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Why are Instrumentation amps so expensive?

R

Roger

Jan 1, 1970
0
It is a question I have been asking myself for a couple of weeks, ever
scince I got a requirement for a low offset drift high impedance true
differential amp with high CMRR. I have a solution in the form of the
$3 AD627, but I cannot really say that I am **satisfied**. There are
some good cheap low voltage single supply solutions as well, but of
course they don't let your inputs float around much!

Can it be possible that a such a device costs more than a 24bit ADC?
The obvious conclusion is that it is the laser trimming that costs, but
keen to get to the bottom of this I went back to basics and blew the
dust off my AOE. Now in ch.7 we have the run down on auto-zero
techniques, chopper stabalized amps and blow off into differential amps
and finally instrumentation amps....and things just dont add up. The
first thing that crossed my mind is why don't FET input amps switch
around thier inputs to get a zeroing effect. Ha! there is an example,
the ICL7605 which uses a capacitor to pass a voltage to a chopper op
amp. The text notes that the drawback is the noise, but I would expect
that 16 years on somebody would have come up with such a device with a
low pass buil in. Better still, why not make an ADC with this sort of
front end built in....go and pick up a voltage somewhere and then eg.
use it in a capacitive SAR, rotating it in opposite polarities each
conversion. Alas, not only can I not find such a device, I can't even
find a ICL7605.

The second thing I find curious is the theoretical **advantage** of the
instrumentation amplifier over a differential amplifer with simple
buffers on each input. The text notes that one of the snags of the
differential amp is that tight resistor matching is required to achieve
high CMRR, whilst in the instrumentation amp configuration this is not
necessary. OK, but let's have a look at the 'INA' range that the text
quotes...the laser trimmed differential amp devices cost less than the
true instrumentation amps. How come? Of course the snag with the
instrumentation amps is the low impedance, of course it is not a
problem in some apps such as thermocouples but if your measuring a
bridge, well it is a disadvantage. Were it not for price the true
instrument amp would win hands down over the diff amps in nearly all
applications....if nothing else they are also easier to protect. So the
fact that a range of instrument amps also includes lowwer cost low
impedance diff amps suggests to me that there is **something** in the
instrument amp that makes them costly.

Clearly I have missed something here, or perhaps there is some new
technique that eclipses all requirements and instrument amps are
considered legacy?
 
M

martin griffith

Jan 1, 1970
0
It is a question I have been asking myself for a couple of weeks, ever
scince I got a requirement for a low offset drift high impedance true
differential amp with high CMRR. I have a solution in the form of the
$3 AD627, but I cannot really say that I am **satisfied**. There are
some good cheap low voltage single supply solutions as well, but of
course they don't let your inputs float around much!

Can it be possible that a such a device costs more than a 24bit ADC?
The obvious conclusion is that it is the laser trimming that costs, but
keen to get to the bottom of this I went back to basics and blew the
dust off my AOE. Now in ch.7 we have the run down on auto-zero
techniques, chopper stabalized amps and blow off into differential amps
and finally instrumentation amps....and things just dont add up. The
first thing that crossed my mind is why don't FET input amps switch
around thier inputs to get a zeroing effect. Ha! there is an example,
the ICL7605 which uses a capacitor to pass a voltage to a chopper op
amp. The text notes that the drawback is the noise, but I would expect
that 16 years on somebody would have come up with such a device with a
low pass buil in. Better still, why not make an ADC with this sort of
front end built in....go and pick up a voltage somewhere and then eg.
use it in a capacitive SAR, rotating it in opposite polarities each
conversion. Alas, not only can I not find such a device, I can't even
find a ICL7605.

The second thing I find curious is the theoretical **advantage** of the
instrumentation amplifier over a differential amplifer with simple
buffers on each input. The text notes that one of the snags of the
differential amp is that tight resistor matching is required to achieve
high CMRR, whilst in the instrumentation amp configuration this is not
necessary. OK, but let's have a look at the 'INA' range that the text
quotes...the laser trimmed differential amp devices cost less than the
true instrumentation amps. How come? Of course the snag with the
instrumentation amps is the low impedance, of course it is not a
problem in some apps such as thermocouples but if your measuring a
bridge, well it is a disadvantage. Were it not for price the true
instrument amp would win hands down over the diff amps in nearly all
applications....if nothing else they are also easier to protect. So the
fact that a range of instrument amps also includes lowwer cost low
impedance diff amps suggests to me that there is **something** in the
instrument amp that makes them costly.

Clearly I have missed something here, or perhaps there is some new
technique that eclipses all requirements and instrument amps are
considered legacy?



do a search for Guy Macon


martin
 
M

martin griffith

Jan 1, 1970
0
Should I be looking at the insult file?
Well, yes, your post was rather verbose, I sort of fell asleep
reading it. Can you repost in sort of 10 lines or less?


martin
 
R

Roger

Jan 1, 1970
0
martin said:
Well, yes, your post was rather verbose, I sort of fell asleep
reading it. Can you repost in sort of 10 lines or less?

Sorry if I offended anyone, but your right, it is wordy. Ver II: (7
lines)

Why are true instrumentation amps so expensive? I understand from AOE
that one of the advantages of the instrumentation amp over a difference
amp is that resistor matching is less critical, and yet if I look at
e.g. the 'INA' range of amps, the differential amps cost less than the
instrumentation amps.

I would have expected technology to have produced some sort of chopper
amp auto zeroing instr. amp by now, or perhaps there is now a 'better'
way to capture a floating differential voltage and present it to an
ADC?
 
M

martin griffith

Jan 1, 1970
0
Sorry if I offended anyone, but your right, it is wordy. Ver II: (7
lines)

Why are true instrumentation amps so expensive? I understand from AOE
that one of the advantages of the instrumentation amp over a difference
amp is that resistor matching is less critical, and yet if I look at
e.g. the 'INA' range of amps, the differential amps cost less than the
instrumentation amps.

I would have expected technology to have produced some sort of chopper
amp auto zeroing instr. amp by now, or perhaps there is now a 'better'
way to capture a floating differential voltage and present it to an
ADC?


Dont know. We will have to wait for Win Hiill or Jim T or other
guru's to reply
Sorry , just making a point



martin
 
J

John Fields

Jan 1, 1970
0
Sorry if I offended anyone, but your right, it is wordy. Ver II: (7
lines)

Why are true instrumentation amps so expensive? I understand from AOE
that one of the advantages of the instrumentation amp over a difference
amp is that resistor matching is less critical, and yet if I look at
e.g. the 'INA' range of amps, the differential amps cost less than the
instrumentation amps.

I would have expected technology to have produced some sort of chopper
amp auto zeroing instr. amp by now, or perhaps there is now a 'better'
way to capture a floating differential voltage and present it to an
ADC?
 
J

Jim Thompson

Jan 1, 1970
0
Dont know. We will have to wait for Win Hiill or Jim T or other
guru's to reply
Sorry , just making a point



martin

Even from the "verbose" version it's not clear what the OP is trying
to accomplish ;-)

Maybe a "flying cap" ??

What input voltage range?

...Jim Thompson
 
J

Joerg

Jan 1, 1970
0
Hello Roger,

Why are true instrumentation amps so expensive? I understand from AOE
that one of the advantages of the instrumentation amp over a difference
amp is that resistor matching is less critical, and yet if I look at
e.g. the 'INA' range of amps, the differential amps cost less than the
instrumentation amps.

I would have expected technology to have produced some sort of chopper
amp auto zeroing instr. amp by now, or perhaps there is now a 'better'
way to capture a floating differential voltage and present it to an
ADC?

It's probably because of the low market volume. They are used in
measurement setups and university projects and that's not a high volume
market.

For high volume products that require a large CM range engineers usually
spend quite some time to make it happen with jelly bean parts. Often the
first stage is discrete.
 
S

Spehro Pefhany

Jan 1, 1970
0
Sorry if I offended anyone, but your right, it is wordy. Ver II: (7
lines)

Why are true instrumentation amps so expensive? I understand from AOE
that one of the advantages of the instrumentation amp over a difference
amp is that resistor matching is less critical, and yet if I look at
e.g. the 'INA' range of amps, the differential amps cost less than the
instrumentation amps.

I would have expected technology to have produced some sort of chopper
amp auto zeroing instr. amp by now,

Yes, they exist, for less than a couple dollars in 1K. They're not
cheap because there are really no high volume applications for them.
I've been able to avoid them in most instrumentation applications, for
example.
or perhaps there is now a 'better'
way to capture a floating differential voltage and present it to an
ADC?

If it's not moving too fast you can just measure the two voltages and
subtract digitally.


Best regards,
Spehro Pefhany
 
S

Stanislaw Flatto

Jan 1, 1970
0
Roger said:
. So the
fact that a range of instrument amps also includes lowwer cost low
impedance diff amps suggests to me that there is **something** in the
instrument amp that makes them costly.

Clearly I have missed something here, or perhaps there is some new
technique that eclipses all requirements and instrument amps are
considered legacy?

Ok, lets say that you are hunting for components to acomplish certain
function in known enviroment. You are as free as a bird to use what fits.
I am test department tech. on hunt for instrumentation amplifiers to
amplify, for computer recording, your ADC, physical parameters like heat
(thermocouples from copper/constantan to platinum/platinum-iridium and
tungsten; strain gauge reading of forces from static to mechanical shock
and so on and on)
So I end (early '80s) with instruments of Vishay at $400/each (50 qty)
and do my testing for more than 8 years without failures. And NOT in lab
but in enviroments where the Tektronix scope displayed a spiral trace
with the trace moving in oposite direction to the flow of time. (Real
SCI-FI).
And the technical design department want accuracy <1% at ANY reading.
Actually you pay for ability to use the equipment to specs.

My 2c.

Stanislaw
Slack user from Uladulla.
 
G

Genome

Jan 1, 1970
0
Wow, up to Version 6.12 ALREADY?

http://www.guymacon.com/FUN/INSULT/INDEX.HTM

"You are not ANSI compliant and your markup doesn't validate. You have
a couple of address lines shorted together. You should be promoted to
Engineering Manager."

That's just wrong. ;)

If true then it's a little bit strange.......

Guy Macon sort of promotes himself as an 'Engineering Manager'.

Oh..... I'm being slow :)

DNA
 
B

Ban

Jan 1, 1970
0
Roger said:
Sorry if I offended anyone, but your right, it is wordy. Ver II: (7
lines)

Why are true instrumentation amps so expensive? I understand from AOE
that one of the advantages of the instrumentation amp over a
difference amp is that resistor matching is less critical, and yet if
I look at e.g. the 'INA' range of amps, the differential amps cost
less than the instrumentation amps.

I would have expected technology to have produced some sort of chopper
amp auto zeroing instr. amp by now, or perhaps there is now a
'better' way to capture a floating differential voltage and present
it to an ADC?

Don't get confused by AoE.
Instrumentation Amps have some indispensable advantages over diffamps or
discrete designs. The input impedance is very high, *equal* for both inputs
and for *common mode* and *differential mode* signals. This maintains the
CMRR for any source unbalancing. The internal resistors are trimmed to very
high precision. A single resistor can vary the gain.
The AD8225 has a gain error of +/-0.1% and 5ppm/K drift max. Offset error is
not so much of an issue today, it can be corrected for in the digital
domain. This amp also has a CMRR of >80dB at 10kHz, very difficult to
achieve with discrete parts, C-trimmers are needed for that.
There are quite a bit of applications where IAs are used: RTI, thermocouple
etc. bridges, professional audio, 4-20mA receivers, EKG front-end....
 
P

Paul E. Schoen

Jan 1, 1970
0
Ban said:
Don't get confused by AoE.
Instrumentation Amps have some indispensable advantages over diffamps or
discrete designs. The input impedance is very high, *equal* for both
inputs and for *common mode* and *differential mode* signals. This
maintains the CMRR for any source unbalancing. The internal resistors are
trimmed to very high precision. A single resistor can vary the gain.
The AD8225 has a gain error of +/-0.1% and 5ppm/K drift max. Offset error
is not so much of an issue today, it can be corrected for in the digital
domain. This amp also has a CMRR of >80dB at 10kHz, very difficult to
achieve with discrete parts, C-trimmers are needed for that.
There are quite a bit of applications where IAs are used: RTI,
thermocouple etc. bridges, professional audio, 4-20mA receivers, EKG
front-end....

Very well said. I use the AD620 by Analog Devices, and it works very well
to measure currents as low as 5 amperes on a 1000A 100 mV shunt. This
corresponds to 5 mV, and my 1% accuracy specification means I must have no
more than 50 uV of noise or common mode error, where common mode voltage
could be as high as one or two volts. I pay about $5 to $8 for these (in
small quantities), but it is quite reasonable in a $3000 instrument that is
used for calibration purposes. I have also used the INA118 or similar
devices, which were somewhat cheaper, but had more noise and drift.

Paul
 
G

Genome

Jan 1, 1970
0
Roger said:
It is a question I have been asking myself for a couple of weeks, ever
scince I got a requirement for a low offset drift high impedance true
differential amp with high CMRR. I have a solution in the form of the
$3 AD627, but I cannot really say that I am **satisfied**.

Clearly I have missed something here, or perhaps there is some new
technique that eclipses all requirements and instrument amps are
considered legacy?

Yes, me neither. I could build one of these out of an LM324 and use the
spare one to drive an LED.

Bastards!

DNA
 
R

Roger

Jan 1, 1970
0
Jim Thompson wrote:

Even from the "verbose" version it's not clear what the OP is trying
to accomplish ;-)

Nothing. In the verbosr version I even mention that I have a solution
for my app.

Mine is purely a curiosity question: "Why are instrumentation amps so
expensive?".

I could narrow down the field and ask "Why do the instrument amps tend
to cost more than the diff amps in a range of precision amplifer
products such as BB's INA range".

Answers here and elsewhere do not convince me:

1) Laser trimming. Well, good diff amps also need laser trimming and in
some respects (according to AoE) the instrument amp cct makes some
resistors matching less critical.

2) Circuit complexity. Scince when has this been a major factor in IC
prices.

3) Quantity. Instrumentation amps have a potentially much larger
market, specificaaly they could be used to improve most designs where
diff amps are used.

Clearly there is something that makes them so much more expensive to
produce than similar precision analog devices.

I also entered into the question of why there are not improved
"chopper" versions of instrumentation amps or cicuits which can do the
same. I specifically noted that the ICL1706 device shown in AoE appears
to be no longer made, nor is there anything like it.
Maybe a "flying cap" ??

What input voltage range?

Let's answer this in one. Most ADC's require an input range within the
5/3v3 supply range and only offer psuedo differential inputs which have
zero CMRR characteristics, yet many sources cannot be readily adapted
to this: Uninsulated thermocouples and bridge sensors optimised for eg.
10V supplies are common examples. Even ground referenced sources (which
in some respects includes uninsulated thermocouples) can benefit from
differential inputs as sensors may be remote and/or in power cct's and
be subject to large CM noise. In low impedence situations it is
possible to use diff amps, but I would only bother to make the
distinction if the diff amp saved me money, one good low priced
instrument amps could eliminate the requirement for a similar diff amp
in the same product range.

But I also mused on the idea that there could be ADC's with a floating
capacitor input, i.e. FETS which attach a cap to a source, and then
switch it into the convertor, so the ADC can measure voltages outside
the supply range which the core converter is running at and, by
flipping the cap, could null out offset errors at the same time.

Well, I would love an ADC like this, but perhaps that is because I do
not know some technique that gets the same results at low cost ;-)
 
J

Joerg

Jan 1, 1970
0
Hello Roger,

Answers here and elsewhere do not convince me:
....

3) Quantity. Instrumentation amps have a potentially much larger
market, specificaaly they could be used to improve most designs where
diff amps are used.

Like where? I have used diff amps in designs that went into production
but I have never used instrumentation amps on any released product.
There was no need to.

Ban has brought up good examples but even there cost often rules. Many
good ECG (EKG) units go sans instrumentation amp.
Let's answer this in one. Most ADC's require an input range within the
5/3v3 supply range and only offer psuedo differential inputs which have
zero CMRR characteristics, yet many sources cannot be readily adapted
to this: Uninsulated thermocouples and bridge sensors optimised for eg.
10V supplies are common examples. Even ground referenced sources (which
in some respects includes uninsulated thermocouples) can benefit from
differential inputs as sensors may be remote and/or in power cct's and
be subject to large CM noise. In low impedence situations it is
possible to use diff amps, but I would only bother to make the
distinction if the diff amp saved me money, one good low priced
instrument amps could eliminate the requirement for a similar diff amp
in the same product range.

But I also mused on the idea that there could be ADC's with a floating
capacitor input, i.e. FETS which attach a cap to a source, and then
switch it into the convertor, so the ADC can measure voltages outside
the supply range which the core converter is running at and, by
flipping the cap, could null out offset errors at the same time.

Well, I would love an ADC like this, but perhaps that is because I do
not know some technique that gets the same results at low cost ;-)

Offsets and extended input voltage ranges are often handled by clamping
circuitry. In the same way that TV sets restore the DC bias for the
video path from a source that isn't DC coupled.
 
J

John Popelish

Jan 1, 1970
0
Roger wrote:
(snip)
But I also mused on the idea that there could be ADC's with a floating
capacitor input, i.e. FETS which attach a cap to a source, and then
switch it into the convertor, so the ADC can measure voltages outside
the supply range which the core converter is running at and, by
flipping the cap, could null out offset errors at the same time.

If you need really good common mode range, this is the way to go. One
pair of reed relays connect the cap to the field voltage, and after
they are opened, a second pair connect the cap to the A/D and signal
ground. You need definite break before make contacts.

I have seen thermocouples, with more than a hundred common mode volts,
read this way.
 
R

Roger

Jan 1, 1970
0
Joerg said:
Hello Roger,



Like where? I have used diff amps in designs that went into production
but I have never used instrumentation amps on any released product.
There was no need to.

Still missing the point :-(

If a precision instrumentation amp cost pretty much what a precision
diff amp does then there would seem little point having both in a
product range.

So that rules out "quantity" as a factor.
 
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