Make a –10V to +10V Adjustable Precision Voltage Source

®APPLICATION BULLETINMailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132Make a –10V to +10V Adjustable Precision Voltage SourceBy R. Mark Stitt, (602) 746-7445Many situations require a precision voltage source whichcan be adjusted through zero to both positive and negativeoutput voltages. An example is a bipolar power supply.Have you ever adjusted your unipolar lab supply down to0V, then swapped the output leads and adjusted it back up toget a negative voltage output? What happened to your circuitwhen the input from the low impedance source went opencircuit? Were you able to actually adjust the output to 0V, ordid a small voltage offset limit the range? This precisionbipolar voltage source can solve these problems.Perhaps the most obvious implementation of a bipolar voltagesource would be to use a bipolar voltage reference.V ININA105Difference Amp25R 125kΩR 225kΩR 3625kΩ3V =10kΩLinear1R 425kΩV OUT= +V IN, Slider at Top of Pot.V OUT= 0V, Slider at Center of Pot.V OUT= –V IN, Slider at Bottom of Pot.OUT+V IN to –V INHowever, a simpler solution is to use a single voltagereference and a precision unity-gain inverting amplifier. Ifyou use a precision difference amplifier for the unity-gaininverting amplifier, the circuit requires just two chips and apotentiometer.To understand how the circuit works, first consider the –1.0V/V to +1.0V/V linear gain control amplifier shown inFigure 1. An INA105 difference amplifier is used in a unitygaininverting amplifier configuration. A potentiometer isconnected between the input and ground. The slider of thepot is connected to the noninverting input of the unity-gaininverting amplifier. (The noninverting input of a unity-gaininverting amplifier would normally be connected to ground.)With the slider at the bottom of the pot, the circuit is anormal precision unity-gain inverting amplifier with a gainof –1.0V/V ±0.01% max. With the slider at the top of thepot, the circuit is a normal precision voltage follower with again 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 normallybe limited by the accuracy of the pot. Precision 10-turn pots are available with 0.01% linearity.The –1.0V/V to +1.0V/V linear gain control amplifier hasmany applications. With the addition of a precision +10.0Vreference as shown in Figure 2, it becomes a –10V to +10Vadjustable precision voltage source.FIGURE 1. –1.0V/V to +1.0V/V Linear Gain Control Amplifier.+V S2V+REF102Gnd410VOut610kΩLinear23R 125kΩR 325kΩINA105Difference AmpR 225kΩ56V OUT =+10V to –10VV OUT= +10V, Slider at Top of Pot.V OUT= 0V, Slider at Center of Pot.V OUT= –10V, Slider at Bottom of Pot.1R 425kΩFIGURE 2. –10V to +10V Adjustable Precision Voltage Source.© 1990 Burr-Brown Corporation AB-006 Printed in U.S.A. April, 1990

In many instances adjustable voltage sources need the abilityto drive high-capacitance loads such as power-supply bypasscapacitors. The additional circuitry needed to drive highcapacitance is shown in Figure 3. For stability, keep C LOAD•R 3< 0.5•R 2•C 2. Since access to the op amp inverting input isneeded, the unity-gain inverting amplifier is made with anop amp and discrete resistors. For precision, R 1and R 2mustbe accurately matched. Also, load current flows in R 3. Theresulting voltage drop adds to the required swing at theoutput of the op amp. Keep the voltage drop across R 3low—less than 1V at full load—to prevent the amplifier outputfrom swinging too close to its power-supply rail.For applications with substantial volume (e.g. 5k ea/year) aversion of the INA105 with the op amp inverting inputbrought out is available as a special (2A660). Inquire withmarketing about price and delivery.2V+REF10210VOut6+10VR 110kΩR 210kΩC 2V OUT= +10V, Slider at Top of Pot.V OUT= 0V, Slider at Center of Pot.V OUT= –10V, Slider at Bottom of Pot.Gnd410kΩLinear0.01µFOPA27R 350Ω+V SV =OUT+10V to –10VC LOADFIGURE 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 assumesno 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 changewithout 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 warrantany BURR-BROWN product for use in life support devices and/or systems.2