Chapter 4 Arb Operation
DAQArb 5411 User Manual 4-20
©
National Instruments Corporation
Figure 4-19. Output Attenuation Chain
By attenuating the output signal, you keep the dynamic range of the
DAC; that is, you do not lose any bits from the digital representation of
the signal because the attenuation is done after the DAC and not
before it.
attenuation (in decibels) = –20 log10 (Vo /Vi)
where,
Vo = desired voltage level for the output signal
Vi = input voltage level.
Note: For the DAQArb 5411, Vi = ±5 V for terminated load and ±10 V for
unterminated load.
For example, to change the output level to ±2.5 V into a terminated
load, use the following formula:
Attenuation = –20*log10 (2.5/5) = 6.020 dB
Note: You can change the output attenuation at any time during waveform
generation.
Output Impedance
As shown in Figure 4-17, before the signal reaches the output
connector, you can select the output impedance to be 50 Ω or 75 Ω. If
the load impedance is 50 Ω and all the attenuators are off (that is, an
output attenuation of 0 dB), the output levels are ±5 V.
A load impedance of 50 Ω is used for most applications but 75 Ω is
required for applications such as testing video devices. If the load is a
very high input impedance load (~1 MΩ), you will see output levels up
to ±10 V.
Note: You can change the output impedance at any time during waveform
generation.
1 dB 16 dB
2 dB 4 dB 8 dB 32 dB
Out
In