Measurement Computing USB-2404-10 - Manual (Page 13)

USB-2404-10 User's Guide Functional Details
13
When making a ground-referenced connection between the signal source and the USB-2404-10, make sure the
voltage on the CH+ and CH– connections are within the channel-to-earth safety voltage range. Refer to the
Specifications chapter for information about operating voltages and overvoltage protection.
Figure 6. Connecting a floating signal source
Don't connect to signals or use for measurements within category III or IV
Refer to the "Safety voltages" section in the Specifications chapter for more information about Measurement
Categories.
Analog input circuitry
The USB-2404-10 analog input channels are floating with respect to earth ground and to each other. The
incoming analog signal on each channel is conditioned, buffered, and then sampled by a 24-bit Delta-Sigma
ADC. Each channel provides an independent signal path and ADC, enabling you to sample all four channels
simultaneously. Figure 7 shows the circuitry for one analog input channel.
Figure 7. Analog input circuitry for one channel
Filtering
The USB-2404-10 uses a combination of analog and digital filtering to provide an accurate representation of
in-band signals while rejecting out-of-band signals. The filters discriminate between signals based on the
frequency range, or bandwidth, of the signal. The passband, stopband, and alias-free bandwidths are important.
The USB-2404-10 represents signals within the passband frequency, as quantified primarily by passband ripple
and phase nonlinearity. All signals within the alias-free bandwidth are either unaliased signals or signals that
have been filtered by at least the amount of the stopband rejection.
Passband bandwidth
The signals within the passband bandwidth have frequency-dependent gain or attenuation. The small amount of
variation in gain with respect to frequency is called the passband flatness. The device's digital filters adjust the
frequency range of the passband to match the data rate. Therefore, the amount of gain or attenuation at a given
frequency depends on the sample rate. Figure 8 shows typical passband flatness for the 50 kS/s sample rate.
Signal
Source
+
CH+
CH-
USB-2404-10
USB-2404-10
CH+
CH-
Overvoltage
Protection
Prefilter
ADC
Amplifier