DC/DC converter for a 16-bit ADC board

[[email protected]]

I am designing my first ever analog to digital board (USB) and have
run into multiple problems trying to gain a handle on sources of
noise. Recently I discovered that the DC/DC converter I am using (C&D
Technologies NMH0512SC) was a source of noise. This part takes +5V
and outputs +12 and -12V. I have been told either to change the DC/DC
I am using or filter the outputs of the current DC/DC mentioned above.

Of course there are other sources of noise within my circuit that I
will get to later but I am focusing on the DC/DC portion of it right
now. All help on this subject is welcome.

Thanks,
Aaron

 

[John Popelish]

I am designing my first ever analog to digital board (USB) and have
run into multiple problems trying to gain a handle on sources of
noise. Recently I discovered that the DC/DC converter I am using (C&D
Technologies NMH0512SC) was a source of noise. This part takes +5V
and outputs +12 and -12V. I have been told either to change the DC/DC
I am using or filter the outputs of the current DC/DC mentioned above.

Of course there are other sources of noise within my circuit that I
will get to later but I am focusing on the DC/DC portion of it right
now. All help on this subject is welcome.

Have you a link to a data sheet for the converter you are using?

 

[linnix]

Have you a link to a data sheet for the converter you are using?

Here it is:

http://www.cd4power.com/data/power/ncl/kdc_nmh.pdf

No info on switching frequency. We decided not to use a similar AM
radio transmitter @600 KHz. That's the main reason why we build it
out of discrete.

 

[Vladimir Vassilevsky]

I am designing my first ever analog to digital board (USB) and have
run into multiple problems trying to gain a handle on sources of
noise. Recently I discovered that the DC/DC converter I am using (C&D
Technologies NMH0512SC) was a source of noise. This part takes +5V
and outputs +12 and -12V. I have been told either to change the DC/DC
I am using or filter the outputs of the current DC/DC mentioned above.

Of course there are other sources of noise within my circuit that I
will get to later but I am focusing on the DC/DC portion of it right
now. All help on this subject is welcome.

It is generally a good idea to synchronize the DC/DC converter and the ADC
clock to the same timing reference (as well as all other clocks in the
system). This allows to filter out the noise. For example, if the DC/DC
operating frequency is the integer multiple of the ADC sample rate, the
noise will be converted to the constant DC offset, which is relatively easy
to get rid off.


Vladimir Vassilevsky
DSP and Mixed Signal Consultant
www.abvolt.com

 

[John Larkin]

I am designing my first ever analog to digital board (USB) and have
run into multiple problems trying to gain a handle on sources of
noise. Recently I discovered that the DC/DC converter I am using (C&D
Technologies NMH0512SC) was a source of noise. This part takes +5V
and outputs +12 and -12V. I have been told either to change the DC/DC
I am using or filter the outputs of the current DC/DC mentioned above.

Of course there are other sources of noise within my circuit that I
will get to later but I am focusing on the DC/DC portion of it right
now. All help on this subject is welcome.

Thanks,
Aaron

LC filter it, input and output! Those cheap dc/dc bricks are very
noisy.

How many bits of RMS noise are you seeing? For a fastish SAR adc, a
few bits should be possible, low 10's being more practical.

John

 

[[email protected]]

In the datasheet, for a 5V input, it says that the switching frequency
is typically 95KHz. My ADC clock sample rate is 10MHz. So the
switching frequency is not an integer multiple of the ADC clock.
Furthermore, this DC/DC is also not regulated. Should I switch to a
switching regulator instead?

I was also confused about isolation with respect to DC/DC converters.
My current DC/DC has isolation but I see many others that don't. Does
isolation refer to the potential difference of the input versus the
output or is it a method to isolate return paths for the input and
output voltages (ie: digital voltage return paths and analog voltage
return paths). In my case, does isolation matter or could I go either
way.

If any information is needed to further understand my problem, I am
more than happy to divulge.

Thanks,
Aaron