National Instruments 5411 - Manual (Page 45)

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 log
10
(V
o
/V
i
)
where,
V
o
= desired voltage level for the output signal
V
i
= input voltage level.
Note: For the DAQArb 5411, V
i
= ±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*log
10
(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 M
Ω
), 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