Supply delivers pin-programmable multiple references
Edited by Bill Travis
V Manoharan, Kochi, India -- EDN, May 1, 2003
In the circuit of Figure 1, the REF01, IC1, is a buried-zener-diode-based, precision 10V reference that features minimal noise and drift over temperature. The circuit provides not only the 10V output of the REF01, but also a 5V output that a REF02 reference would deliver. In addition, the circuit provides –5V, –10V, and an unbalanced dual reference, the sum of whose voltages is precisely 10V. In addition to the REF01, the circuit uses a highly precise, unity-gain inverting amplifier, IC2. Table 1 and Table 2 define the output voltages as a function of the jumper connections and as a function of the optional use of a REF02 reference in place of the REF01. In Figure 1, assume the use of a REF01 reference, and that Point 1 connects to Point 2. (Pin 4 of IC1 connects to ground.) IC2 inverts the 10V output of IC1 to deliver –10V at VOUT2.
Now assume that Point 1 connects to Point 3. (Pin 4 of IC1 connects to the output of IC2). If VOUT1 is at X volts, VOUT2 assumes a level of –X volts. The REF01 forces exactly 10V between its output and Pin 4. Therefore, X–(–X)=10, 2X=10, and X=5V. In this arrangement, 5V and –5V are simultaneously available at VOUT1 and VOUT2, respectively. To obtain precisely –5V at VOUT2, you must ratio-match R1 and R2 and also match their temperature coefficients. Now assume R2/R1=A and Point 1 connects to Point 3. In this case, the gain of the inverting amplifier is A. Therefore, VOUT1 and VOUT2 deliver unbalanced outputs, the sum of which is 10V. You can easily derive that VOUT1=10/(1+A) and VOUT2=–10A/(1+A).
The flexibility of this circuit eliminates the need to design and inventory several voltage sources. Moreover, the circuit can serve as a dual reference. The circuit finds application in D/A converters needing external references, portable instruments, digital multimeters, and A/D converters. It is advisable to use the ultralow-offset-voltage OP07 or ultralow-noise OP27 for the inverting amplifier.
