NI-9237 and Voltage Divider

[ngthai]

I'm still limited to make the best out what i have. Using Voltage Divider, I have a quite consistent Modified Factor = Theoretical Factor / 1.11
Rd = Divider Resistance (e.g., 350)
RL= Load Resistance
RdL = Divider+Load leg resistance = (1/Rd + 1/RL)^-1
Applied Excitation (original) = 1V
Updated Excitation on that leg = 1 * RdL  / (Rd + RdL) and as I mentioned earlier, I checked using DVM and this Updated Excitation is matching with calculation.
Thus, theoretically signal will be reduced to this value: Vout_theoretical = Vout_supposed_to_if_no_Divider * RdL  / (Rd + RdL).
               Theoretical Factor = (Rd + RdL) / Rdl
But in reality, Modified Factor = Theoretical Factor / 1.11
I just need to be able to track down why on Signal, it is quite consistent that the signal is increased by about 9% to 11% (compared to the Vout_supposed_to_if_no_Divider)

[santo_13]

Since, you're adding external components (whose performance is not characterized), your accuracy specifications are no longer valid for the units of measure as your external components is altering the performance unpredictably (a case you already noticed why your calculations don't match the actual measurement).

If you're relying on these measurements for internal, non-critical observations, it may be fine. But if you're going to rely on these numbers to characterize or design other systems or guarantee some numbers to customers, I advise against doing so.