APPLICATION BULLETIN ® Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132 Make a –10V to +10V Adjustable Precision Voltage Source By R. Mark Stitt, (602) 746-7445 Many situations require a precision voltage source which can be adjusted through zero to both positive and negative output voltages. An example is a bipolar power supply. Have you ever adjusted your unipolar lab supply down to 0V, then swapped the output leads and adjusted it back up to get a negative voltage output? What happened to your circuit when the input from the low impedance source went open circuit? Were you able to actually adjust the output to 0V, or did a small voltage offset limit the range? This precision bipolar voltage source can solve these problems. However, a simpler solution is to use a single voltage reference and a precision unity-gain inverting amplifier. If you use a precision difference amplifier for the unity-gain inverting amplifier, the circuit requires just two chips and a potentiometer. To understand how the circuit works, first consider the – 1.0V/V to +1.0V/V linear gain control amplifier shown in Figure 1. An INA105 difference amplifier is used in a unitygain inverting amplifier configuration. A potentiometer is connected between the input and ground. The slider of the pot is connected to the noninverting input of the unity-gain inverting amplifier. (The noninverting input of a unity-gain inverting amplifier would normally be connected to ground.) With the slider at the bottom of the pot, the circuit is a normal precision unity-gain inverting amplifier with a gain of –1.0V/V ±0.01% max. With the slider at the top of the pot, the circuit is a normal precision voltage follower with a gain of +1.0V/V ±0.001% max. With the slider in the center, there is equal positive and negative gain for a net gain of 0V/ V. The accuracy between –1.0V/V and +1.0V/V will normally be limited by the accuracy of the pot. Precision 10turn pots are available with 0.01% linearity. Perhaps the most obvious implementation of a bipolar voltage source would be to use a bipolar voltage reference. INA105 Difference Amp 2 VIN 5 R1 25kΩ 10kΩ Linear 3 R2 25kΩ R3 25kΩ 6 VOUT = +V IN to –V IN The –1.0V/V to +1.0V/V linear gain control amplifier has many applications. With the addition of a precision +10.0V reference as shown in Figure 2, it becomes a –10V to +10V adjustable precision voltage source. 1 R4 25kΩ VOUT = +VIN, Slider at Top of Pot. VOUT = 0V, Slider at Center of Pot. VOUT = –VIN, Slider at Bottom of Pot. FIGURE 1. –1.0V/V to +1.0V/V Linear Gain Control Amplifier. +VS 2 V+ INA105 Difference Amp REF102 10V 6 Out 2 Gnd 4 VOUT = +10V, Slider at Top of Pot. VOUT = 0V, Slider at Center of Pot. VOUT = –10V, Slider at Bottom of Pot. 10kΩ Linear 5 R1 25kΩ 3 R3 25kΩ R2 25kΩ 6 VOUT = +10V to –10V 1 R4 25kΩ FIGURE 2. –10V to +10V Adjustable Precision Voltage Source. 1990 Burr-Brown Corporation AB-006 Printed in U.S.A. April, 1990 resulting voltage drop adds to the required swing at the output of the op amp. Keep the voltage drop across R 3 low —less than 1V at full load—to prevent the amplifier output from swinging too close to its power-supply rail. In many instances adjustable voltage sources need the ability to drive high-capacitance loads such as power-supply bypass capacitors. The additional circuitry needed to drive high capacitance is shown in Figure 3. For stability, keep C LOAD•R3 < 0.5•R2•C2. Since access to the op amp inverting input is needed, the unity-gain inverting amplifier is made with an op amp and discrete resistors. For precision, R 1 and R2 must be accurately matched. Also, load current flows in R 3. The For applications with substantial volume (e.g. 5k ea/year) a version of the INA105 with the op amp inverting input brought out is available as a special (2A660). Inquire with marketing about price and delivery. +VS 2 V+ REF102 10V 6 Out +10V R1 10kΩ R2 10kΩ C2 Gnd 0.01µF 4 VOUT = +10V, Slider at Top of Pot. VOUT = 0V, Slider at Center of Pot. VOUT = –10V, Slider at Bottom of Pot. OPA27 10kΩ Linear R3 50Ω VOUT = +10V to –10V CLOAD FIGURE 3. –10V to +10V Adjustable Precision Voltage Source with High Capacitance-Load Drive Capability. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. 2