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

In article ,
Lostgallifreyan scribeth thus
George Herold wrote in news:11271e1e-5ded-4bee-9852-
:

further. :) I doubt that any input I'll put into this system will have mu
ch
signal above audio band.

Oh, your opamps and resistors will have noise that goes out to MHz and
beyond. And will only be fall off when you reach the gain BW limit of
the opamp.


Yes, but I won't be adding to it in any obvious or fixable way, and the
systems I'm adapting a DC coupler to will already have filtering for their
signal converters unless they weren't designed well.

The TI website is now working for me. The 'new' audio opamp is the
OPA1641, 1642 and 1644. The 'audio' specs on these look pretty good.
(You should at least give them a glance.) The only down side is the
DC performance. The offset voltage is 0.5mV or 1 mV.

George H.


Thanks. I'll look at them, and if I can afford to, buy some at some point to
get used to them. I'm staying with OPA2277 for now though, as DC is what I
want, I just wanted to be sure it didn't let me down for AC, and my slew rate
and sample rate calculations suggest it won't. I'm not building high gain mic
preamps with them..

Is it possible to say more precisely what you are doing with this
application at all JOOI?...
--
Tony Sayer

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

tony sayer wrote in
:

In article ,
Lostgallifreyan scribeth thus
George Herold wrote in
news:11271e1e-5ded-4bee-9852- :

further. :) I doubt that any input I'll put into this system will
have mu
ch
signal above audio band.

Oh, your opamps and resistors will have noise that goes out to MHz and
beyond. And will only be fall off when you reach the gain BW limit of
the opamp.


Yes, but I won't be adding to it in any obvious or fixable way, and the
systems I'm adapting a DC coupler to will already have filtering for
their signal converters unless they weren't designed well.

The TI website is now working for me. The 'new' audio opamp is the
OPA1641, 1642 and 1644. The 'audio' specs on these look pretty good.
(You should at least give them a glance.) The only down side is the
DC performance. The offset voltage is 0.5mV or 1 mV.

George H.


Thanks. I'll look at them, and if I can afford to, buy some at some
point to get used to them. I'm staying with OPA2277 for now though, as
DC is what I want, I just wanted to be sure it didn't let me down for
AC, and my slew rate and sample rate calculations suggest it won't. I'm
not building high gain mic preamps with them..

Is it possible to say more precisely what you are doing with this
application at all JOOI?...

No. Without disclosing a diagram I have no right to pass on, there isn't. In
the posts here I described it in a lot of detail too, so I won't repeat
myself.

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

In article ,
Lostgallifreyan scribeth thus
tony sayer wrote in
:

In article ,
Lostgallifreyan scribeth thus
George Herold wrote in
news:11271e1e-5ded-4bee-9852- :

further. :) I doubt that any input I'll put into this system will
have mu
ch
signal above audio band.

Oh, your opamps and resistors will have noise that goes out to MHz and
beyond. And will only be fall off when you reach the gain BW limit of
the opamp.


Yes, but I won't be adding to it in any obvious or fixable way, and the
systems I'm adapting a DC coupler to will already have filtering for
their signal converters unless they weren't designed well.

The TI website is now working for me. The 'new' audio opamp is the
OPA1641, 1642 and 1644. The 'audio' specs on these look pretty good.
(You should at least give them a glance.) The only down side is the
DC performance. The offset voltage is 0.5mV or 1 mV.

George H.


Thanks. I'll look at them, and if I can afford to, buy some at some
point to get used to them. I'm staying with OPA2277 for now though, as
DC is what I want, I just wanted to be sure it didn't let me down for
AC, and my slew rate and sample rate calculations suggest it won't. I'm
not building high gain mic preamps with them..

Is it possible to say more precisely what you are doing with this
application at all JOOI?...

No. Without disclosing a diagram I have no right to pass on, there isn't. In
the posts here I described it in a lot of detail too, so I won't repeat
myself.

OK Fair comment!...
--
Tony Sayer

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

tony sayer wrote in
:

Is it possible to say more precisely what you are doing with this
application at all JOOI?...

No. Without disclosing a diagram I have no right to pass on, there
isn't. In the posts here I described it in a lot of detail too, so I
won't repeat myself.

OK Fair comment!...

Cool. Thought you might be going to argue. :) I can tell you this much... I'm
making a device that lets me turn a sound card into a logging tool. Assuming
the ADC's have reasonable DC performance, I can use it to map out changes
that are unique, too fast for a multimeter, too slow and unrepeatable for an
oscilloscope. By using something like Sound Forge, which has really nice
keyboard shortcuts for zooming, locating, selecting, etc, it makes fun easy
work of sifting through truly enormous amounts of data, so watching for all
sorts of drifting in meter circuits can be done. Further, you can do
experiments logging to one channel while speaking a commentary into the
other, so on playback you can know what you did that caused the responses you
see. This could be a very useful diagnostic tool, and very cheap.

All the earlier specifics are basically about trying to do this while not
screwing up the sound card performance, so the adapter can be left in
circuit.

Out of general interest, Sparkfun Electronics do a nice multichannel logger
called the Logomatic V2, which (with Kwan's firmware) can log two serial
inputs and 8 (10?) analog inputs at up to 1KHz at 12 bits, but if, for a
similar price, you can get two or more channels sampling at 48 KHz at 16
bits, it has to be worth trying... A lot of people have done this sort of
thing for laser show control by modifying the outputs, but I haven't seen
much to suggest anyone's doing it to inputs.

Should be enough here now, with the other posts, to allow anyone to figure
out what to do. Or come up with a better way, in which case, please post it.

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

Lostgallifreyan wrote:
tony sayer wrote in
:

Is it possible to say more precisely what you are doing with this
application at all JOOI?...
No. Without disclosing a diagram I have no right to pass on, there
isn't. In the posts here I described it in a lot of detail too, so I
won't repeat myself.
OK Fair comment!...

Cool. Thought you might be going to argue. :) I can tell you this much... I'm
making a device that lets me turn a sound card into a logging tool. Assuming
the ADC's have reasonable DC performance, I can use it to map out changes
that are unique, too fast for a multimeter, too slow and unrepeatable for an
oscilloscope. By using something like Sound Forge, which has really nice
keyboard shortcuts for zooming, locating, selecting, etc, it makes fun easy
work of sifting through truly enormous amounts of data, so watching for all
sorts of drifting in meter circuits can be done. Further, you can do
experiments logging to one channel while speaking a commentary into the
other, so on playback you can know what you did that caused the responses you
see. This could be a very useful diagnostic tool, and very cheap.

All the earlier specifics are basically about trying to do this while not
screwing up the sound card performance, so the adapter can be left in
circuit.

Out of general interest, Sparkfun Electronics do a nice multichannel logger
called the Logomatic V2, which (with Kwan's firmware) can log two serial
inputs and 8 (10?) analog inputs at up to 1KHz at 12 bits, but if, for a
similar price, you can get two or more channels sampling at 48 KHz at 16
bits, it has to be worth trying... A lot of people have done this sort of
thing for laser show control by modifying the outputs, but I haven't seen
much to suggest anyone's doing it to inputs.

Should be enough here now, with the other posts, to allow anyone to figure
out what to do. Or come up with a better way, in which case, please post it.

What I did was to buy a second-hand HP 35665A for $350. A boat anchor,
but a goodie.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

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

Phil Hobbs wrote in
:

What I did was to buy a second-hand HP 35665A for $350. A boat anchor,
but a goodie.


A bonny wee beastie. :) I like HP Agilent stuff, I have a nice 1740A scope
that people compare favourable with a Tektronix 265B scope but having seen
both, I think the HP wins tenfold.

But here, I bet I could pay the equivalent of $350 just getting that HP
35665A shipped to me. Joking.. but it does look heavy. I'll settle for the
adapted soundcard because the bang per buck is so good, and I can do it with
no added weight of gear, or extra space found for it. And I can use all kinds
of software to handle the data easily.

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

Although I got your idea exactly backwards, AFAICS now, it's
still true that your slew rate argument is bonkers, as far
as it goes. More or less for the same reason, exactly
backwards.

You seem to argue that, if the opamp is able to slew
full-scale between samples of the following audio ADC, then
the opamp's max slew rate must be adequate for audio input.
Why should this be true?

Perhaps someone could point me in the direction of
enlightenment?

Ian


"Ian Iveson" wrote in
message news:kj6%n.3277$xf1.2298@hurricane...

"Lostgallifreyan" wrote in message
. ..
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.

I'm struggling with this maths, perhaps because you've
left
quite a bit out. If you're amplifying before any
filtering,
why don't you need the same slew rate as the soundcard's
DAC? Take your worst case of a switch between max +ve and
max -ve from one sample to the next. That could happen
with
a 24kHz sine input. Assume the DAC outputs a slightly
slew-rate
limited square wave, so that a filter can then be used to
extract a sine wave, at a certain amplitude relative to
the
square wave. If your max slew rate is less than the DAC,
so
the square tends towards a triangle, then the amplitude of
the sine wave extracted by the same filter may be reduced.

Looked at another way, a 15Vpk 24kHz sine wave has a
maximum
slew rate of 2.26V/µS (?). In order for the filter to have
this output, what is the minimum slew rate required at its
input? Is it the same? Could be there's some law that
everyone knows but me. Anyway, in the limiting case of
your argument, where the time to "fully settle" is zero,
you would have a triangle wave, and the fundamental sine
would be considerably attenuated.

I also wonder if it's OK to actively use the slew rate
limit like this? Are there no penalties, like recovery
time or power dissipation? Presumably one stage within the
opamp consumes max current whenever it is in the process
if limiting slew rate.

All in all, this seems close enough so you need to make
sure, by going through each stage of the process from
input to filtered output, to make sure your design gets
from one to the other as well as the original.

Maybe the "10-1" rule of thumb applies somehow...works for
impedance.

Ian

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?
 

Here's a followup to this, because it seems to work, so if anyone's still
interested, they might like to know how it went...

First, I noticed noise, about 270µV of it, so I put a LPF filter in the
reference voltage (10K and 10µF ceramic). I also changed the buffer amp from
LF412 to another of the OPA2277A's I'm using. (The other channel buffers a
negative voltage for when my board is to remove a DC offset instead of adding
one). I also had to desolder a pin on the ADC and bend it to a conveniently
grounded pin next to it, and solder it there to disable an onboard digital
HPF.

I now have DC coupling, with noise on an empty channel within 3dB of best
unmodified system performance, which is better than I'd hoped. (-78.3dB as
opposed to -80.8dB originally).

Most of the existing DC offset is in the rest of the original system, I know
this because I can see it change as the device warms up, with all external
signals being absent or constant.

The DC offset remaining is around 700 values on a scale of 32768 so I'm ok
with that, especially as Sound Forge makes a truly neat way to remove it
immediately prior to record. It's so good that when detecting laser power on
the meter all this is aimed at testing, I won't need to tweak its own offset,
I can just record the output and do that in Sound Forge, as well as any extra
filtering I might want.

My conclusion is that modifying a decent studio audio interface for data
logging at arbitrary sample rates from 2000 Hz to 96000 Hz is well worth
doing. One ideal unit is the Echo Layla24, often found on eBay for less than
£100 now. Given the bang per buck, I prefer this to any other method because
I can still use it as a viable multichannel audio I/O when I want to.

(Incidentally, DC coupling on those units is even easier, as they don't have
DC on either side of the DC blocking caps, so just put a wire link where
those are now, and get accurate voltage generation up to around ±13.5V, with
fast and accurate changes, from wave file players or other software... All
kinds of uses for that, no doubt).

One last point: I can get decent audio band through an OPA2277 despite the
modest slew rate, but there are limits. Full scale differential input is
possible for sample rates up to 48 KHz, but for 96 KHz only non-balanced
input will allow this cleanly, so if the input is differential on a system
with a ±V supply, attenuate the signal by 6dB, or choose a faster low noise
amp. The low offset might not seem so important now, but the low noise and
drift still are.

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

Lostgallifreyan wrote in
:

so if the input is differential on a system
with a ±V supply, attenuate the signal by 6dB, or choose a faster low
noise amp.

Correction: 'with a ±15v supply'...