Chapter3 DigitizerBasics
©NationalInstruments Corporation 3-9 NI5102 User Manual
Analytically,obtaining a flat frequency response means:
Rin/(Rin+R
p)=Cp/(C
p+C
in +C
c)
It can be shownthat:
Rin(Cin +C
c)=C
pRp
ort het ime constant of the probe equals the time constant of the digitizer
input.
How to Compensate Your Probe
Adjusting the tunable probe capacitor to get a flat frequency response
is called probe compensation. On the NI 5102, you can select a 0–5V,
1kHzpulsetrainasreferencetooutputonPFI1orPFI2.Calling
niScope_ProbeCompensationin LabVIEW, how ever, will only allow
output to PFI1. Refer to Figure 3-7 as you follow these instructions to
compensate your probe:
1. Connect the BNC end of the probe to an input channel, either CH0
orCH 1 and select X10 attenuation on the body of the probe tip.
2. Attachthe BNC adapter (probeaccessory) to the tip of the probe.
3. Connectthe SMB100 probe-compensation cableto one of the
PFIl ines. On the NI 5102 (PXI) this line is PFI1.
4. Attachthe probe with the BN C adapter to the BNC female end of the
SMB100 cable.
5. Enablethe probe compensation signal onthe PFI line you selected in
step 3.See your application software documentation for more
information howto perform t his step.
6. Digitize data on the input channel, amplifying the signal until the
signalstarts to clip. Then go back one step so it does not clip anymore.
This step ensures thaty ou use the main dynamic range of the ADC.
7. Adjustthe tunable capacitor to make the waveform look as square as
possible.
8. Forthe most accurate measurements, compensate probes for each
channel(CH0 and CH1) and use them on that channel only.
Recompensate whenusing thesame probe on a different channel.