input capacitance of scxi-1313

[NAUSHICA]

Is there any value for input capacitance when SCXI-1125 is used with SCXI-1313?
Someone told me that when you want to measure pure resistances using high
frequency injection currents, input capacitance of the acquisition board  together
with capacitances of the cables used to measure may appear as a pure resistive
load therefore giving you a value different than what you are trying to measure.

Can some one help me in this matter?

Rg.

Naushica

[Kurt_NISalesEngineer]

Hi,

I know that for example thermocouples are typically considered low impedance sources, while SCXI signal conditioning modules usually have very high input impedances. i.e. SCXI modules designed and recommended for use with thermocouples have input impedances of 1 Gohm or more.

For the 1125:
Page 47 of 71 specify's the amplifiers input impedance 
Input impedance is defined as the measured resistance and capacitance between the input terminals of a circuit

The gain of the 1125 won't affect the input impedance. The effect perhaps could be driven by the 1313? for example page 44 0f 71 says:
The SCXI-1327 does reduce the overall input impedance to 1 MOhm, but this is acceptable in most applications. Other SCXI-1125 analog specifications are also affected by the SCXI-1327. The documentation does not mention 1313.

Can you tell me more about how the person who told you this came to their conclusion? What are you attempting to measure?

Kind Regards,

Kurt
Applicaitons Engineer
NIUK

Message Edited by Kurt NI AE on 11-07-2006 12:15 PM