somewhat OT: op-amp parameters
Archiver
Posts: 46,084
This may be kind of a dumb question, but I'd like to hear the opinion of
the analog gurus on the list. The question concerns how to interpret some
op-amp characteristics when trying to select a particular device. For example:
1) Input offset voltage: It seems to me that if the "system" will have an
offset adjustment somewhere for the final output, why should you care about
10-100 uV of offset in one or more intervening op-amps? Especially if the
op-amps are used at low gains? And especially if the input signals are large?
2) Drift over temperature: Again, if the gain is low (say 2-10) and lots of
op-amps can do < 1uV/C, so over say 20 degrees with a gain of 10 you'd get
200uV of offset referred to the output. But this seems small if the output
is volts.
3) Input voltage noise: Typically spec'ed as peak-to-peak or RMS noise
voltage at 0.1 to 10 Hz. Lots of op-amps have a 0.5uV p-p noise spec. But
again, with modest gains and largish signals, does this matter? If so,
under what conditions?
4) Input bias current: Here I can see that low input bias current lets you
handle higher source impedances, and FET inputs are good for very high
source impedances. Is this the main idea here?
And finally, can anyone recommend a good summary of how to select a device
based on what's so good about each of the parameters? I've got Horowitz and
Hill, and Jung's op-amp book, and that's where I get much of my
information, but if possible I'd like to see a consise 2-page summary of
how to select op-amps. Any comments and/or recommendations would be welcome.
I know this isn't strictly Stamp-related, but questions about applying
op-amps come up pretty regularly on the list, so there might be some
general interest here. Thanks, and I apologize if this is felt to be too
far off-topic.
Steve
Steve Roberts: sroberts@s...
the analog gurus on the list. The question concerns how to interpret some
op-amp characteristics when trying to select a particular device. For example:
1) Input offset voltage: It seems to me that if the "system" will have an
offset adjustment somewhere for the final output, why should you care about
10-100 uV of offset in one or more intervening op-amps? Especially if the
op-amps are used at low gains? And especially if the input signals are large?
2) Drift over temperature: Again, if the gain is low (say 2-10) and lots of
op-amps can do < 1uV/C, so over say 20 degrees with a gain of 10 you'd get
200uV of offset referred to the output. But this seems small if the output
is volts.
3) Input voltage noise: Typically spec'ed as peak-to-peak or RMS noise
voltage at 0.1 to 10 Hz. Lots of op-amps have a 0.5uV p-p noise spec. But
again, with modest gains and largish signals, does this matter? If so,
under what conditions?
4) Input bias current: Here I can see that low input bias current lets you
handle higher source impedances, and FET inputs are good for very high
source impedances. Is this the main idea here?
And finally, can anyone recommend a good summary of how to select a device
based on what's so good about each of the parameters? I've got Horowitz and
Hill, and Jung's op-amp book, and that's where I get much of my
information, but if possible I'd like to see a consise 2-page summary of
how to select op-amps. Any comments and/or recommendations would be welcome.
I know this isn't strictly Stamp-related, but questions about applying
op-amps come up pretty regularly on the list, so there might be some
general interest here. Thanks, and I apologize if this is felt to be too
far off-topic.
Steve
Steve Roberts: sroberts@s...
Comments
>The question concerns how to interpret some
>op-amp characteristics when trying to select a particular device. For
example:
>
>1) Input offset voltage: It seems to me that if the "system" will have an
>offset adjustment somewhere for the final output, why should you care
about
>10-100 uV of offset in one or more intervening op-amps? Especially if the
>op-amps are used at low gains? And especially if the input signals are
large?
>
>2) Drift over temperature: Again, if the gain is low (say 2-10) and lots
of
>op-amps can do < 1uV/C, so over say 20 degrees with a gain of 10 you'd get
>200uV of offset referred to the output. But this seems small if the output
>is volts.
Hi Steve,
You've answered your own questions!
Conversely, when the input signal is low and the gain is high, that is when
you need the low offset. Examples would be an amplifier for a thermopile
or an oxygen probe. The gain may have to be x1000 or more. Starting with
a low offset chip simplifies the circuit and gets rid of a pesky offset
adjustments. It all comes down to a comparison of the signal to the error
sources. A 20 microvolt signal amounts to about 1 degree Fahrenheit for a
thermocouple. That may be significant for an incubator, but not so for a
kiln.
>3) Input voltage noise: Typically spec'ed as peak-to-peak or RMS noise
>voltage at 0.1 to 10 Hz. Lots of op-amps have a 0.5uV p-p noise spec. But
>again, with modest gains and largish signals, does this matter? If so,
>under what conditions?
Again it is a comparison of signal to noise. Trying to acquire a signal
from the stars or from a gas analyzer or a long data transmission link will
push the limits. In your gas analyzers, you chop the signal so as to
narrow the bandwidth and bring the signal up out of the noise at DC. What
limits the parts per million you can detect? Is it noise limited?
>4) Input bias current: Here I can see that low input bias current lets you
>handle higher source impedances, and FET inputs are good for very high
>source impedances. Is this the main idea here?
Right.
>And finally, can anyone recommend a good summary of how to select a device
>based on what's so good about each of the parameters? I've got Horowitz
and
>Hill, and Jung's op-amp book, and that's where I get much of my
>information, but if possible I'd like to see a consise 2-page summary of
>how to select op-amps. Any comments and/or recommendations would be
welcome.
Don't forget the AC parameters like bandwidth and slew rate, or things like
common mode range, or power supply specs, not to mention packaging.
Availabilty. Cost. I often find that my choices are dictated by factors
other than 1-4 above. The best education is to understand what each
parameter in the data sheet means. In addition to the sources you
mentioned (which are excellent), there are the National Semi application
notes. I like the books by Jerald G. Graeme (Burr-Brown guru), also
anything by Bob Pease (National) or Jim Williams (Linear Tech). It is not
only a matter of parameters, but circuits. All the major manufacturers
have parameter trees on their web sites, and there are industry wide sites
like specsearch.com. But they assume you know what you are doing and know
how to rank your requirements.
You start with the input signal source, the desired output, and the other
restraints like the available power supplies. Then you narrow down the
selection. If you are quickly down to only a few, that means you have a
specialized requirement. Then go down the list of parameters on the data
sheet one by one to see if there is anything you have overlooked that could
come back to bite you later. Then you order some samples and put them
through the paces to see if they really work. Or maybe you have a
jellybean requirement and almost any op-amp will do. I find that most of
my requirements can be met by a few op amps I keep in stock. It is when
those fall short on some parameter that it is time to go shopping again.
-- Tracy
http://www.emesystems.com
Tracy, thanks, especially for the tips on the ref author's backgrounds.
Its going to make it easier to hook more stuff up to my stamp.
"you chop the signal so as to narrow the bandwidth and bring the signal up
out of the noise at DC."
"noise at DC", what does this mean?
"bandwidth and slew rate" Could you explain this in terms of AC?
Mark
Original Message
From: Tracy Allen <emesys@c...>
To: INTERNET:basicstamps@egroups.com <basicstamps@egroups.com>
Date: Sunday, October 29, 2000 8:35 AM
Subject: [noparse][[/noparse]basicstamps] somewhat OT: op-amp parameters
>
>Steve Roberts: sroberts@s... wrote:
>>The question concerns how to interpret some
>>op-amp characteristics when trying to select a particular device. For
>example:
>>
>>1) Input offset voltage: It seems to me that if the "system" will have an
>>offset adjustment somewhere for the final output, why should you care
>about
>>10-100 uV of offset in one or more intervening op-amps? Especially if the
>>op-amps are used at low gains? And especially if the input signals are
>large?
>>
>>2) Drift over temperature: Again, if the gain is low (say 2-10) and lots
>of
>>op-amps can do < 1uV/C, so over say 20 degrees with a gain of 10 you'd get
>>200uV of offset referred to the output. But this seems small if the output
>>is volts.
>
>Hi Steve,
>You've answered your own questions!
>
>Conversely, when the input signal is low and the gain is high, that is when
>you need the low offset. Examples would be an amplifier for a thermopile
>or an oxygen probe. The gain may have to be x1000 or more. Starting with
>a low offset chip simplifies the circuit and gets rid of a pesky offset
>adjustments. It all comes down to a comparison of the signal to the error
>sources. A 20 microvolt signal amounts to about 1 degree Fahrenheit for a
>thermocouple. That may be significant for an incubator, but not so for a
>kiln.
>
>
>>3) Input voltage noise: Typically spec'ed as peak-to-peak or RMS noise
>>voltage at 0.1 to 10 Hz. Lots of op-amps have a 0.5uV p-p noise spec. But
>>again, with modest gains and largish signals, does this matter? If so,
>>under what conditions?
>
>Again it is a comparison of signal to noise. Trying to acquire a signal
>from the stars or from a gas analyzer or a long data transmission link will
>push the limits. In your gas analyzers, you chop the signal so as to
>narrow the bandwidth and bring the signal up out of the noise at DC. What
>limits the parts per million you can detect? Is it noise limited?
>
>>4) Input bias current: Here I can see that low input bias current lets you
>>handle higher source impedances, and FET inputs are good for very high
>>source impedances. Is this the main idea here?
>
>Right.
>
>>And finally, can anyone recommend a good summary of how to select a device
>>based on what's so good about each of the parameters? I've got Horowitz
>and
>>Hill, and Jung's op-amp book, and that's where I get much of my
>>information, but if possible I'd like to see a consise 2-page summary of
>>how to select op-amps. Any comments and/or recommendations would be
>welcome.
>
>Don't forget the AC parameters like bandwidth and slew rate, or things like
>common mode range, or power supply specs, not to mention packaging.
>Availabilty. Cost. I often find that my choices are dictated by factors
>other than 1-4 above. The best education is to understand what each
>parameter in the data sheet means. In addition to the sources you
>mentioned (which are excellent), there are the National Semi application
>notes. I like the books by Jerald G. Graeme (Burr-Brown guru), also
>anything by Bob Pease (National) or Jim Williams (Linear Tech). It is not
>only a matter of parameters, but circuits. All the major manufacturers
>have parameter trees on their web sites, and there are industry wide sites
>like specsearch.com. But they assume you know what you are doing and know
>how to rank your requirements.
>
>You start with the input signal source, the desired output, and the other
>restraints like the available power supplies. Then you narrow down the
>selection. If you are quickly down to only a few, that means you have a
>specialized requirement. Then go down the list of parameters on the data
>sheet one by one to see if there is anything you have overlooked that could
>come back to bite you later. Then you order some samples and put them
>through the paces to see if they really work. Or maybe you have a
>jellybean requirement and almost any op-amp will do. I find that most of
>my requirements can be met by a few op amps I keep in stock. It is when
>those fall short on some parameter that it is time to go shopping again.
>
> -- Tracy
> http://www.emesystems.com
>
>
>
>
>
>
As a minor addition to Tracy Allen's excellent summary, using 2 or 3 op
amps in a multi-stage design can simplify or eliminate considerations of
individual op amp characteristics. Configure the first amp to match the
sensor input impedance characteristics, using a FET-input amp if necessary,
but with only part of the overall gain. This will prevent any input offset
of the op amp from saturating the output. Then, use follower stages for
filtering, auto zeroing, or setting a new zero level for A/D conversion,
along with the remainder of the required overall gain. The later stages can
be done with low-cost op amps.
For example, my application amplifies bipolar 5 mV signals of
electrode-generated human eye movements, drifting over a wide range, to
0-4.096 V for A/D conversion. Stage 1 is a FET-input instrumentation amp
with a gain of 220. Stage 2 adds auto-rest with an additional gain of 10,
combined with analog low-pass filtering and a pot input to offset zero to
about 2 volts. Stage 3 is a 0-5 V rail-to-rail op amp which brackets the
signal for input to a A/D converter. It is all controlled by a PIC, which
also sends a pulse to a Stamp BS1 generating an auditory tone driving an
audio amp and a speaker. One great advantage of using a PIC controller, is
that if the eye movements drift toward either 0 or 5 volts, the program
detects it and activates the auto-reset back toward the 2-volt center
position. If anyone would like details of this sample-and-hold auto reset
circuit, contact me off line.
Dennis
Original Message
From: Tracy Allen [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=vqE68BWZfXot8TA2mSJZRWbQQ8LGKtiCdBvbb90CDAI2MeVOIu5vd_URlvXmqnY4ON4cEmam-YL1tdDGasI]emesys@c...[/url
Sent: Sunday, October 29, 2000 8:34
To: INTERNET:basicstamps@egroups.com
Subject: [noparse][[/noparse]basicstamps] somewhat OT: op-amp parameters
Steve Roberts: sroberts@s... wrote:
>The question concerns how to interpret some
>op-amp characteristics when trying to select a particular device. For
example:
>
>1) Input offset voltage: It seems to me that if the "system" will have an
>offset adjustment somewhere for the final output, why should you care
about
>10-100 uV of offset in one or more intervening op-amps? Especially if the
>op-amps are used at low gains? And especially if the input signals are
large?
>
>2) Drift over temperature: Again, if the gain is low (say 2-10) and lots
of
>op-amps can do < 1uV/C, so over say 20 degrees with a gain of 10 you'd get
>200uV of offset referred to the output. But this seems small if the output
>is volts.
Hi Steve,
You've answered your own questions!
Conversely, when the input signal is low and the gain is high, that is when
you need the low offset. Examples would be an amplifier for a thermopile
or an oxygen probe. The gain may have to be x1000 or more. Starting with
a low offset chip simplifies the circuit and gets rid of a pesky offset
adjustments. It all comes down to a comparison of the signal to the error
sources. A 20 microvolt signal amounts to about 1 degree Fahrenheit for a
thermocouple. That may be significant for an incubator, but not so for a
kiln.
>3) Input voltage noise: Typically spec'ed as peak-to-peak or RMS noise
>voltage at 0.1 to 10 Hz. Lots of op-amps have a 0.5uV p-p noise spec. But
>again, with modest gains and largish signals, does this matter? If so,
>under what conditions?
Again it is a comparison of signal to noise. Trying to acquire a signal
from the stars or from a gas analyzer or a long data transmission link will
push the limits. In your gas analyzers, you chop the signal so as to
narrow the bandwidth and bring the signal up out of the noise at DC. What
limits the parts per million you can detect? Is it noise limited?
>4) Input bias current: Here I can see that low input bias current lets you
>handle higher source impedances, and FET inputs are good for very high
>source impedances. Is this the main idea here?
Right.
>And finally, can anyone recommend a good summary of how to select a device
>based on what's so good about each of the parameters? I've got Horowitz
and
>Hill, and Jung's op-amp book, and that's where I get much of my
>information, but if possible I'd like to see a consise 2-page summary of
>how to select op-amps. Any comments and/or recommendations would be
welcome.
Don't forget the AC parameters like bandwidth and slew rate, or things like
common mode range, or power supply specs, not to mention packaging.
Availabilty. Cost. I often find that my choices are dictated by factors
other than 1-4 above. The best education is to understand what each
parameter in the data sheet means. In addition to the sources you
mentioned (which are excellent), there are the National Semi application
notes. I like the books by Jerald G. Graeme (Burr-Brown guru), also
anything by Bob Pease (National) or Jim Williams (Linear Tech). It is not
only a matter of parameters, but circuits. All the major manufacturers
have parameter trees on their web sites, and there are industry wide sites
like specsearch.com. But they assume you know what you are doing and know
how to rank your requirements.
You start with the input signal source, the desired output, and the other
restraints like the available power supplies. Then you narrow down the
selection. If you are quickly down to only a few, that means you have a
specialized requirement. Then go down the list of parameters on the data
sheet one by one to see if there is anything you have overlooked that could
come back to bite you later. Then you order some samples and put them
through the paces to see if they really work. Or maybe you have a
jellybean requirement and almost any op-amp will do. I find that most of
my requirements can be met by a few op amps I keep in stock. It is when
those fall short on some parameter that it is time to go shopping again.
-- Tracy
http://www.emesystems.com
<<<snip>>>...
>I find that most of
>my requirements can be met by a few op amps I keep in stock. It is when
>those fall short on some parameter that it is time to go shopping again.
Thanks Tracy and Dennis for your comments. I'll bet we all would learn
something if we knew which op-amps Tracy keeps in stock, and a brief
comment on why for each one! Tracy, could we see your op-amp list please?
Dennis also if possible. I think these would be good general guidelines.
Any other comments? Thanks!
Steve
Steve Roberts: sroberts@s...
Thanks for the comments to the group. I'm interested in your circuit as
it sounds robust and well-engineered. I have needs to magnify a 0 - 30mv
signal from a sensor but the sensitivity of the sensor is down in the 100
microvolt range. I need it converted by an 8-bit ADC, and the auto-reset
and centering around 2v sounds useful--not sure I fully understand the
ramifications of all that. A lot of what you said is on the periphery of my
hardware skills.
Anyway, I'd be interested in finding out more about your circuit. What
resources could you make available to me?
Thanks,
Rich
>From: "Dennis P. O'Leary" <doleary@h...>
>Reply-To: basicstamps@egroups.com
>To: <basicstamps@egroups.com>
>Subject: RE: [noparse][[/noparse]basicstamps] somewhat OT: op-amp parameters
>Date: Sun, 29 Oct 2000 21:18:29 -0800
>
>Stever,
> As a minor addition to Tracy Allen's excellent summary, using 2 or 3 op
>amps in a multi-stage design can simplify or eliminate considerations of
>individual op amp characteristics. Configure the first amp to match the
>sensor input impedance characteristics, using a FET-input amp if necessary,
>but with only part of the overall gain. This will prevent any input offset
>of the op amp from saturating the output. Then, use follower stages for
>filtering, auto zeroing, or setting a new zero level for A/D conversion,
>along with the remainder of the required overall gain. The later stages can
>be done with low-cost op amps.
> For example, my application amplifies bipolar 5 mV signals of
>electrode-generated human eye movements, drifting over a wide range, to
>0-4.096 V for A/D conversion. Stage 1 is a FET-input instrumentation amp
>with a gain of 220. Stage 2 adds auto-rest with an additional gain of 10,
>combined with analog low-pass filtering and a pot input to offset zero to
>about 2 volts. Stage 3 is a 0-5 V rail-to-rail op amp which brackets the
>signal for input to a A/D converter. It is all controlled by a PIC, which
>also sends a pulse to a Stamp BS1 generating an auditory tone driving an
>audio amp and a speaker. One great advantage of using a PIC controller, is
>that if the eye movements drift toward either 0 or 5 volts, the program
>detects it and activates the auto-reset back toward the 2-volt center
>position. If anyone would like details of this sample-and-hold auto reset
>circuit, contact me off line.
>Dennis
>
>
Original Message
>From: Tracy Allen [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=R5TfDCSOKJJ0qaitH9jA03pugMmNSMQDfxx2m1-c21bL9TRAFyU0Chd-n0X8A5s-SOsHK77AR5aDoHIr371big]emesys@c...[/url
>Sent: Sunday, October 29, 2000 8:34
>To: INTERNET:basicstamps@egroups.com
>Subject: [noparse][[/noparse]basicstamps] somewhat OT: op-amp parameters
>
>
>
>Steve Roberts: sroberts@s... wrote:
> >The question concerns how to interpret some
> >op-amp characteristics when trying to select a particular device. For
>example:
> >
> >1) Input offset voltage: It seems to me that if the "system" will have an
> >offset adjustment somewhere for the final output, why should you care
>about
> >10-100 uV of offset in one or more intervening op-amps? Especially if the
> >op-amps are used at low gains? And especially if the input signals are
>large?
> >
> >2) Drift over temperature: Again, if the gain is low (say 2-10) and lots
>of
> >op-amps can do < 1uV/C, so over say 20 degrees with a gain of 10 you'd
>get
> >200uV of offset referred to the output. But this seems small if the
>output
> >is volts.
>
>Hi Steve,
>You've answered your own questions!
>
>Conversely, when the input signal is low and the gain is high, that is when
>you need the low offset. Examples would be an amplifier for a thermopile
>or an oxygen probe. The gain may have to be x1000 or more. Starting with
>a low offset chip simplifies the circuit and gets rid of a pesky offset
>adjustments. It all comes down to a comparison of the signal to the error
>sources. A 20 microvolt signal amounts to about 1 degree Fahrenheit for a
>thermocouple. That may be significant for an incubator, but not so for a
>kiln.
>
>
> >3) Input voltage noise: Typically spec'ed as peak-to-peak or RMS noise
> >voltage at 0.1 to 10 Hz. Lots of op-amps have a 0.5uV p-p noise spec. But
> >again, with modest gains and largish signals, does this matter? If so,
> >under what conditions?
>
>Again it is a comparison of signal to noise. Trying to acquire a signal
>from the stars or from a gas analyzer or a long data transmission link will
>push the limits. In your gas analyzers, you chop the signal so as to
>narrow the bandwidth and bring the signal up out of the noise at DC. What
>limits the parts per million you can detect? Is it noise limited?
>
> >4) Input bias current: Here I can see that low input bias current lets
>you
> >handle higher source impedances, and FET inputs are good for very high
> >source impedances. Is this the main idea here?
>
>Right.
>
> >And finally, can anyone recommend a good summary of how to select a
>device
> >based on what's so good about each of the parameters? I've got Horowitz
>and
> >Hill, and Jung's op-amp book, and that's where I get much of my
> >information, but if possible I'd like to see a consise 2-page summary of
> >how to select op-amps. Any comments and/or recommendations would be
>welcome.
>
>Don't forget the AC parameters like bandwidth and slew rate, or things like
>common mode range, or power supply specs, not to mention packaging.
>Availabilty. Cost. I often find that my choices are dictated by factors
>other than 1-4 above. The best education is to understand what each
>parameter in the data sheet means. In addition to the sources you
>mentioned (which are excellent), there are the National Semi application
>notes. I like the books by Jerald G. Graeme (Burr-Brown guru), also
>anything by Bob Pease (National) or Jim Williams (Linear Tech). It is not
>only a matter of parameters, but circuits. All the major manufacturers
>have parameter trees on their web sites, and there are industry wide sites
>like specsearch.com. But they assume you know what you are doing and know
>how to rank your requirements.
>
>You start with the input signal source, the desired output, and the other
>restraints like the available power supplies. Then you narrow down the
>selection. If you are quickly down to only a few, that means you have a
>specialized requirement. Then go down the list of parameters on the data
>sheet one by one to see if there is anything you have overlooked that could
>come back to bite you later. Then you order some samples and put them
>through the paces to see if they really work. Or maybe you have a
>jellybean requirement and almost any op-amp will do. I find that most of
>my requirements can be met by a few op amps I keep in stock. It is when
>those fall short on some parameter that it is time to go shopping again.
>
> -- Tracy
> http://www.emesystems.com
>
>
>
>
>
>
>
>
>
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Original Message
From: Stephen Roberts [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=dJ0PmK81GCQ_SagswX2N4VRiY4RfjLj3bJAGcNDguA4A1JBT_T_uoEIlXcxz9sRr9NX5Flv3NN0p9CV77V9HbWOYgs4]sroberts@s...[/url
Sent: Monday, October 30, 2000 12:44 PM
To: basicstamps@egroups.com
Subject: [noparse][[/noparse]basicstamps] Re: somewhat OT: op-amp parameters
When possible, I like to reduce parts count on projects by using op amps
that are included in chips with other useful functions. One example is the
differential instrumentation amplifier, which we used to build from 3 op
amps, but can now be found as a single 8-dip chip (e.g., Burr-Brown INA111,
or 121, which include FET inputs). Another that I use a lot is a Burr-Brown
UAF42 general-purpose filter chip, that includes a spare op amp. I also like
the Burr-Brown INA105, which is an op amp with 4 integral 25K resistors that
can be configured in various ways.
Dennis
Thanks Tracy and Dennis for your comments. I'll bet we all would learn
something if we knew which op-amps Tracy keeps in stock, and a brief
comment on why for each one! Tracy, could we see your op-amp list please?
Dennis also if possible. I think these would be good general guidelines.
Any other comments? Thanks!
You seem to have a good handle on the realities of opamp specifications.
Yes, you are correct, if the system can handle the errors inserted by the
imperfections of a certain opamp, then the imperfections can be basically
ignored. That idea is in fact the basis for opamp selection. Just be sure
to read all specifications carefully before considering an opamp for a
particular application. Be sure to also study the specifications for
non-linearity, power consumption, phase distortion, and phase stability as
well.
Chuck
Original Message
From: Stephen Roberts <sroberts@s...>
To: basicstamps@egroups.com <basicstamps@egroups.com>
Date: Saturday, October 28, 2000 6:53 PM
Subject: [noparse][[/noparse]basicstamps] somewhat OT: op-amp parameters
>This may be kind of a dumb question, but I'd like to hear the opinion of
>the analog gurus on the list. The question concerns how to interpret some
>op-amp characteristics when trying to select a particular device. For
example:
>
>1) Input offset voltage: It seems to me that if the "system" will have an
>offset adjustment somewhere for the final output, why should you care about
>10-100 uV of offset in one or more intervening op-amps? Especially if the
>op-amps are used at low gains? And especially if the input signals are
large?
>
>2) Drift over temperature: Again, if the gain is low (say 2-10) and lots of
>op-amps can do < 1uV/C, so over say 20 degrees with a gain of 10 you'd get
>200uV of offset referred to the output. But this seems small if the output
>is volts.
>
>3) Input voltage noise: Typically spec'ed as peak-to-peak or RMS noise
>voltage at 0.1 to 10 Hz. Lots of op-amps have a 0.5uV p-p noise spec. But
>again, with modest gains and largish signals, does this matter? If so,
>under what conditions?
>
>4) Input bias current: Here I can see that low input bias current lets you
>handle higher source impedances, and FET inputs are good for very high
>source impedances. Is this the main idea here?
>
>And finally, can anyone recommend a good summary of how to select a device
>based on what's so good about each of the parameters? I've got Horowitz and
>Hill, and Jung's op-amp book, and that's where I get much of my
>information, but if possible I'd like to see a consise 2-page summary of
>how to select op-amps. Any comments and/or recommendations would be
welcome.
>
>I know this isn't strictly Stamp-related, but questions about applying
>op-amps come up pretty regularly on the list, so there might be some
>general interest here. Thanks, and I apologize if this is felt to be too
>far off-topic.
>
>Steve
>
>
> Steve Roberts: sroberts@s...
>
>
>
>
>