ADLINK PXIe-9848 - Manual (Page 24)

14 Operations
As shown, in the signal channel analog input path of a digitizer,
each path provides 50 Ω input impedance or high impedance and
DC couple or AC couple. The gain amplifier is optimized for each
input range with low noise and high dynamic range. An anti-alias-
ing filter with a choice of 100MHz or 20MHz further eliminates high
frequency noise. The 14-bit ADC provides not only accurate DC
performance but also high signal-to-noise ratio and high non-spu-
rious dynamic range.
For auto-calibration, internal calibration provides stable and accu-
rate reference voltage to the AI.
Input Impedance Configuration
When acquiring high frequency signals (of at least MHz fre-
quency), to prevent reflection from the line, set “Characteristic
Impedance” to equal 50Ω from the signal source, such as a
high speed function generator, transmission line, such as a
coaxial cable, to the digitizer input. For this application, 50Ω
impedance switch is provided, matching the Characteristic
Impedance from the connector, PCB trace and input of the gain
amplifier equaling 50Ω.
If the signal source is not terminated with low impedance and
the transmission line is short, PXIe-9848 provides overall high
impedance of 1MΩ.
AC and DC Input Coupling
With DC coupling, DC offset present in the input signal is
passed to ADC, and is indicated if the signal source has a low
level of offset voltage or if DC content of the signal is important.
In AC coupling, the DC offset present in the input signal
is erased, and is indicated if the DC content of the input signals
is to be rejected. AC coupling enables a high pass R-C filter
through the input signal path. Corner frequency (-3dB) is about
10Hz.
100MHz and 20MHz Bandwidth
For applications that require lower bandwidth for low frequency
input signals, PXIe-9848 provides a bandwidth limit of 20MHz
to eliminate high frequency noise and raise the SNR ratio.