Measuring signals with spectrum analyzer

Hallo,

I tried to measure the noise of a DSP-output when no signal is applied
to the output. I measured the "rest"-noise with an oszilloscope. The
amplitude of the signal seems to be about 2mV. Then i wanted to
analyse the power spectrum of the noise. I connected the DSP-output to
a spectrum analyzer.
Now the problem is that i get different results when changing the
frequancy span of the spectrum analyzer. When i measure from 10Hz to
3.2KHz, the sprectrum of the measured noise lies at about -100dBVrms.
Increasing the span (10Hz to 25,6KHz) leads to a spectrum that lies at
about -90dBVrms. Compared to the previous measurement it is
approximately 10dB higher. Has anybody an explanation for this effect?
How can i ensure a precise measurement of the noise?

Thanks in advance

Jens

.

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