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?
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?