I got was 138.50 rand, or $22... Lot of money, lot of performance
though...

Cheers
Roland
{Original Message removed}

2000\01\20@044358 by wwl

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On Wed, 19 Jan 2000 08:12:45 -0800, you wrote:

>I reread the requirements, AFAIK there are no op amps that will drive a
>100 ohm load.
(assuming we're still talking about precision amps)
Certainly not precision ones - driving heavy loads causes thermal
problems on-chip. you could buffer the output with a high-drive amp -
if you put the output amp inside the feedback loop you preserve the
accuracy.

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