A stable 10-V reference is a valuable asset for calibrating oscilloscopes and other laboratory equipment. The 10-V reference was selected because it can be used in decade fashion (multiplied or divided by 10). One of the major requirements for a laboratory reference is not only initial accuracy but long-term stability. This requires precision low-drift components. An OP-07 bipolar operational amplifier was chosen because it has low offset and long-term stability. The offset voltage drift is approximately 0.3µ/oC. The OP-07 is excellent because of its low noise and high-accuracy amplification of very low-level signals. Figure 3-12 illustrates an accurate 10-V reference circuit.
The accuracy of the circuit can be enhanced by using precision resistors. The 1N4579A zener diode was chosen because of its 0.0005%/oC temperature coefficient. The resistor values were calculated from the following formulas:
Assuming a zener diode voltage of 6.4 V, resistors R2 and R3 total 10 kΩ from the 10-V output to ground. The values of R2 and R3 are calculated to have 6.4 V between their junction and ground. This voltage is applied to the inverting input of the OP-07. Resistor R1 has 0.002 A of current and a 3.6 V drop across it, hence a value of 1800 Ω. This establishes a stable reference at the noninverting input of the OP-07. If the output voltage moves either higher or lower, the operational amplifier holds it at 10 V. By using the recommended components good long-term stability at the desired output voltage can be expected. If other voltages are needed, they can be calculated with the same formulas. However, the output voltage can never be lower than the reference zener diode voltage. To compensate for zener and other component variations, a multiturn potentiometer may be used at the junction of R2 and R3.