Fluke Biomedical 07-487 - Manual (Page 9)

Calibration
Bias Adjustment
2
2-1
Section 2
Calibration
2.1 Bias Adjustment
With no input signal, connect the oscilloscope probe to pin 7 of the CA3160E Operational Amplifier, U1,
which feeds the 555 relaxation oscillator. Set the scope for 1 v/cm and 100 usec/cm. Adjust the bias
potentiometer, R12, until a complex sawtooth pulse waveform is seen on the scope. Rotate the
potentiometer CCW so that the oscillation stops and the indicated voltage just returns to 0 and then about
5 degrees farther. This will be adjusted again during the final calibration phase.
2.2 Use of Precision Calibrator
The special calibration tool uses a crystal controlled gate circuit feeding a constant current generator.
Gate times are 0.50 and 1.00 sec and currents are 75, 300, 450, and 750 mA (approx.). The calibrator
provides precise gate times, but the mA values must be measured and listed with a precision of 0.1%
before use. The mA measurement can be made with a precision milliampere meter with the gate circuit
bypassed. The calibration values and limits given below assume that the measured mA values are
exactly as above. In practice, the calibration limits will depend on the measured mA values. When a
decimal point is shown in the calibration value, the range switch of the mAs meter should be set to the
high sensitivity range.
Use the calibrator to inject a 75 mA signal for 1.00 sec. Set the calibration potentiometer, R14, for 74.3 ±
0.2 mAs (0.7 mAs less than the measured value) displayed and reset the bias potentiometer, R12, for a
return to zero time of the signal at U1-7 of 0.5 to 1.0 sec. Reset the meter after each test.
Inject signals of 75, 300, 450, & 750 mA for 0.500 and 1.00 sec and record the readings. Adjust the
calibration potentiometer, R14, if necessary to keep the precision within the required range. Record the
final values on the calibration sheet for the meter. The most common causes of failure to meet the
calibration standards are: defective input CA3160E, defective C6, or improper bias adjustment.
2.3 Alternative Calibration Method
A calibrator can be built out of common laboratory instruments and can be used when the recommended
calibrator is not available. The alternative calibrator can be assembled using a medium voltage power
supply, some resistors, a relay, a precision milliammeter, an interval counter, and a few common parts.
A pushbutton switch is used to cause the electrolytic capacitor to discharge through the relay which
closes for a period of between 0.30 and 1.1 sec. The value of the capacitor, relay load resistor, and the
relay must be selected to set that interval. The interval counter must read the period in increments of less
than 0.001 sec. Resistors are chosen for current values close to those defined above. The calibration
proceeds in the same way as described previously.