Circuit Note CN-0370 Devices Connected/Referenced Serial-Input, Voltage Output, Unbuffered AD5542A Circuits from the Lab reference designs are engineered and 16-Bit DAC tested for quick and easy system integration to help solve todays Rail-to-Rail Input/Output, Zero Input analog, mixed-signal, and RF design challenges. For more ADA4500-2 Crossover Distortion Amplifier information and/or support, visit www.analog.com/CN0370. Ultralow Noise, High Accuracy, 2.5 V ADR4525 Voltage Reference 16-Bit, Single-Supply LED Current Driver with Less than 1 LSB Integral and Differential Nonlinearity and differential nonlinearity and has a 0.1 Hz to 10 Hz noise of EVALUATION AND DESIGN SUPPORT less than 45 nA p-p for a full-scale output current of 20 mA. Circuit Evaluation Boards CN-0370 Circuit Evaluation Board (EVAL-CN0370-PMDZ) The innovative output driver amplifier eliminates the crossover System Demonstration Platform (EVAL-SDP-CB1Z) nonlinearity normally associated with most rail-to-rail input op PMOD to SDP Interposer Board (SDP-PMD-IB1Z) amps that can be as high as 4 LSBs or 5 LSBs for a 16-bit system. Design and Integration Files This industry-leading solution is ideal for pulse oximetry Schematics, Layout Files, Bill of Materials applications where 1/f noise superimposed on the LED brightness levels affects the overall accuracy of the CIRCUIT FUNCTION AND BENEFITS measurement. The circuit in Figure 1 is a complete single-supply, low noise Total power dissipation for the three active devices is less than LED current source driver controlled by a 16-bit digital-to- 20 mW typical when operating on a single 5 V supply. analog converter (DAC). The system maintains 1 LSB integral VPMOD VPMOD VPMOD VEXT 3.3V TO 5.5V 10F ADR4525 VPMOD 2.5V V V IN OUT 0.1F 10F 0.1F GND 1/2 V LED ADA4500-2 I OUT SERIAL 1/2 REFS REFF V INTERFACE DD ADA4500-2 D CS SCLK V OUT AD5542A G DIN S LDAC DGND AGND AGND V OUT R LIMIT 124 AGND Figure 1. 1 LSB Linear 16-Bit LED Current Source Driver (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. 0 Circuits from the Lab reference designs from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause Fax: 781.461.3113 2015 Analog Devices, Inc. All rights reserved. whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) + 13447-001CN-0370 Circuit Note A typical rail-to-rail input amplifier uses two differential pairs CIRCUIT DESCRIPTION (PNP and NPN, or PMOS and NMOS) to achieve rail-to-rail In a typical pulse oximetry application, an LED is pulsed from a input swing (see the MT-035 Tutorial). One differential pair is high level of current (for example, 3/4 scale) to a lower level of active at the low range of the input common-mode voltage, and current (for example, 1/4 scale). The on-time of these pulses is the other pair is active at the high end. This classic complementary typically in the order of several hundred microseconds. Peak- dual differential pair topology introduces crossover distortion to-peak 1/f noise superimposed on the LED brightness levels during the transition between one differential pair to the other. during the on-time affects the accuracy of the overall The change in offset voltage causes nonlinearity when the measurement and must be minimized. amplifier is used as a DAC buffer. Figure 1 shows the single-supply signal chain that consists of a The ADA4500-2 uses an integrated charge pump in its input voltage reference, a DAC, a DAC output buffer, and a current structure to achieve rail-to-rail input swing without the need for source. a second differential pair. Therefore, it does not exhibit The DAC is the AD5542A 16-bit, serial input, voltage output crossover distortion. Using a zero crossover distortion amplifier segmented R-2R CMOS DAC. The output voltage of the DAC is in this single-supply system provides wide dynamic output dependent on the reference voltage, as shown in the following range while maintaining linearity over the entire input equation: common-mode range. Details of the operation of the ADA4500-2 can be found in the ADA4500-2 data sheet. V D REF V OUT N 2 The output impedance of the DAC is constant (typically 6.25 k) and code independent. The output buffer must therefore have a where: high input impedance and low input bias current to minimize D is the decimal data word loaded in the DAC register. errors. The ADA4500-2 is a suitable candidate with high input N is the number of bits. impedance and 2 pA maximum of input bias current at room For a reference of 2.5 V and N = 16, the equation simplifies to temperature, and 190 pA maximum of input bias current over the following: temperature. This results in 1.2 V worst-case error due to input 2.5 D 2.5 D bias current flowing through the 6.25 k DAC impedance, V OUT 16 which is significantly less than 1 LSB. 2 65,536 The output of the DAC is buffered and used to drive the power This gives a V of 1.25 V at mid scale, and 2.5 V at full scale. OUT MOSFET (IRLMS2002TRPBF). The MOSFET converts the DAC The LSB size is 2.5 V/65,536 = 38.1 V. output voltage into current that drives the LED. The MOSFET One LSB at 16 bits is also 0.0015% of full scale or 15 ppm full in the circuit is able to handle currents up to 6.5 A however, the scale. current is limited to 20 mA, which is the maximum rated current of the LED supplied on the EVAL-CN0370-SDPZ board. The The DAC reference pin is driven by an 2.5 V ADR4525 voltage board has provisions to easily change the full-scale current to reference buffered with the ADA4500-2. The ADR4525 voltage the LED by changing the R resistor. The maximum current reference provides a high precision, low noise (1.25 V p-p, LIMIT 0.1 Hz to 10 Hz), and stable reference to the DAC. The can be calculated by ADR4525 uses an innovative core topology to achieve high IMAX = 2.5 V/RLIMIT accuracy while offering industry-leading temperature stability Jumper options allow the LED to be connected to either the and noise performance. The low output voltage temperature PMOD voltage (VPMOD) or an external voltage (VEXT). coefficient (2 ppm/C maximum) and low long-term output The VEXT option is required to provide sufficient headroom voltage drift of the device also improve system accuracy over for the MOSFET when operating with VPMOD = 3.3 V. For time and temperature variations. The initial room temperature example, if V = 2.5 V, V = 0.7 V, and V = 0.7 V, then error of the ADR4525B is 0.02% maximum, which is OUT DS LED VEXT must be greater than 2.5 V + 0.7 V + 0.7 V = 3.9 V. approximately 13 LSBs at 16 bits. The dual ADA4500-2 is selected as the DAC output buffer as well An alternative that allows 3.3 V supply operation is to limit the as the voltage reference buffer. The ADA4500-2 is a high precision full-scale output voltage to approximately 1.9 V and only use amplifier with maximum offset voltage of 120 V, offset drift of 76% of the DAC output range. The RLIMIT resistor must be changed to approximately 95 to maintain 20 mA full-scale less than 5.5 V/C, 0.1 Hz to 10 Hz noise of 2 V p-p, and output current at 1.9 V output. maximum input bias current of 2 pA. Its innovative rail-to-rail input structure eliminates crossover distortion and therefore The AD5542A is available in a 10-lead MSOP or 10-lead LFCSP. makes it an excellent choice as a DAC buffer. The ADR4525 is available in an 8-lead SOIC, and the ADA4500-2 is available in an 8-lead MSOP or 8-lead LFCSP. Rev. 0 Page 2 of 6