Digital I/O
NI-9237 and Voltage Divider
I'm a civil engineer, so please forgive if I misunderstand electrical engineering stuffs.
My NI-9237: 2 RJ50 ports (each port is connected to a 10-position connector) are connected to 1 Strain Gauge and 1 PR Accelerometer (both are Full Bridge). PR Accel sensitivity is 0.03 mV/V/g, and the NI-DAQ limitation is 25mV/V, thus I'm able to measure only to 25/ 0.03 = 833 g before the signal clips. However, I need to measure acceleration up to 1600 g.
As such, I have to add a Voltage Divider (350 + 350 ohm) to prior to the P+, P- excitation ports (G and E) of the Accel. No Divider is needed for the Strain Gauge (excitations are pin G
Without power, without load, I measured Resistance across G-K (future strain gauge) on the 10-position connector, it was supposed to be infinite, but my measurement is 350 ohm. Resistance across D-E (future Accel) was measured 263 ohm. So I back-calculated and found out that it looks like NI-9237 has a 700-ohm resistance there.
Thus the parallel resistance calculations are :
D-E: (1/1050 + 1/350)^-1 = 263 ohm (match measured value)
G-K: (1/700 + 1/700)^-1 = 350 (also match measured value)
I need to calculate how much Excitation is reduced when I plugged Accel and Strain Gauge on.
Accel input resistance = 6000 ohm.
My calculation is: Equivalent resistance (1/350 + 1/6000)^-1 = 330.7 ohm
Thus, on 1V excitation from NI-9237, the P+ P- on Accel shall be 1V * 330.7 / (350 + 330.7) = 0.4858
Thus, whatever voltage from S+ S- measured on the Accel, I need to multiply by a FACTOR of 1/ 0.4858 = 2.06 to get the actual acceleration.
I tested this "Voltage Divider" device with another Accel that does not require Voltage Divider, and I found out that the multiplying FACTOR needs to be 1.87 for the results to match with the non-divider accel.
Please note, I then attached my accelerometer to the system (twist-lock connectors), but stripped the wires so that I can measure voltage with DVM between P+P- while the load is connected.
Supplying Vex = Vgk = 1 V
Measured Vde = 0.486 V -> it matches perfectly with the theoretical calculation above.
I did induce different accelerations to see if Vde stay constant. It did stay constant at 0.486 V.
1) Why on S+S-, my output is INCREASED by approximately 9%? (thus overall Modifying Factor changes from 2.06 to 1.87).
(e.g., S+S- supposed to read 0.486mV so that when I multiply by the supposed 2.06 factor, I get the true 1mV of signal - assuming no Dividers connected,
but it actually read more at 0.535 mV, which causes the Modifying Factor to be 1.87).
I have another setup, where the Voltage divider is 1000ohm+1000oh. GK reads 917 ohm, D-E reads 733 ohm. Thus I was able to back calculate and found out the supposed "internal" resistance from NI-9237 is 1700 ohm. Based on this 1700 ohm, my calculation on GK is 919 ohm (very close to 917), and on DE is 730 ohm (very close to 733).
Additional question is:
2) why "internal" resistance is now 1700 ohm?
3) similar to above, when LOAD is attached, I'm expecting FACTOR of 2.17 to be multiplied to my acceleration output, but it appears a FACTOR of 1.94 will give reasonable matching results. Why this FACTOR does not match my calc (of 2.17)?
