Eugenio Mejia, Kevin Duke, Navin Kommaraju TI Precision Designs: Verified Design ±10V 4-Quadrant Multiplying DAC TI Precision Designs Circuit Description TI Precision Designs are analog solutions created by TI’s analog experts. Verified Designs offer the theory, component selection, simulation, complete PCB schematic & layout, bill of materials, and measured performance of useful circuits. Circuit modifications that help to meet alternate design goals are also discussed. This four-quadrant multiplying DAC (MDAC) circuit conditions the current output of an MDAC into a symmetrical bipolar voltage. The design uses an op amp in a transimpedance configuration to convert the MDAC current into a voltage. This stage is followed by an additional amplifier in a summing configuration to apply an offset voltage. The fundamentals of this design can be extended to realize any symmetric or non-symmetric output voltage. Design Resources Design Archive TINA-TI™ DAC8811 OPA2277 Ask The Analog Experts WEBENCH® Design Center TI Precision Designs Library All Design files SPICE Simulator Product Folder Product Folder RG2 VREF REFIN RFB IOUT RG1 + MDAC RFB2 A1 VDAC + VOUT A2 An IMPORTANT NOTICE at the end of this TI reference design addresses authorized use, intellectual property matters and other important disclaimers and information. TINA-TI is a trademark of Texas Instruments WEBENCH is a registered trademark of Texas Instruments TIDU031-October 2013-Revised October 2013 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated 1 www.ti.com 1 Design Summary The design requirements are as follows: DAC Supply Voltage: +5V dc Amplifier Supply Voltage: ±15V dc Input: 3-wire, 16-bit SPI Output: ±10V dc The design goals and performance are summarized in Table 1. TUE is defined as the total unadjusted error of the system, including errors from each component in the system. Figure 1 depicts the measured transfer function of the design. Table 1. Comparison of Design Goals, Simulation, and Measured Performance System Total Unadjusted Error (%FSR) Goal Simulated Measured 0.1% 0.087135 0.053985 Figure 1: Full-Scale Ramp of Output 2 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated TIDU031-October 2013-Revised October 2013 www.ti.com 2 Theory of Operation The first stage of the design converts the current output of the MDAC (Iout) to a voltage (Vout) using an amplifier in a transimpedance configuration. A typical MDAC features an on-chip feedback resistor sized appropriately to match the ratio of the resistor values used in the DAC R-2R ladder. This resistor is available using the input shown in Figure 2 called RFB on the MDAC. The MDAC reference and the output of the transimpedance stage are then connected to the inverting input of the amplifier in the summing stage to produce the output that is defined by Equation 1. Trans-Impedance Stage Gain & Offset Stage RG2 VREF REFIN RFB IOUT RG1 + MDAC RFB2 A1 VDAC + VOUT A2 Figure 2: System Diagram R V Code RFB 2 VOUT Code FB 2 REF bits VREF 2 RG1 RG2 (1) The resulting system is commonly referred to as a four-quadrant MDAC configuration. A system only including the MDAC and transimpedance stage, highlighted in Figure 2, would be referred to as a twoquadrant configuration because the output is only able to swing positive or negative by changing the reference voltage polarity, illustrated in Figure 3(a). The four-quadrant system is capable of positive or negative output voltages by changing either the reference voltage or by changing DAC codes as illustrated in Figure 3(b). 0V 0V (a) Two-Quadrant Positive Reference Quadrant I +VREF Codes 0x0000 to 0x7FFF Codes 0x8000 to 0xFFFF Quadrant III Positive Reference Quadrant IV Negative Reference 0V Negative Output Negative Output -VREF Quadrant IV Positive Reference Negative Reference Quadrant II Positive Output Positive Output +VREF Positive Output Codes 0x0000 to 0xFFFF +VREF Negative Output Negative Reference Quadrant I -VREF -VREF (b) Four-Quadrant Figure 3: Two-Quadrant vs. Four-Quadrant Output TIDU031-October 2013-Revised October 2013 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated 3 www.ti.com 2.1 Transimpedance Amplifier Stage The transimpedance amplifier converts the current output of the MDAC to voltage. This voltage, VDAC, is opposite in polarity to VREF, making the output range of VDAC between 0 and -VREF. Amplifier selection for this stage is one of the most critical decisions for this design. This design is focused on delivering a highly accurate, un-calibrated, dc signal. Input offset voltage and input bias current are the two most critical op amp specifications to achieve accuracy. Ideally the amplifier selected will contribute negligible error to the system. 2.1.1 Input Bias Current The output of the MDAC is a current, so any amplifier input bias current, IB, directly adds or subtracts from the MDAC output. This results in an offset error at the voltage output of the amplifier. The equation for this offset is shown below: VDAC _OFFSET i B RFB (2) The value of RFB is not always explicitly listed in the MDAC’s datasheet but it is typically equal to the input impedance of the reference pin, which is listed in the theory of operation section or in the electrical characteristics table. Input bias current for CMOS amplifiers tends to be very small, usually on the order of picoamperes, so finding one with small input bias current can be an easy task with most modern amplifier portfolios. Because the offset contribution of input bias current is constant, its effects will be more significant with small reference values or with high-resolution devices because of the reduced LSB size. 2.1.2 Input Offset Voltage IOUT The effect of input offset voltage is a linearity error at the output of the transimpedance stage, VDAC. Input offset voltage introduces non-linearity because the output impedance of the IOUT pin of the MDAC creates a code-dependent gain on the amplifier input offset voltage. In the circuit shown in Figure 4, the input offset voltage of the amplifier is subject to non-inverting amplification with a gain of (1+RFB/ROF), where ROF changes based on the DAC code. ROF represents the output impedance of the IOUT pin. IDAC RFB + ROF VDAC A1 VOS Figure 4: DAC + Transimpedance Stage Model 4 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated TIDU031-October 2013-Revised October 2013 www.ti.com In order to calculate the impedance seen at IOUT at each code, the following generic procedure can be used: 1. Analyze the R-2R ladder architecture of the DAC 2. Write nodal equations for each rung of the ladder 3. Write equations to define IOUT impedance versus code 4. Iterate through all the codes To be practical, this process requires the use of software to iterate through all of the codes available for a modern DAC which typically has 8-bits or more of resolution. The design archive for this document includes MATLAB files that were used for this analysis. A simplified example using a 5-bit DAC with 2 MSBs segmented will be used to step through the above procedure. In section 2.1.2.5 results will be shown from the MATLAB simulations used to model the DAC8811. 2.1.2.1 Internal Architecture The specific topology implemented in the R-2R ladder of the MDAC will impact the impedance seen at Iout for each code. Segmentation is frequently implemented for R-2R ladder designs to improve linearity and the segmentation scheme and is a key concern in studying the MDACs topology. Although some devices may implement complex trimming schemes to deliver highly matched resistors, a practical approximation can be made by using just the number of segmented and non-segmented bits along with the ratio of resistor values for each leg. Non-segmented bits are normal R-2R ladder legs where there is a resistor of value 2R that connects between VREF and either IOUT or GND. In between each 2R leg of the non-segmented ladder is a resistor of value R. The R resistor causes a binary weighted current divider effect on each of the 2R legs. Segmented bits are similar to regular R-2R legs, except there is no R resistor between each leg, causing each segmented leg to be equally weighted current dividers. An example is shown in Figure 5 for a 5-bit MDAC with 2 bits of segmentation. Each bit of the 5-bit DAC in Figure 5 is labeled as Bn, where B1 controls the first two segmented switches. The segmentation scheme and the values of the resistor may vary slightly from device to device. The theory of operation section of the chosen MDAC datasheet will describe the architecture of the R-2R ladder. Segmented Bits R-2R Bits VREF 2R 2R B4 2R B3 R R R 2R 2R 2R B2 B1 2R B0 IOUT Figure 5: 5-Bit R-2R Ladder, 2 segmented bits TIDU031-October 2013-Revised October 2013 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated 5 www.ti.com 2.1.2.2 Nodal Equations In order to iterate through all the bit combinations it is necessary to write the nodal equations for this ladder network. Equations for each node in the ladder that is not a switch and is not part of the segmentation scheme will be written. The segmented node equations are straightforward. In this case the resistors connected to switches controlled by bits B3, B4 and B5 will receive nodal equations. Since the goal is to calculate the equivalent dc resistance seen from IOUT, VREF can be effectively grounded, as shown in Figure 6. V3 R V2 2R R V1 2R B2 R V0 2R B1 2R B0 IOUT Figure 6: Simplified Equivalent model for 5-Bit, 2 Bit Segmented Ladder The equations for this example are shown below. Although V2 is equal to zero it remains in Equation 3 to highlight the pattern that occurs in all the nodes except for the LSB node. In these equations the Bn coefficients represent the binary value (0 or 1) of the respective bits of the DAC data register. VIOUT is the voltage at the IOUT terminal. B2 VIOUT 2 V3 V1 5 5 B1 VIOUT 2 V1 V2 V0 5 5 B0 VIOUT V1 V0 4 2 V2 (3) (4) (5) This method can be extended for any MDAC with an R-2R ladder as follows: n: number of non-segmented bits i: bit number Bi VIOUT 2 Vi 1 Vi 1 5 5 Bi VIOUT Vi 1 i 0 Vi 4 2 0 i n Vi 6 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated (6) (7) TIDU031-October 2013-Revised October 2013 www.ti.com 2.1.2.3 Input Impedance of IOUT First Equations 3, 4, and 5 simplified to be defined in terms of bits. B B B V2 VIOUT 2 1 0 8 16 4 5 B1 5 B0 B V1 VIOUT 2 16 32 8 5 B1 21 B0 B V0 VIOUT 2 32 64 16 (8) (9) (10) Using the voltage equations for each node of the R-2R ladder, equations can be written that define the current going through each leg of the ladder and finally an equation can be written defining the sum of all ladder currents seen at IOUT. Iterating through all possible bit combinations will show the changing input impedance of the IOUT pin versus code. Note that leading B coefficients are added to these equations when necessary to ensure that the bits that are LOW will not be summed to the current going into IOUT. i 4 B4 VIOUT V V V2 V V1 V V0 , i 3 B3 IOUT , i 2 B2 IOUT , i1 B1 IOUT , i 0 B0 IOUT R 2R 2R 2R 2R iOUT i 0 i1 i 2 i 3 i 4 ROS (11-16) VIOUT i OUT (17) (18) ROS is recorded for each code in order to analyze the results. Matrix math in MATLAB is used to iterate through all possible combinations to generate the curves in Figures 7-10. 2.1.2.4 Iterations & Results The plots below approximate what ROS looks like, normalized to one unit of resistance R as defined by the R-2R ladder of the DAC, across all codes for both the 5-Bit, 2 MSB segmented example and the 16-Bit, 3 MSB segmented DAC8811. Figure 7: 5-Bit DAC, Code vs. ROS TIDU031-October 2013-Revised October 2013 Figure 8: 16-Bit DAC, Code vs. ROS ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated 7 www.ti.com The gain applied to the input offset voltage for each code can then be calculated based on the value of ROS and RFB. The value of RFB must be equal to the parallel and series combination of all of the resistors in the R-2R ladder design. For the 5-Bit, 2 MSB segmented example, RFB=R/2. For a 16-Bit, 3 MSB segmented DAC, RFB=R/4. GainVos 1 Figure 9: 5-Bit DAC, Code vs. VOS Gain RFB ROS (19) Figure 10: 16-Bit DAC, Code vs. VOS Gain This variable gain on the offset voltage will cause a linearity error at the output of the transimpedance stage. There is a linear component to this gain error and the values calculated in this analysis could be used to calibrate the linear component of this error. The MATLAB code to generate the curves shown in Figure 9 and Figure 10 are included in this documents design archive. In this case, the non-linear gain error is simply treated as an INL error and only the maximum error is recorded for the circuit to calculate the worst case INL contributions due to VOS. The worst case INL error occurs when VOS experiences the highest gain, shown in Figure 10. For the DAC8811, this occurs at approximately code 64171. 8 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated TIDU031-October 2013-Revised October 2013 www.ti.com 2.2 Summing Amplifier Stage The summing amplifier outputs the difference between the two inputs with individual gain applied to each of them based on their respective input impedances. The reference input (VREF) acts as a DC offset multiplied by a gain of -RFB2/RG2. The output of the transimpedance stage (VDAC) receives a gain equal to RFB2/RG1. VDAC has an output of 0 to -VREF and the system has an output of ±VREF. The resistor ratios are then determined using two end point equations derived from Equation 1. Case 1: Code = 0, VOUT = -VREF R VREF FB 2 VREF RG2 RG2 RFB 2 Case 2: Code = 2 bits , VOUT = VREF R VREF FB 2 VREF VREF RG1 RFB 2 RG1 2 2.3 (20) (21) Passive Component Values Large resistors will introduce noise and therefore decrease system accuracy. Small resistors will draw more current, and subsequently increase power, which may affect load regulation of the reference. The base values for the resistors used in this design are based on the resistor ratios discussed in Section 2.2 while limiting the current drawn from the reference to 500µA. The maximum current that the amplifier in the transimpedance stage will sink is 1mA. The precision required is determined from the simulation results shown in Section 4.2. RG1 = 10kΩ ±0.1% RG2 = 20kΩ ±0.1% RFB2 = 20kΩ ±0.1% There is a small 12 pF capacitor between the IOUT pin and the RFB pin that is not installed by default. This is a compensation capacitor that may be needed if gain peaking is observed due to parasitics. Since the exact value is not critical for this component, 10% tolerance is acceptable. TIDU031-October 2013-Revised October 2013 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated 9 www.ti.com 3 Component Selection The goal of this design is to achieve 0.1% TUE (%FSR). The error contributions of each component can be subtracted from the overall error budget to ensure that the design goal is met. Each component subtracts from the error budget for the rest of the components. 3.1 DAC Selection Generally, MDACs are used in high performance applications that take full advantage of their strong dc specifications. Since the typical MDAC does not feature an on-board output amplifier, they do not exhibit an offset error. Instead MDACs are only specified with INL, DNL, and gain errors and usually they show very strong linearity specifications. Other differences are application related, such as resolution, number or channels, control interface, or other auxiliary features. The DAC is chosen first in this system and sets the error baseline. All the other components will be chosen using the remaining head-room for the system. For this design the DAC8811 is chosen. DAC8811 delivers excellent linearity and low gain error to leave much of the error budget to the rest of the discrete components. DAC8811 also features a serial interface, which is often preferred over a parallel bus since it uses fewer pins. 3.2 Amplifier Selection Two amplifiers must be selected in this design: one for the transimpedance stage and one for the summing amplifier stage. For the transimpedance stage, low input bias current and low input offset voltage are the most critical parameters to deliver accurate dc operation. Input bias current will create a dc offset across the transfer function. Input offset voltage will create an integral non-linearity error. Both of these error sources may be increased by gain in the summing amplifier stage. Full details on the implications of each of these specifications are explained in sections 2.1 and 2.2 of this document. Similar concerns are applicable to the summing stage of this design. Input bias current is not as critical since the impedance in the summing stage is typically be small enough, making the impact of input bias current negligible compared to other error sources. Input offset voltage should still be considered since VOS of the summing amplifier directly contributes to offset error of the system. The OPA277 core was selected for both stages because it delivers very low input offset voltage and very low input bias current. The OPA2277 is the dual package offering of the OPA277 core with very similar specifications and can help reduce PCB area. Other amplifier considerations such as bandwidth, temperature drift and slew rate may also be relevant depending on the application requirements. 3.3 Passive Component Selection Resistor matching is very important on the amplifying stage since resistor mismatch can cause both offset and gain errors in the system. High tolerance components must be used to keep the error within the allowance. In this design tolerance resistors were suitable to meet the accuracy requirements, but this can be adjusted to enhance performance. The capacitor between the IOUT pin and the RFB pin is a compensation capacitor that does not require high tolerance. 10 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated TIDU031-October 2013-Revised October 2013 www.ti.com 4 Simulation Simulation is split into two sections, one for the DAC + transimpedance stage and one for the summing stage. The results from both simulations will be combined to calculate overall system performance. 4.1 DAC + Transimpedance Stage The INL and offset error effects of the MDAC and transimpedance stage were simulated using a separate model that was developed in MATLAB, as described in Section 2.1.1 and 2.1.2. These results, along with the INL and gain error specifications from the MDAC datasheet, are used to model the MDAC + transimpedance stage in TINA-TI. The DAC + Transimpedance stage model uses an ideal summing stage in order to obtain the error contribution of just the DAC and transimpedance amplifier at the output of the system. The TINA-TI™ schematic shown in Figure 11 represents the DAC + transimpedance stage model. The results are shown in Table 2 and Figure 12. The results of this stage will be used with the results of the summing stage in order to determine the overall system offset error, gain error and total unadjusted error (TUE). R2 20k R5 10k R4 1 - + + VREF 10 R3 20k IOP1 R1 10k Gerror_dac - + Vos_dac -2.5u Vout + VDAC DAC8811 + Trans-impedance Stage Figure 11: DAC + Transimpedance Stage Model Table 2. Simulated DAC & Transimpedance Stage Performance Parameter Simulated Value Negative Full-Scale Voltage (V) -9.999995 Zero-Scale Voltage (mV) 1.005 Positive Full-Scale Voltage (V) 10.002008 Offset Error (%FSR) 0.000025 Gain Error (%FSR) 0.010000 INL Error (%FSR) 0.001530 Total Unadjusted Error (%FSR) 0.010116 TIDU031-October 2013-Revised October 2013 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated 11 www.ti.com T 10.00 10.002005 V 5.00 Vout (V) 1.005 mV 0.00 -5.00 -9.999995 V -10.00 -10.00 -7.50 -5.00 Vdac (V) -2.50 0.00 Figure 12: DAC + Transimpedance Stage, Output Transfer Function The following equations are used to calculate the error parameters in Table 2 based on the information in Figure 12. The total unadjusted error equation uses a root sum squared (RSS) technique to sum uncorrelated error sources. OffsetError%FSR GainError%FSR V OUTSIM ( MAX ) VOUTSIM ( MIN ) VOUTIdeal ( MIN ) VOUTIdeal ( MAX ) VOUTIdeal ( MIN ) (22) 100 VOUTSIM ( MIN ) VOUTSIM ( MAX ) VOUTIDEAL ( MIN ) VOUTIDEAL ( MAX ) VOUTIDEAL ( MIN ) V RFB 2 INL _ ErrorDAC _ LSBs REF bits 2 RG1 INL _ Error%FSR VOUTIDEAL ( MAX ) VOUTIDEAL ( MIN ) 100 100 TUE%FSR OffsetERROR 2%FSR GainERROR 2%FSR INL ERROR 2%FSR 12 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated (23) (24) (25) TIDU031-October 2013-Revised October 2013 www.ti.com 4.2 Summing Stage The TINA-TI™ schematic shown in Figure 13 uses the OPA277 model and Monte-Carlo analysis for the resistor network to simulate the summing stage and to select appropriate resistor tolerances to meet the system accuracy goals. In this model the DAC and transimpedance stage are represented by an ideal voltage source sweeping from 0 to -10V. Resistors of 0.1% tolerance were found suitable to meet 0.1% system TUE, but tighter tolerance resistors could be used to enhance results. VCC R3 20k C1 1u R2 20k V1 15 VSS C4 100p C5 100n V2 -15 + VDAC Vout + Vs+ C2 1u VREF 10 Vs- OPA277 TRM8 VSS TRM1 R1 10k U2 OPA277 C6 100p C7 100n VCC Figure 13: Summing Stage Model T 10.00 Vout[1] A:(-10; 9.995963) Vout[2] A:(-10; 10.001644) Vout[9] A:(-10; 9.988845) Vout[3] Vout[4] A:(-10; 10.002055) Vout[5] A:(-10; 10.002546) Vout[6] A:(-10; 9.998891) Vout[7] A:(-10; 9.996261) Vout[8] A:(-10; 9.9931) Vout[9] A:(-10; 10.001131) Vout[10] A:(-10; 9.995206) (V) Voltage Vout (V) 5.00 0.00 Vout[1] A:(-18.790525f; -10.00216) Vout[2] A:(-18.790525f; -9.997218) Vout[3] A:(-18.790525f; -10.00045) Vout[4] A:(-18.790525f; -10.005341) Vout[5] A:(-18.790525f; -10.001578) Vout[6] A:(-18.790525f;Vout[9] -10.002096) Vout[7] A:(-18.790525f; -10.001387) Vout[8] A:(-18.790525f; -9.997481) Vout[9] A:(-18.790525f; -10.003574) Vout[10] A:(-18.790525f; -9.999098) -5.00 -10.00 -10.00 -7.50 -5.00 Vdac (V) (V) Input voltage -2.50 0.00 Figure 14: Monte Carlo Results TIDU031-October 2013-Revised October 2013 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated 13 www.ti.com The results from 10 iterations of the Monte-Carlo simulation of the summing stage are shown in Tables 3 & 4. Figure 14 shows a subset of the Monte-Carlo dc transfer function simulations. Table 3. Simulated Summing Stage Value Min Max Average Std. Dev. ( ) Offset error (mV) 0.4841 5.8023 0.8000 2.5298 Full-Scale Range (V) 19.9938 20.0062 19.9995 0.0153 |Full-Scale Error| (mV) 0.0730 6.2250 0.0041 2.9640 The standard deviation of the Monte-Carlo results can be used to generate a realistic error figure for the system by multiplying the standard deviation by 3, commonly referred to as a 3-σ system. This error should encompass 99.7% of systems, leaving out absolute worst-case resistor mismatches that are highly unlikely to occur. These errors are summarized in Table 4. The equations used to calculate the error values are shown below: OffsetError%FSR GainError%FSR Table 4. 3 OffsetError * 100 VOUTIDEAL ( MAX ) VOUTIDEAL ( MIN ) 3 GainError * 100 VOUTIDEAL ( MAX ) VOUTIDEAL ( MIN ) (27) Simulated Summing Stage Performance Parameter 4.3 (26) Simulated Value Offset Error (%FSR) 0.0379 Gain Error (%FSR) 0.0763 INL Error (%FSR) 0.0000 Total Unadjusted Error (%FSR) 0.0852 System Simulation The DAC+ transimpedance stage results are root sum squared with the summing stage simulation results in order to see the results of the combined stages. INL error is taken directly from the DAC + transimpedance simulation since the summing stage is completely linear. Table 5. Simulated System Performance Parameter 14 Simulated Value Goal Offset Error (%FSR) 0.037948 n/a Gain Error (%FSR) 0.076931 n/a INL Error (%FSR) 0.001530 n/a Total Unadjusted Error (%FSR) 0.087135 0.1 OffsetErrorSystem OffsetError2DAC Trans OffsetError2Sum min g (28) GainErrorSystem GainError2DAC Trans GainError2Sum min g (29) ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated TIDU031-October 2013-Revised October 2013 www.ti.com 5 PCB Design The PCB schematic and bill of materials can be found in Appendix A. 5.1 PCB Layout General PCB layout best-practices should be followed for this design. The transimpedance stage summing node should be kept as small as possible and a pour cut-out should be placed underneath to reduce parasitics. Similar guidelines should be followed for the summing amplifier stage. Figure 15: PCB Layout TIDU031-October 2013-Revised October 2013 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated 15 www.ti.com 6 Verification & Measured Performance 6.1 Transfer Function The graph in Figure 16 was collected by applying input codes from 0 to 65535 to the DAC and measuring the output voltage on a single system. 10.00000 Output Voltage (Volts) 5.00000 0.00000 -5.00000 -10.00000 0 8192 16384 24576 32768 40960 49152 57344 65536 Input Code (Decimal) Figure 16: Measured Transfer Function To easily visualize the error of the system, the difference between the ideal output voltage and measured output voltage of the circuit in %FSR is plotted in Figure 17. 16 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated TIDU031-October 2013-Revised October 2013 www.ti.com 0.014 0.012 Output Error (%FSR) 0.01 0.008 0.006 0.004 0.002 0 0 8192 16384 24576 32768 40960 49152 57344 65536 Input Code (Decimal) Figure 17: Output Voltage Error vs. Input Code Table 6 summarizes the average results observed over 10 units. These results were measured using a two-point line of best fit measured at codes 485 and 64714. The equations used to calculate these values are shown in Equations X and Y Table 6. Average Measured Circuit Performance Measured Value Simulated Goal Offset Error (%FSR) Parameter 0.001374 0.037900 n/a Gain Error (%FSR) 0.053619 0.076900 n/a INL Error (%FSR) 0.001330 0.001530 n/a Total Unadjusted Error (%FSR) 0.053985 0.087135 0.1 GainError(%FSR ) OffsetError(% FSR ) V OUTREAL ( 64714 ) VOUTREAL ( 485 ) VOUTIDEAL ( 64714 ) VOUTIDEAL ( 485 ) VOUTIDEAL ( 64714 ) VOUTIDEAL ( 485 ) * 100 VOUTREAL ( 64714 ) VOUTREAL ( 485 ) VOUTREAL ( 485 ) * 485 VOUTIDEAL ( MIN ) 64714 485 * 100 VOUTIDEAL ( MAX ) VOUTIDEAL ( MIN ) TIDU031-October 2013-Revised October 2013 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated (30) (31) 17 www.ti.com 7 Modifications The components in this design were selected based on the design goals outlined at the beginning of this document. The components may differ depending on the constraints of a different design. The resistor tolerance was selected to meet the 0.1%FSR goal. By improving the tolerance of the resistors a lower TUE can be achieved. Most alternative MDACs will offer comparable linearity, gain error, and offset error but may show different interface options, channel count, resolution, and other auxiliary features. Table 7 offers options to expand the channel count of this design. This design was not simulated or measured over temperature. The OPA277 features excellent input offset voltage drift specifications. This drift could be improved by selecting a zero-drift chopper amplifier, but additional noise may be introduced at the output of the system. Table 7. DAC Resolution Channel Count Interface INL Error DAC8811 16-Bit 1 SPI ±1 LSB ±1 mV DAC8812 16-Bit 2 SPI ±1 LSB ±0.75 mV DAC8814 16-Bit 4 SPI ±1 LSB ±0.75 mV DAC8822 16-Bit 2 Parallel ±1 LSB ±1 mV Table 8. Amplifier 18 Alternate DAC Options Supply Voltage Full Scale Error Alternate Amplifier Options Bandwidth Offset Voltage (Typ) Offset Drift (Typ) Quiescent Current Input Bias Current (Typ) Input Voltage Noise (0.1Hz to 10Hz) OPA2277 ±18 V 1MHz ±10 µV 0.1 µV/°C 790 µA ±0.5 nA 220 nVpp OPA211 ±18 V 80 MHz ±30 µV 0.35 µV/°C 3.6 mA ±50 nA 80 nVpp OPA188 ±18 V 2 MHz ±6 µV 0.085 µV/°C 450 µA ±160 pA 250 nVpp OPA170 ±18 V 1.2 MHz ±250 µV 0.3 µV/°C 110 µA ±8 pA 2 µVpp ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated TIDU031-October 2013-Revised October 2013 www.ti.com 8 About the Authors Eugenio Mejia is an applications engineer intern in the precision digital to analog converters group at Texas Instruments. Kevin Duke is an applications engineer in the precision digital to analog converters group at Texas Instruments where he supports industrial and catalog products and applications. Kevin received his BSEE from Texas Tech University in 2010. Navin Kommaraju is the systems and applications manager in the precision digital to analog converters group at Texas Instruments. Navin received his BTech in Electrical and Electronics Engineering from the Indian Institute of Technology, India, an MS in Computer Engineering from Iowa State University and an MBA from the University of Texas at Austin. TIDU031-October 2013-Revised October 2013 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated 19 www.ti.com Appendix A. A.1 Electrical Schematic Figure A-1: Electrical Schematic Passive Name in Text Passive Name in Schematic R3 R4 R5 C13 20 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated TIDU031-October 2013-Revised October 2013 www.ti.com A.2 Bill of Materials Figure A-2: Bill of Materials TIDU031-October 2013-Revised October 2013 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated 21 www.ti.com IMPORTANT NOTICE FOR TI REFERENCE DESIGNS TI reference designs are intended to help buyers of semiconductor products (also referred to herein as “components”) sold by Texas Instruments Incorporated and its subsidiaries (TI). TI reference designs have been created using standard laboratory conditions and engineering practices, including specific testing (where applicable) described in the reference design documentation. 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TI has specifically designated certain components as meeting ISO/TS16949 requirements, mainly for automotive use. In any case of use of non-designated products, TI will not be responsible for any failure to meet ISO/TS16949. 22 ±10V 4-Quadrant Multiplying DAC Copyright © 2013, Texas Instruments Incorporated TIDU031-October 2013-Revised October 2013 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. 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