Instrumentation op-amp for DC-coupling to audio input?
 

I'm considering an op-amp for making a DC coupling adapter to a soundcard to
convert it to signal logging purposes while retaining its audio performance.
It uses a passive adder and a gain of 2 to add a bias voltage to the signal
before an ADC input.

The sound card is one with external analog circuitry in a rack unit, it has
20 bit signal conversion, so this op-amp will have to be good to maintain
that and the other specs this unit has.

I looked first at a few audio amps and noticed that their claims for CMRR and
open-loop gain often fall well short of the claims made for the equipment
they go into, but never mind, that's another issue for another day.. :)

Then I looked at a DC instrumentation amp (OPA2277) I'm using in a laser
power meter design. If I can use it, it saves me buying varieties of
expensive chips in small quantities. Audio boffs high, wide and plentiful
will say don't do it, slew rate is slow, etc, but is it?? 0.8V/µS. It doesn't
sound a lot when people are saying I need 16V/µS or whatever, but I
calculated it, and it looks fine to me. The sound unit I'm adapting to is
considerably better than CD quality, sampling with 20 bits at up to 48 KHz,
and I calculated that this means a sample at intervals of a tad over 20 µS.
As 20 µS of 0.8V/µS is 16V, and as the device I'm adapting to has a ±15V
supply and a differential input design that halves the input, the largest
possible voltage change will occur, and fully settle, in the time between
samples at highest sample rate available.

As all the other figures for dynamic range and noise are so good that they
will allow the original specs for the entire unit to remain intact, is there
any reason I should not use this op-amp? It's a lot cheaper than any audio
amp that looks like it will do as well as this. And as I'm after DC as well
as AC capability, it seems that this is the right decision, but I'm
interested in other views before I decide anything. (I could just use
sockets, but for a low profile board I'll be soldering it in, and don't want
to have to mess with that later. :)

Instrumentation op-amp for DC-coupling to audio input?
 

Hi,

I'm considering an op-amp for making a DC coupling adapter to a soundcard
to
convert it to signal logging purposes while retaining its audio
performance.
It uses a passive adder and a gain of 2 to add a bias voltage to the
signal
before an ADC input.

The sound card is one with external analog circuitry in a rack unit, it
has
20 bit signal conversion, so this op-amp will have to be good to maintain
that and the other specs this unit has.

So did you think about noise?

As all the other figures for dynamic range and noise are so good that they
will allow the original specs for the entire unit to remain intact,

are you shure? What's about your supply?

Marte

Instrumentation op-amp for DC-coupling to audio input?
 

On Jul 13, 8:48*am, Lostgallifreyan wrote:
I'm considering an op-amp for making a DC coupling adapter to a soundcard to
convert it to signal logging purposes while retaining its audio performance.
It uses a passive adder and a gain of 2 to add a bias voltage to the signal
before an ADC input.

The sound card is one with external analog circuitry in a rack unit, it has
20 bit signal conversion, so this op-amp will have to be good to maintain
that and the other specs this unit has.

I looked first at a few audio amps and noticed that their claims for CMRR and
open-loop gain often fall well short of the claims made for the equipment
they go into, but never mind, that's another issue for another day.. :)

Then I looked at a DC instrumentation amp (OPA2277) I'm using in a laser
power meter design. If I can use it, it saves me buying varieties of
expensive chips in small quantities. Audio boffs high, wide and plentiful
will say don't do it, slew rate is slow, etc, but is it?? 0.8V/µS. It doesn't
sound a lot when people are saying I need 16V/µS or whatever, but I
calculated it, and it looks fine to me. The sound unit I'm adapting to is
considerably better than CD quality, sampling with 20 bits at up to 48 KHz,
and I calculated that this means a sample at intervals of a tad over 20 µS.
As 20 µS of 0.8V/µS is 16V, and as the device I'm adapting to has a ±15V
supply and a differential input design that halves the input, the largest
possible voltage change will occur, and fully settle, in the time between
samples at highest sample rate available.

As all the other figures for dynamic range and noise are so good that they
will allow the original specs for the entire unit to remain intact, is there
any reason I should not use this op-amp? It's a lot cheaper than any audio
amp that looks like it will do as well as this. And as I'm after DC as well
as AC capability, it seems that this is the right decision, but I'm
interested in other views before I decide anything. (I could just use
sockets, but for a low profile board I'll be soldering it in, and don't want
to have to mess with that later. :)

Hi, I usually reserve the name 'instrumentation amp' for those
differential input three opamp things that I use for bridge circuits.
That said, I do like the OPA277 for DC measurments. Do you need the
10uV offset? The OPA227 is a bit faster, but has a bit more DC offset
voltage.
Have you looked at the OPA134? There is also a newer version... I
think the number is OPA164??, (I couldn't get the TI website to work)
smaller voltage noise than the 134 but more cuurent noise and large
capacitance on the input. What's the source impedance driving the
opamp?

George H.

Instrumentation op-amp for DC-coupling to audio input?
 

"Marte Schwarz" wrote in -
berlin.de:

Hi,

I'm considering an op-amp for making a DC coupling adapter to a soundcard
to
convert it to signal logging purposes while retaining its audio
performance.
It uses a passive adder and a gain of 2 to add a bias voltage to the
signal
before an ADC input.

The sound card is one with external analog circuitry in a rack unit, it
has
20 bit signal conversion, so this op-amp will have to be good to maintain
that and the other specs this unit has.

So did you think about noise?


Yes, several times. This amp has THD+N of 0.00005% and is going into a device
specificied as 0.002%. Good enough? I think so.

As all the other figures for dynamic range and noise are so good that they
will allow the original specs for the entire unit to remain intact,

are you shure? What's about your supply?


I described that already, with regard to slew rate and maximum possible
voltage change in a given time at maximum sample rate, so yes, I think I
considered that too.

Instrumentation op-amp for DC-coupling to audio input?
 

George Herold wrote in news:c9ea2d68-021e-4387-8389-
:

Hi, I usually reserve the name 'instrumentation amp' for those
differential input three opamp things that I use for bridge circuits.
That said, I do like the OPA277 for DC measurments. Do you need the
10uV offset? The OPA227 is a bit faster, but has a bit more DC offset
voltage.

It makes life easier if I have that low offset. :) I'd also strongly prefer
to use what I have, it means I can buy more cheaply and try to encourage
makers to persist in making and selling certain devices by staying with
those. I also like dual-amp IC's a lot, I find them very practical to lay out
compact boards for them.

I see what you mean about the three-amp devices, with no compromise on input
resistance between the two inputs of a single amp. I guess I use the term
'instrumentation' fairly loosely, based on intended purpose rather than the
device itself.

Have you looked at the OPA134? There is also a newer version... I
think the number is OPA164??, (I couldn't get the TI website to work)
smaller voltage noise than the 134 but more cuurent noise and large
capacitance on the input. What's the source impedance driving the
opamp?


Good question, and one I've yet to follow up, my understanding of these
things has only just reached that bit.. I learned that two input noise
figures can be divided one by the other to find out the ideal source
impedance to feed a given input with, but I only read that last night, these
things take time to explore... So far I've always used the basic logic that
is usually applied to avoid precise impedance matching: make sure the source
is very low, and the input very high. This is apparently fine for readio
reception and most audio couplings, so I assumed I could do it too. I took
the idea further, I assumed that if I keep the adapter as simple as possible
I can reduce noise more than it will rise due to thermal noise in large
resistor values, hence I used a passive adder with 1Meg resistors. I can
change this to 100K perhaps, at risk of drawing more power. This method
already works fine in my power meter design so I guess it's ok here too.

Instrumentation op-amp for DC-coupling to audio input?
 

Lostgallifreyan wrote in
:

What's the source impedance driving the opamp?


Good question, and one I've yet to follow up, my understanding of these
things has only just reached that bit.. I learned that two input noise
figures can be divided one by the other to find out the ideal source
impedance to feed a given input with, but I only read that last night,
these things take time to explore... So far I've always used the basic
logic that is usually applied to avoid precise impedance matching: make
sure the source is very low, and the input very high. This is apparently
fine for readio reception and most audio couplings, so I assumed I could
do it too. I took the idea further, I assumed that if I keep the adapter
as simple as possible I can reduce noise more than it will rise due to
thermal noise in large resistor values, hence I used a passive adder
with 1Meg resistors. I can change this to 100K perhaps, at risk of
drawing more power. This method already works fine in my power meter
design so I guess it's ok here too.


I just found this via Google:
"Typical low-noise work with bipolar input transistors depends upon having a
low-impedance signal source to shunt and thus reduce noise developed in the
input transistor. The transistor noise flows out the preamp input into the
signal source output. That means the impedance between the signal source and
the preamp op amp must be low at the frequency of interest, say, a tenth of
the target impedance."

I guess that means my idea falls foul of ideal practise because the OPA2277
is a bipolar input device, and I usually do that passive adder trick with
high values and a JFET input amp. The laser power meter showed no apparent
noise problems despit using a differentiator in its design that might amplify
any noise problem that did exist, but even so, that can't be expected to hold
true for audio up to 24 KHz I guess. I still think it will work ok though,
I'll just not be soldering any IC's in till I've tested stuff.

There's an upgrade to the LF412 called AD712 that I liked in principle, but
its only ideally aimed at 12 bit systems, so while it will be fine for my
logging intents, it won't preservew the audio specs of ther device I'm
modifying..

I'll look up the OPA134 and OPA164 amps, but I think knowing more about the
limits imposed by context will tell me more about what I need to know that
looking up yet more op-amps. :)

Instrumentation op-amp for DC-coupling to audio input?
 

On Jul 13, 9:55*am, Lostgallifreyan wrote:
George Herold wrote in news:c9ea2d68-021e-4387-8389-
:

Hi, *I usually reserve the name 'instrumentation amp' for those
differential input three opamp things that I use for bridge circuits.
That said, I do like the OPA277 for DC measurments. *Do you need the
10uV offset? *The OPA227 is a bit faster, but has a bit more DC offset
voltage.

It makes life easier if I have that low offset. :) I'd also strongly prefer
to use what I have, it means I can buy more cheaply and try to encourage
makers to persist in making and selling certain devices by staying with
those. I also like dual-amp IC's a lot, I find them very practical to lay out
compact boards for them.

I see what you mean about the three-amp devices, with no compromise on input
resistance between the two inputs of a single amp. I guess I use the term
'instrumentation' fairly loosely, based on intended purpose rather than the
device itself.

Have you looked at the OPA134? *There is also a newer version... I
think the number is OPA164??, (I couldn't get the TI website to work)
smaller voltage noise than the 134 but more cuurent noise and large
capacitance on the input. *What's the source impedance driving the
opamp?

Good question, and one I've yet to follow up, my understanding of these
things has only just reached that bit.. I learned that two input noise
figures can be divided one by the other to find out the ideal source
impedance to feed a given input with, but I only read that last night, these
things take time to explore... So far I've always used the basic logic that
is usually applied to avoid precise impedance matching: make sure the source
is very low, and the input very high. This is apparently fine for readio
reception and most audio couplings, so I assumed I could do it too. I took
the idea further, I assumed that if I keep the adapter as simple as possible
I can reduce noise more than it will rise due to thermal noise in large
resistor values, hence I used a passive adder with 1Meg resistors. I can
change this to 100K perhaps, at risk of drawing more power. This method
already works fine in my power meter design so I guess it's ok here too.

Oh I've been doing all sorts of noise stuff lately. I don't do much A-
D, so if I make a mistake there I hope someone will correct me.

So 20 bits is about 10^6, if you have 10 volts full scale that means
10 uV is your LSB. So if your noise is much greater than 10uV you are
losing resolution. Lets do the voltage noise first and use the
opa277. The voltage noise is 8nV/rtHz. Your band width is maybe
100kHz? (Do you have any filtering before the A-D) So the rms noise
from the opamp will be about 8nV *sqrt (100k Hz) ~ 2.5 uV. That looks
fine. What about the Johnson noise of your 100 kohm resistor?
(forget the 1Meg!) it's got 40 nV/rtHz. or about 14uV of noise...
that's starting to have an impact... And now the current noise. The
current noise ofthe opa277 is 0.2pA/rtHz. (Hey that's pretty good for
a BJT front end) times 100kohm is 20nV/rtHz of voltage noise or
about 7uV rms. (assuming I guessed you bandwidth correctly.) To get
the total noise you have to add in quadrature. (sqrt of the sum of
squares) Which means it's only the big one that matters. The 14
uVrms from the 100k resistor. Reduce the resistance a bit more and
this looks fine. (unless I've made some hugh blunder.)

George H.

Instrumentation op-amp for DC-coupling to audio input?
 

George Herold wrote in news:a02bdb85-1047-4ba3-aaec-
:

On Jul 13, 9:55*am, Lostgallifreyan wrote:
George Herold wrote in news:c9ea2d68-021e-4387-83
89-
:

Hi, *I usually reserve the name 'instrumentation amp' for those
differential input three opamp things that I use for bridge circuits.
That said, I do like the OPA277 for DC measurments. *Do you need the
10uV offset? *The OPA227 is a bit faster, but has a bit more DC offse
t
voltage.

It makes life easier if I have that low offset. :) I'd also strongly pref
er
to use what I have, it means I can buy more cheaply and try to encourage
makers to persist in making and selling certain devices by staying with
those. I also like dual-amp IC's a lot, I find them very practical to lay
out
compact boards for them.

I see what you mean about the three-amp devices, with no compromise on in
put
resistance between the two inputs of a single amp. I guess I use the term
'instrumentation' fairly loosely, based on intended purpose rather than t
he
device itself.

Have you looked at the OPA134? *There is also a newer version... I
think the number is OPA164??, (I couldn't get the TI website to work)
smaller voltage noise than the 134 but more cuurent noise and large
capacitance on the input. *What's the source impedance driving the
opamp?

Good question, and one I've yet to follow up, my understanding of these
things has only just reached that bit.. I learned that two input noise
figures can be divided one by the other to find out the ideal source
impedance to feed a given input with, but I only read that last night, th
ese
things take time to explore... So far I've always used the basic logic th
at
is usually applied to avoid precise impedance matching: make sure the sou
rce
is very low, and the input very high. This is apparently fine for readio
reception and most audio couplings, so I assumed I could do it too. I too
k
the idea further, I assumed that if I keep the adapter as simple as possi
ble
I can reduce noise more than it will rise due to thermal noise in large
resistor values, hence I used a passive adder with 1Meg resistors. I can
change this to 100K perhaps, at risk of drawing more power. This method
already works fine in my power meter design so I guess it's ok here too.

Oh I've been doing all sorts of noise stuff lately. I don't do much A-
D, so if I make a mistake there I hope someone will correct me.

So 20 bits is about 10^6, if you have 10 volts full scale that means
10 uV is your LSB. So if your noise is much greater than 10uV you are
losing resolution. Lets do the voltage noise first and use the
opa277. The voltage noise is 8nV/rtHz. Your band width is maybe
100kHz? (Do you have any filtering before the A-D) So the rms noise
from the opamp will be about 8nV *sqrt (100k Hz) ~ 2.5 uV. That looks
fine. What about the Johnson noise of your 100 kohm resistor?
(forget the 1Meg!) it's got 40 nV/rtHz. or about 14uV of noise...
that's starting to have an impact... And now the current noise. The
current noise ofthe opa277 is 0.2pA/rtHz. (Hey that's pretty good for
a BJT front end) times 100kohm is 20nV/rtHz of voltage noise or
about 7uV rms. (assuming I guessed you bandwidth correctly.) To get
the total noise you have to add in quadrature. (sqrt of the sum of
squares) Which means it's only the big one that matters. The 14
uVrms from the 100k resistor. Reduce the resistance a bit more and
this looks fine. (unless I've made some hugh blunder.)

George H.


Thanks a lot, it's a good example for me to learn from. I don't know enough
to be sure but I worked it through and it looks right to me. I can certainly
lower the passive adder resistances, the only penalty is increased current
draw (there might be 8 or more channels of this), and I'm hoping to hitch a
ride on existing PSU's of anything I adapt this to, but I don't think they'll
begrudge me a couple of tens of mA.

BTW, I think the device I'm adapting already has noise enough to reduce its
claim to useful 20 bit depth but I appreciate the rigour, I wouldn't want to
be adding to the problem.

Instrumentation op-amp for DC-coupling to audio input?
 

George Herold wrote in news:a02bdb85-1047-4ba3-aaec-
:

Your band width is maybe
100kHz? (Do you have any filtering before the A-D)

Sorry, I didn't remember to directly answer that... They just have a
differential input stage that adds the two differential signals (doubles)
then quarters the output in the gain of the second of two amps they do this
with, so halving the total signal. After that it goes through a digitally
controlled analog variable resistor (CS3310-KS) followed by a DC blocking
capacitor and then an op-amp stage to make a differential signal for the ADC
(CS5335) with a 2.2nF cap with two 150R resistors to remove anything above a
few hundred KHz.

Apart from using that variable resistor IC they kept it as simple as they
could. Total dynamic range through the system is 98 dB, so only a tad more
than 16 bits worth, but aiming for better in my DC coupler board means I can
adapt other devices reliably.

Instrumentation op-amp for DC-coupling to audio input?
 

Lostgallifreyan writes:

I'm considering an op-amp for making a DC coupling adapter to a soundcard to
convert it to signal logging purposes while retaining its audio performance.
It uses a passive adder and a gain of 2 to add a bias voltage to the signal
before an ADC input.

The sound card is one with external analog circuitry in a rack unit, it has
20 bit signal conversion, so this op-amp will have to be good to maintain
that and the other specs this unit has.

Audio ADCs usually have bad DC specifications - why wouldn't they. You
may want to verify this before you try too hard to find the perfect
opamp.

[...]


--

John Devereux