National Instruments 321645c-01 用户手册

Chapter 2

Function Reference — AO_Change_Parameter

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 National Instruments Corporation

2-33

NI-DAQ FRM for PC Compatibles

Some devices have attenuators after the final amplifier stage. By attenuating the output signal, 
you do not lose any dynamic range of the signal; 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 mdB) = - [20 log

10

 (V

o

/V

i

)]*1000

V

o

 = The voltage level that you want for the output signal.

V

i

 

= The input voltage level.

For DAQArb5411 devices, V

i

 = -5 to +5 V for terminated load and -10 to +10 V for 

unterminated load. For example, to change the output levels to –2.5 to +2.5 V into a 
terminated load, then: 

Attenuation = –[20*log

10

(

2.5/5)]*1000 = 6020 mdB

The 4451 and 4551 devices have three levels of attenuation providing voltage ranges of
–10 to + 10 V, –1 to +1 V, and –100 to +100 mV.

To change the output attenuation setting set paramID to 

ND_ATTENUATION

. You can change 

the attenuation at any time.

Some devices have an analog lowpass filter in their output stage. To correct for 
the abnormalities of this filter at a particular frequency, set paramID to 

ND_FILTER_CORRECTION_FREQ

. You can set the paramValue to 0 to disable the 

frequency correction for the analog filter. If you have disabled the analog filter, you also 
must disable the frequency correction. 

Table 2-11.  Parameter Setting Information for Output Attenuation

DAQArb AT-5411 
DAQArb PCI-5411

Yes

0 through 

74,000

0

4451 and 4551 devices

Yes

0, 20,000, 

40,000

0