Chapter4 HardwareOverview
©NationalInstruments Corporation 4-7 NI5102 User Manual
The CH0, CH1,and TR IG channelshave a software-programmable
coupling selectionb etween ACand DC. Use AC coupling when your
ACsignal contains a large DC component. Without AC coupling, it is
difficultto view details of the AC component with a large DC offset and
asm all ACcomponent, such as switching noise on a DC supply.If you
enableAC coupling, you remove the large DC offset for the input amplifier
andamplify only the AC component. This technique makes effective use of
dynamic range to digitize the signal of interest.
Thelow-frequency corner in an AC-coupled circuit is the frequency below
which signals are attenuated by at least 3 dB.The low-frequencycorner
is 11 Hz with an X1 probe, 1.1 Hz with an X10 probe, 0.11 Hz with
anX100 probe, and 4 Hz with an X1000 probe.
Whenchan ging coupling on the NI 5102 digitizers, the input stage takes a
finitetime to settle, as shown in Table 4-3.
Caution Whenswitching coupling from DC to AC, returned data is accurate about 20 time
constants after switching to AC.This delay is based on switching to AC and, at the same
time,switching from a gain of 1 to a gain of 100. NI-SCOPE does not provide the delay to
accountfor settling time; therefore, acquisitions immed iately followinga coupling c hange
may yield incorrect data.
Table4-3. AC/DC Coupling Change Settling Rates with NI Probes
Action Time Constant
Switching from ACto DC settling ti me 0.5 ms
Switching from DC to AC
X1 probe time constant
X10probe time constant
15 ms
150 ms