When a Burr-Brown OPA2277 op-amp is used with a NI 6052E and a sensor with a 1 MW source impedance, the NI 6052E acquires data at a gain of 0.5, which provides an input range of -10 to +10 V. Using these settings, 1 LSB is equivalent to 305 mV.
Determine the added noise by calculating :Where:
The new system noise can now be calculated. Note that the added op-amp noise must be counted twice for a differential measurement. Where:
The increased system noise due to the addition of the unity gain buffer to the system can be compensated for by increasing the number of points that you average when making a DC measurement. Averaging improves the relative accuracy of measurements.
Calculate the new system DC specifications:
B = (1 MW)(1 nA) = 1 mV or 6.41 LSBs
For this configuration, the bias offset voltage is over 6 LSBs. However, the offset voltage drift for this op-amp is +/- 0.25 mV/oC which means that DC offset error can be calibrated out of the system if the system temperature remains relatively constant. Calculate the Unity Gain Offset Error (UGOE). Be sure to multiply RsIB by 2 for a differential measurement, if needed:
UGOE (V) = V
os + R
B = 1.025 mV
Where Vos = Op-Amp Input Offset Voltage = 25 mV.
To determine the new absolute accuracy of the system, the UGOE must be added to the absolute accuracy of the NI 6052E. Absolute accuracy is the specification that you use to determine the overall maximum error of the measurement. The absolute accuracy of the NI 6052E at +/- 10V is 4.747 mV.
Absolute Accuracy at Full Scale = 6052E Absolute Accuracy + UGOE = 5.772 mV
Depending on the individual system requirements, the increased amount of error may or may not be acceptable for the measurement system.