Improving the noise floor of a Delta44 A/D converter. Measurements.
(Sept 3 2002)
When placing the computer in a metal box to avoid radio
interference a lot of low frequency interference, 1kHz and down
enters the Delta44 if the screen of the cable from the Delta44
is allowed to be in electrical contact with the wall of the box.
This is an indication of significant potential differences between
different parts of the (double) metal case surrounding the computer.
Some of this interference is present also when nothing is connected to
the Delta44 or when the inputs are grounded right at the d-sub
connector of the board.
Before taking proper action to avoid currents on the screen of the
audio cable I decided to try to understand how the interference
reaches the input when nothing is connected and to eliminate
the interference if possible.
There are three different modifications which together eliminate
the low frequency spurs and lower the noise floor by a little more
When using Delta44 as the A/D converter for a software radio 3dB
is a significant improvement so it is worthwile to do these
The board is labelled M-AUDIO DELTA 1999 Rev-B
Fig. 1 shows the Delta44 in its original shape.
Figures 2 to 4 show the effect of the modifications when they
are added one by one.
Figures 1 to 4 have the same colour scale in the waterfall graph
while Fig.5 shows the board in its final state, same as fig.4
but here the zero point of the waterfall graph is changed by 3dB
to give a quantitative measure of the qualitative differences
shown in the previous figures.
The noise floor can be lowered by one more dB by
bypassing the input circuitry
of the Delta44.
Here are some measurements on
the dynamic range of modified Delta 44
showing the performance when the input amplifier/filter is still in place.
The Delta44 in its original shape.
The screen is produced by linrad operating in direct conversion
mode for two radio channels so the spectra contain contributions
from all four audio channels.
At 48.000kHz the audio frequency is zero and no signal comes
out from the A/D converter at this frequency because
the board is AC coupled.
Low frequency noise is shown in the baseband graph with high
Besides 1/F noise one can see an overtone rich signal around
There is also a spur at about 750Hz, it is best seen in the
normal spectrum where it is about 3dB above the noise floor.
The first modification.
A 2.5V reference voltage is generated by a 7805 followed
by a voltage divider and a capacitor.
This capacitor is inadequate and so is the capacitor
connected directly to the output of the 7805.
These capacitors are C25 and C61 and fig.2 shows the noise floor
when both of them are increased to 470 microfarad. (The
original value is 10 microfarad.)
Not even 470 microfarad is quite enough, a very small but clearly
observable improvement can be observed when these capacitors are
made even bigger. I am using 3300 microfarads.
The second modification.
The 15 pin d-sub connects the outer screen and the chassis
potential to analog ground on the A/D board.
This is a design error.
The reference zero of the A/D converter is taken from one pin on the
bus and taking it from a second place as well introduces currents
in the analog ground reference with interference pick up
as a consequence.
Disconnecting the grounding to the metal back-plate at the d-sub
gives the performance of fig. 3.
The strong spurs have disappeared - but there are some new although
very weak spurs.
The third modification.
The A/D converters use 2.5V and a 5V DC. For each chip both
voltages are decoupled with 10 microfarads.
When these capacitors are increased to 470 microfarads the
noise floor is lowered just a little and the spurs are eliminated.
The capacitors are C45, C47, C51 and C53.
Same as fig. 4 but with the zero point for the waterfall
lowered by 3dB.
Compare to fig.1! It is quite clear that the improvement
is over 3dB - and all spurs are gone.