DDS Selection

On Jan 25, 12:18 am, John Devereux <j...@devereux.me.uk> wrote:
spflanze <art...@wavenet.org> writes:
On Jan 23, 10:37 pm, spflanze <art...@wavenet.org> wrote:

There has been mention in another reply of an LTZ1000 reference. I
have looked at the specs and it looks like a very good one. I plan to
use one them to voltage reference both the ADCs and the square wave
generators. Thanks for bringing this one up.

That was me I think.

Overkill for most applications, expensive, power-hungry and awkward to
use. But it is the king of references. It is still unbeaten after nearly
30 years for long term drift and probably noise too.

I changed my mind about it. Its voltage is too high.

spflanze <artist_at_wavenet.org> writes:

On Jan 25, 12:18 am, John Devereux <j...@devereux.me.uk> wrote:
spflanze <art...@wavenet.org> writes:
On Jan 23, 10:37 pm, spflanze <art...@wavenet.org> wrote:

[...]


There has been mention in another reply of an LTZ1000 reference. I
have looked at the specs and it looks like a very good one. I plan to
use one them to voltage reference both the ADCs and the square wave
generators. Thanks for bringing this one up.

That was me I think.

Overkill for most applications, expensive, power-hungry and awkward to
use. But it is the king of references. It is still unbeaten after nearly
30 years for long term drift and probably noise too.

I changed my mind about it. Its voltage is too high.

Like I said, awkward to use. That is the least of it.

There is LTC6655 if you are looking for low noise.

On Jan 23, 1:28 pm, spflanze<art...@wavenet.org> wrote:
On Jan 23, 8:03 am, George Herold<gher...@teachspin.com> wrote:





On Jan 22, 12:47 am, spflanze<art...@wavenet.org> wrote:

I am designing designing a system that uses changes in the
transmissivity light in a detection chemistry to detect gas. The light
source is a sine wave modulated LED source. After the light passes
through the detection chemistry it will be measured by a photodiode
and a transimpedance amplifier circuit. The output of the
transimpedance amplifier is measured by a 16 bit AD7606 ADC which does
a DMA data transfer into a Blackfin DSP where a Fourier Transform is
done to extract an amplitude at the LED's modulation frequency.

There will be six detection channels. Each channel is to be modulated
at a different frequency to reduce crosstalk. The length of time the
Fourier Transform is done over will be in multiples of 100ms for
maximum rejection of both 50Hz and 60Hz. The modulation frequencies
are chosen in multiples of the inverse of this length of time for
maximum crosstalk rejection.

Between the output of the transimpedance amplifier and the ADC is an
anti-aliasing filter. Its cutoff frequency and number of polls are
chosen so the attenuation at the ADC's sampling frequency is equal or
less than the ADC's LSB divided by the ADC's total number of states
(2^16).

The system needs an option for battery power so current draw is an
issue.

I need to choose a DDS chip to generate the sine wave reference for
the LED's modulation. I have been looking at 14 Bit DDS chips at:http://www.analog.com/ps/psthandler.aspx?pstid=10068&la=en
I notice there is a dramatic increase in power requirements the more
bits the DAC part of the DDS has. I would like to choose a 10 bit one
such as AD9838 that uses little power but I am concerned about the
quantization noise that appears as a spurs in the DDS output beginning
at twice the output frequency. That spur would be within the anti-
alias filter's passband. Theoretically this spur would appear right in
one of the notches of the sync function that is the frequency response
of a Fourier Transform and so would be taken out by it. In practice
can I actually count on it doing that? The DDS has the same frequency
reference as the ADC's sample rate so the spur would be precisely
located there.

Or do I need a reconstruction filter for the DDS that would have a cut
off between the modulation frequency and the first spur at twice this
frequency? I am sure it would need many poles. If so could the anti-
alias filter, although it is at the output of the transimpedance
amplifier instead of the DDS, double as a reconstruction filter if it
would have the same cutoff frequency and poles?

Fun discussion,
Sorry, but I can't help you with the DDS selection.
I'd fourth the lockin/ demod approach.
Are your six LED's all different wavlenghts?
(and if not why six?)

To the group... is there some point where it is advantageous to use
the FFT technique.
(what if he had more sources)
(I'm thinking of FIR spectroscopy and the Fellgett advantage.)
At least with one detection channel you don't have to worry about
channel to channel variation.

George H.

Not all the LEDs will be the same wavelength. Each will illuminate a
different chemistry. For those LEDs of the same wavelength it is too
difficult to split the beam to where the light needs to go.

Also it is known there will be cross talk. How much isn't known yet.- Hide quoted text -

- Show quoted text -

Cool, sounds like a fun project. One thing that stinks about LED's is
you don't get a lot of light out of them.

George H.

On 23/01/2012 21:07, George Herold wrote:





On Jan 23, 1:28 pm, spflanze<art...@wavenet.org>  wrote:
On Jan 23, 8:03 am, George Herold<gher...@teachspin.com>  wrote:

On Jan 22, 12:47 am, spflanze<art...@wavenet.org>  wrote:

I am designing designing a system that uses changes in the
transmissivity light in a detection chemistry to detect gas. The light
source is a sine wave modulated LED source. After the light passes
through the detection chemistry it will be measured by a photodiode
and a transimpedance amplifier circuit. The output of the
transimpedance amplifier is measured by a 16 bit AD7606 ADC which does
a DMA data transfer into a Blackfin DSP where a Fourier Transform is
done to extract an amplitude at the LED's modulation frequency.

There will be six detection channels. Each channel is to be modulated
at a different frequency to reduce crosstalk. The length of time the
Fourier Transform is done over will be in multiples of 100ms for
maximum rejection of both 50Hz and 60Hz. The modulation frequencies
are chosen in multiples of the inverse of this length of time for
maximum crosstalk rejection.

Between the output of the transimpedance amplifier and the ADC is an
anti-aliasing filter. Its cutoff frequency and number of polls are
chosen so the attenuation at the ADC's sampling frequency is equal or
less than the ADC's LSB divided by the ADC's total number of states
(2^16).

The system needs an option for battery power so current draw is an
issue.

I need to choose a DDS chip to generate the sine wave reference for
the LED's modulation. I have been looking at 14 Bit DDS chips at:http://www.analog.com/ps/psthandler.aspx?pstid=10068&la=en
I notice there is a dramatic increase in power requirements the more
bits the DAC part of the DDS has. I would like to choose a 10 bit one
such as AD9838 that uses little power but I am concerned about the
quantization noise that appears as a spurs in the DDS output beginning
at twice the output frequency. That spur would be within the anti-
alias filter's passband. Theoretically this spur would appear right in
one of the notches of the sync function that is the frequency response
of a Fourier Transform and so would be taken out by it. In practice
can I actually count on it doing that? The DDS has the same frequency
reference as the ADC's sample rate so the spur would be precisely
located there.

Or do I need a reconstruction filter for the DDS that would have a cut
off between the modulation frequency and the first spur at twice this
frequency? I am sure it would need many poles. If so could the anti-
alias filter, although it is at the output of the transimpedance
amplifier instead of the DDS, double as a reconstruction filter if it
would have the same cutoff frequency and poles?

Fun discussion,
Sorry, but I can't help you with the DDS selection.
I'd fourth the lockin/ demod approach.
Are your six LED's all different wavlenghts?
(and if not why six?)

To the group... is there some point where it is advantageous to use
the FFT technique.
(what if he had more sources)
(I'm thinking of FIR spectroscopy and the Fellgett advantage.)
At least with one detection channel you don't have to worry about
channel to channel variation.

George H.

Not all the LEDs will be the same wavelength. Each will illuminate a
different chemistry. For those LEDs of the same wavelength it is too
difficult to split the beam to where the light needs to go.

Also it is known there will be cross talk. How much isn't known yet.- Hide quoted text -

- Show quoted text -

Cool, sounds like a fun project.  One thing that stinks about LED's is
you don't get a lot of light out of them.

George H.

That's not really a valid generalisation for LEDs any more - there is a
wide selection of single colour LEDs with a few hundred mW of output power.

Right now I'm pretty impressed with the LEDEngin range and using some of
there parts in a couple of projects. I'll be building a driver soon to
control their 40 and 90W parts and modulate them at a few hundred Hz.

These higher power parts are multple die devices but very impressive.

cheers

David- Hide quoted text -

- Show quoted text -

Jamie wrote:

spflanze wrote:
I am designing designing a system that uses changes in the
transmissivity light in a detection chemistry to detect gas. The light
source is a sine wave modulated LED source. After the light passes
through the detection chemistry it will be measured by a photodiode
and a transimpedance amplifier circuit. The output of the
transimpedance amplifier is measured by a 16 bit AD7606 ADC which does
a DMA data transfer into a Blackfin DSP where a Fourier Transform is
done to extract an amplitude at the LED's modulation frequency.

There will be six detection channels. Each channel is to be modulated
at a different frequency to reduce crosstalk. The length of time the
Fourier Transform is done over will be in multiples of 100ms for
maximum rejection of both 50Hz and 60Hz. The modulation frequencies
are chosen in multiples of the inverse of this length of time for
maximum crosstalk rejection.

Between the output of the transimpedance amplifier and the ADC is an
anti-aliasing filter. Its cutoff frequency and number of polls are
chosen so the attenuation at the ADC's sampling frequency is equal or
less than the ADC's LSB divided by the ADC's total number of states
(2^16).

The system needs an option for battery power so current draw is an
issue.

I need to choose a DDS chip to generate the sine wave reference for
the LED's modulation. I have been looking at 14 Bit DDS chips at:
http://www.analog.com/ps/psthandler.aspx?pstid=10068&la=en
I notice there is a dramatic increase in power requirements the more
bits the DAC part of the DDS has. I would like to choose a 10 bit one
such as AD9838 that uses little power but I am concerned about the
quantization noise that appears as a spurs in the DDS output beginning
at twice the output frequency. That spur would be within the anti-
alias filter's passband. Theoretically this spur would appear right in
one of the notches of the sync function that is the frequency response
of a Fourier Transform and so would be taken out by it. In practice
can I actually count on it doing that? The DDS has the same frequency
reference as the ADC's sample rate so the spur would be precisely
located there.

Or do I need a reconstruction filter for the DDS that would have a cut
off between the modulation frequency and the first spur at twice this
frequency? I am sure it would need many poles. If so could the anti-
alias filter, although it is at the output of the transimpedance
amplifier instead of the DDS, double as a reconstruction filter if it
would have the same cutoff frequency and poles?

All that just to modulate a LED with sine wave output? Sounds a bit
over kill to me.

One item on the list you need to consider, the LED is not linear in
its transition to current verses emissions. The first item on the list
should be a driving circuit using a photo feed back to ensure you have a
linear representation of photons emitting through your chemistry.

Generating the SINE wave in short, can be done with out the use of DDS
technology, which just adds to the pile of over kill.

Using an analog method of generating that sine wave with a
synchronous signal for the receiver would be more to what I would expect
to use.

Jamie


Nah, LEDs are pretty linear above about 10 uA. I second the suggestion
to use square wave drive and a lock-in per channel, i.e. an AM radio
with synchronous detection. (AM radios are pretty good at adjacent
channel rejection, for such simple devices.) For instance, the OP can
pick drive frequencies of, say, 12, 13, 14, 15, 16, and 17 kHz, with 100
Hz-ish lowpass filters after the demodulator. One wants the low order
harmonics of each modulation frequency to miss each other, because that
helps a lot with the spurious signal reduction. If he doesn't want to
lose all that nice software work, he can write a program to find the
optimum channel assignments that minimize crosstalk.

Square wave lock-ins are sensitive to odd harmonics, but a reasonably
good lowpass filter after the TIA will get rid of those, as well as
noise contributions from their neighbourhoods.

Cheers

Phil Hobbs

On Jan 26, 3:34 pm, David<da...@nospam.com> wrote:
On 23/01/2012 21:07, George Herold wrote:





On Jan 23, 1:28 pm, spflanze<art...@wavenet.org> wrote:
On Jan 23, 8:03 am, George Herold<gher...@teachspin.com> wrote:

On Jan 22, 12:47 am, spflanze<art...@wavenet.org> wrote:

I am designing designing a system that uses changes in the
transmissivity light in a detection chemistry to detect gas. The light
source is a sine wave modulated LED source. After the light passes
through the detection chemistry it will be measured by a photodiode
and a transimpedance amplifier circuit. The output of the
transimpedance amplifier is measured by a 16 bit AD7606 ADC which does
a DMA data transfer into a Blackfin DSP where a Fourier Transform is
done to extract an amplitude at the LED's modulation frequency.

There will be six detection channels. Each channel is to be modulated
at a different frequency to reduce crosstalk. The length of time the
Fourier Transform is done over will be in multiples of 100ms for
maximum rejection of both 50Hz and 60Hz. The modulation frequencies
are chosen in multiples of the inverse of this length of time for
maximum crosstalk rejection.

Between the output of the transimpedance amplifier and the ADC is an
anti-aliasing filter. Its cutoff frequency and number of polls are
chosen so the attenuation at the ADC's sampling frequency is equal or
less than the ADC's LSB divided by the ADC's total number of states
(2^16).

The system needs an option for battery power so current draw is an
issue.

I need to choose a DDS chip to generate the sine wave reference for
the LED's modulation. I have been looking at 14 Bit DDS chips at:http://www.analog.com/ps/psthandler.aspx?pstid=10068&la=en
I notice there is a dramatic increase in power requirements the more
bits the DAC part of the DDS has. I would like to choose a 10 bit one
such as AD9838 that uses little power but I am concerned about the
quantization noise that appears as a spurs in the DDS output beginning
at twice the output frequency. That spur would be within the anti-
alias filter's passband. Theoretically this spur would appear right in
one of the notches of the sync function that is the frequency response
of a Fourier Transform and so would be taken out by it. In practice
can I actually count on it doing that? The DDS has the same frequency
reference as the ADC's sample rate so the spur would be precisely
located there.

Or do I need a reconstruction filter for the DDS that would have a cut
off between the modulation frequency and the first spur at twice this
frequency? I am sure it would need many poles. If so could the anti-
alias filter, although it is at the output of the transimpedance
amplifier instead of the DDS, double as a reconstruction filter if it
would have the same cutoff frequency and poles?

Fun discussion,
Sorry, but I can't help you with the DDS selection.
I'd fourth the lockin/ demod approach.
Are your six LED's all different wavlenghts?
(and if not why six?)

To the group... is there some point where it is advantageous to use
the FFT technique.
(what if he had more sources)
(I'm thinking of FIR spectroscopy and the Fellgett advantage.)
At least with one detection channel you don't have to worry about
channel to channel variation.

George H.

Not all the LEDs will be the same wavelength. Each will illuminate a
different chemistry. For those LEDs of the same wavelength it is too
difficult to split the beam to where the light needs to go.

Also it is known there will be cross talk. How much isn't known yet.- Hide quoted text -

- Show quoted text -

Cool, sounds like a fun project. One thing that stinks about LED's is
you don't get a lot of light out of them.

George H.

That's not really a valid generalisation for LEDs any more - there is a
wide selection of single colour LEDs with a few hundred mW of output power.

Right now I'm pretty impressed with the LEDEngin range and using some of
there parts in a couple of projects. I'll be building a driver soon to
control their 40 and 90W parts and modulate them at a few hundred Hz.

These higher power parts are multple die devices but very impressive.

cheers

David- Hide quoted text -

- Show quoted text -

Cool, Thanks. and in stock at Mouser. I'll have to order some of the
~$3 660nm ones... ~500mW! (I'll need a heat sink.)

http://www.ledengin.com/files/products/LZ1/LZ1-00R200.pdf

Run at low current ~200mA it's almost 25% efficient. (~100mW of
photons at 2.3Vf)

George H.

Another idea I will look into is to have a surface mount led mounted
over a hole in the PCB, and on the other side of the hole have a
surface mount monitoring photodiode. What I am not sure about is how
much light comes out the underside of a surface mount LED.

This also
means there will be no lens in the LED package, and I am not sure how
accepting the optics designer will be of that.