RE: [linrad] OSS and Delta 44

[Leif Åsbrink <leif.asbrink@xxxxxxxxxxxxxxxxxxxx]

Hello Niels,

> The i/p to the Delta 44 appears to be way too 
> high - I run ossmix on the card and I have tried 
> a couple of settings but nothing seems to change
> the input level. Any ideas / suggestions ?

Set the parameter gain.in1/2 to +4DB with ossmix.

Use the command ossmix -d0 or ossmix -d1 to get
a listing of the parameters.

Please observe that the maximum peak voltage into the
Delta44 is 10 V with this minimum gain setting.
This means that the maximum input signal is 3.5 V RMS.
Nothing is destroyed, but the AD saturates if you allow 
more input signal. I do not know how much overvoltage
you can apply without damage - but I never managed to
destroy anything here;)

With saturation at 3.5V RMS, the internal noise floor
of the Delta44 is 144 dB lower in 1 Hz bandwidth. 
(147 dB if you modify the board). If you send a 3kHz wide
passband into the Delta44 from the FT1000, the internal
noise floor of the Delta44 will be 144-35 = 109 dB
below 3.5V RMS in this bandwidth. This is about 13 
microvolts RMS.

You should make an attenuator by use of resistors to
bring the voltage down to a suitable level, something
like 20 dB above the internal noise floor of the
Delta44. That is 130 microvolts RMS !!

The FT1000 does of course not have the dynamic range 
to make full use of the Delta44. If you attenuate down to 
the 130 uV level, the FT1000 will not be able to come anywhere
near the saturation point of the Delta44, but the discussion
above was intended to illustrate principles. It becomes 
important when you increase the bandwidth and improve the
dynamic range.


73

Leif  /  SM5BSZ





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