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From: Lostgallifreyan on 13 Jul 2010 08:48
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/�S. It doesn't
sound a lot when people are saying I need 16V/�S 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 �S.
As 20 �S of 0.8V/�S 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. :)
From: Marte Schwarz on 13 Jul 2010 09:12
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


From: George Herold on 13 Jul 2010 09:34
On Jul 13, 8:48 am, Lostgallifreyan <no-...(a)nowhere.net> 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.

From: Lostgallifreyan on 13 Jul 2010 09:43
"Marte Schwarz" <marte.schwarz(a)gmx.de> wrote in news:8a371iFcjgU1(a)mid.uni-
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.
From: Lostgallifreyan on 13 Jul 2010 09:55
George Herold <gherold(a)teachspin.com> wrote in news:c9ea2d68-021e-4387-8389-
a53e590bf3e7(a)b35g2000yqi.googlegroups.com:

> 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.
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