AN-1208 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com Programmable Bidirectional Current Source Using the AD5292 Digital Potentiometer and the ADA4091-4 Op Amp CIRCUIT FUNCTION AND BENEFITS CIRCUIT DESCRIPTION The circuit in Figure 1 provides a programmable bidirectional Howland current source using the AD5292 digital potentiometer in conjunction with the quad ADA4091-4 op amp and the ADR512 voltage reference. This circuit offers 10-bit resolution over an output current range of ±18.4 mA. The AD5292 is programmable over an SPI-compatible serial interface. Table 1. Devices Connected/Referenced The ±1% resistor tolerance of the AD5292 allows it to be placed in series with external divider resistors, as shown in Figure 5, to reduce the maximum output current without the need to match the resistors in the circuit. Reducing the IOUT range serves to increase the sensitivity of the output current. ADR512 Product AD5292 ADA4091-4 This circuit employs the AD5292 digital potentiometer in conjunction with the ADR512 reference and the ADA4091-4 op amp, providing a 10-bit, programmable, bidirectional current source. The circuit guarantees monotonicity, ±1 LSB DNL, and has an integral nonlinearity of ±2 LSB, typical. The AD5292 has an internal 20-times programmable memory that allows a customized IOUT at power-up. The circuit provides accurate, low noise, and low tempco output voltage capability and is well suited for digital calibration applications. U1A Description Digital potentiometer, 10 bits, 1% resistor tolerance Micropower, overvoltage protected (OVP), rail-to-rail op amp Low noise, precision 1.2 V reference The bipolar high voltage regulator consists of a low voltage reference followed by a noninverting and an inverting amplifier whose gains are set by the ratio of R1 to R2.and R3 to R4. The ADR512 1.200 V voltage reference has low temperature drift, high accuracy, and ultralow noise performance. +V +15V 1/4 ADA4091-4 V1 +13.8V VDD –15V +15V –V VDD +15V RBIAS ADR512 3.9kΩ +1.2V R2 R1 RAB 20kΩ 16.5kΩ ± 1% 3/4 1.05kΩ ± 1% R6B 50Ω ± 1% C1 10pF IOUT R6A U1D 3/4 RLOAD ADA4091-4 –15V VSS R9 1kΩ R6 ADA4091-4 SERIAL INTERFACE U1B 10nF VW 10.5kΩ ± 1% 1kΩ ± 1% U1C AD5292 2/4 ADA4091-4 –V2 –13.8V R7 R8 16.5kΩ ± 1% 1.1kΩ ± 1% 1kΩ ± 1% R4 10pF 11.5kΩ ± 1% Figure 1. Programmable Bidirectional Current Source (Simplified Schematic: Decoupling and All Connections Not Shown) Rev. C | Page 1 of 3 08459-001 C2 R3 AN-1208 Application Note 20 (1) 10 IOUT (mA) As shown in Figure 1, the RBIAS resistor is 3.9 kΩ, which sets the bias current of the ADR512 at 3.5 mA. The ADA4091-4 is an op amp that offers low offset voltage and rail-to-rail output. The ADR512, in combination with the ADA4091, offers a low tempco and low noise output voltage. The R1 and R2 resistors adjust the gain in the noninverting amplifier, U1A. The output voltage, V1, defines the maximum positive output current range. Equation 2 and Equation 3 are used to calculate the resistor values. I OUT (2) V1 ≈ 1.33 × 10 − 3 0 –10 –20 08459-002 The maximum resistor that ensures an ADR512 minimum operating current is defined in Equation 1. V − 1.2 V RBIAS = DD 1.5 mA 500 0 1000 CODE (Decimal) Figure 2. Maximum Output Current Versus Decimal Code Typical INL and DNL plots are shown in Figure 3 and Figure 4. 1.5 (3) 1.0 The maximum negative output current range is adjusted by R3 and R4, which define the output voltage, V2, in the inverting amplifier, U1B. Equation 4 and Equation 5 are used to calculate the resistor values. I OUT 1.33 × 10− 3 V2 = 1.2 × (− R1 ) R2 INL (LSB) (4) –0.5 –1.0 (5) –1.5 0 The resistors, which are shown in Figure 1, are chosen to provide a gain of +11.5 and −11.5 in the noninverting and the inverting amplifier, respectively. This provides a bipolar regulated voltage of ±13.8 V. These voltages can be used to power other circuits with a maximum output current of +17 mA. = D × (V1 − V2 ) D × 27.6 + V2 = − 13.8 1024 1024 600 800 1000 800 1000 Figure 3. INL Versus Decimal Code 0.7 0.2 (6) –0.3 (1.05 kΩ + 50 Ω) × VW = 1.33 × 10 −3 × VW 16.5 kΩ × 50 Ω VW = 400 DNL (LSB) R6 A + R6 B × VW = R7 × R6 B 200 CODE (Decimal) Equation 6 and Equation 7 calculate the output current of the Howland current source, and Figure 2 shows the maximum IOUT versus code. I OUT = 0 08459-003 V2 ≈ 0.5 (7) 08459-004 R V1 = 1.2 × (1 + 1 ) R2 –0.8 0 200 400 600 CODE (Decimal) where D is the code loaded in the digital potentiometer. R6 A + R6 B = R8 (8) R5 = R7 (9) Figure 4. DNL Versus Decimal Code As shown in Figure 1, the bidirectional current source operates over the maximum output range of ±18.4 mA. To improve the circuit accuracy the maximum output current, IOUT, should be decreased by recalculating the resistor value in the U1C and U1D op amps or by reducing the voltage reference across the AD5292. This gives the full 10-bit resolution over a limited output current range. Rev. C | Page 2 of 3 Application Note AN-1208 The U1C and U1D op amp resistors can be recalculated using Equation 6 and Equation 7, but care should be taken to minimize errors when selecting standard resistor values from the calculated values. Decreasing the reference voltages, V1 and V2, in the AD5292 can be accomplished by recalculating the bipolar output regulator and the U1A and U1B output voltages or by using two external resistors, as shown in Figure 5. V1 +13.8V R10 17.8kΩ ± 1% V'1 +4.96V VW V'2 –4.96V R11 17.8kΩ ± 1% V2 –13.8V The ADA4091-2 dual op amp can be used when the voltage references, V1 and V2, are not necessary. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND," Analog Devices. MT-032 Tutorial, Ideal Voltage Feedback (VFB) Op Amp, Analog Devices. MT-087 Tutorial, Voltage References, Analog Devices. 08459-005 20kΩ The AD5291 (eight bits with 20-times programmable power-up memory) and the AD5293 (10 bits, no power-up memory) are both ±1% tolerance digital potentiometers that are suitable for this application. LEARN MORE AD5292 SERIAL INTERFACE COMMON VARIATIONS MT-091 Tutorial, Digital Potentiometers, Analog Devices. MT-095 Tutorial, EMI, RFI, and Shielding Concepts, Analog Devices. Figure 5. Improvement in Accuracy Using Reduced Reference Voltages (Simplified Schematic: Decoupling and All Connections Not Shown) The resistors in series with the AD5292 are useful when the voltage references, V1 and V2, are the main system power supplies. Traditionally, digital potentiometers have a ±20% end-to-end resistor tolerance error. This affects the circuit accuracy because of the mismatch error between the digital potentiometer and the external resistors. The industry leading ±1% resistor tolerance performance of the AD5292 helps to overcome the mismatch resistance error. MT-101 Tutorial, Decoupling Techniques, Analog Devices. Data Sheets AD5292 Data Sheet AD5291 Data Sheet AD5293 Data Sheet ADR512 Data Sheet The AD5292 has 20-times programmable memory, which allows presetting the circuit output current to a specific value at power-up. ADA4091-2 Data Sheet Excellent layout, grounding, and decoupling techniques must be used to achieve the desired performance from the circuits discussed in this note (see MT-031 Tutorial and MT-101 Tutorial). As a minimum, a 4-layer PCB should be used with one ground plane layer, one power plane layer, and two signal layers. REVISION HISTORY ADA4091-4 Data Sheet 4/13—Rev. B to Rev. C Changed Document Title from CN-0177 to AN-1208 .............................................................................. Universal 3/11—Rev. A to Rev. B Change to Figure 1 ............................................................................ 1 3/10—Rev. 0 to Rev. A Changes to Circuit Function and Benefits Section....................... 1 9/09—Revision 0: Initial Version ©2009–2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. AN08459-0-4/13(C) Rev. C | Page 3 of 3

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