Selecting Op Amps for Precision 16-Bit DACs – Design Note 214 Kevin R. Hoskins, Jim Brubaker and Patrick Copely The LTC ®1597 16-bit current output DAC offers a new level of accuracy and cost efﬁciency. It has exceptionally high accuracy and low drift: ±1LSB (max) INL and DNL over temperature. It eliminates costly external resistors in bipolar applications because it includes tightly trimmed, on-chip 4-quadrant resistors. To help achieve the best circuit DC performance, this design note offers two sets of easy-to-use design equations for evaluating an op amp’s effects on the DAC’s accuracy in terms of INL, DNL, offset (unipolar), zero (bipolar), unipolar and bipolar gain errors. With this information, selecting an op amp that gives the accuracy you need in a unipolar or bipolar application is easy. Figure 1 shows a unipolar application that combines the LTC1597 DAC and LT®1468 fast settling op amp. The equations for evaluating the effects of the op amp on the DAC’s accuracy are shown in Table 1. Quick work on Table 1’s equations with a calculator gives the results shown in Table 2. These are the changes the op amp can cause to the INL, DNL, unipolar offset and gain error of the DAC. Note, all are substantially less than 1LSB. Thus, the LT1468 is an excellent choice. The LTC1597’s internal design is very insensitive to offset-induced INL and DNL changes. An op amp VOS as large as 0.5mV causes just 0.55LSB INL and 0.15LSB DNL in the output voltage for 10VFS. So, how L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Table 2. Changes to Figure 1’s Accuracy in Terms of INL, DNL, Offset and Gain Error When Using the Fast Settling LT1468. VREF = 10V INL TYPICAL (LSB) LT1468 VOS (mV) 0.03 AVOL (V/V) OFFSET GAIN ERROR (LSB) 0.009 0.2 0.21 3 0.0017 0.0005 0.2 0 9000k 0.0011 0.0003 0 0.015 0.4 0.23 IB (nA) 0.036 DNL (LSB) TOTALS 0.039 0.01 Table 1. Easy-to-Use Equations Determine Op Amp Effects on DAC Accuracy in Unipolar Applications OP AMP INL (LSB) VOS (mV) IB (nA) AVOL (V/V) DNL (LSB) UNIPOLAR OFFSET (LSB) UNIPOLAR GAIN ERROR (LSB) VOS • 1.2 • (10V/VREF) VOS • 0.3 • (10V/VREF) VOS • 6.6 • (10V/VREF) VOS • 6.9 • (10V/VREF) IB • 0.00055 • (10V/VREF) IB • 0.00015 • (10V/VREF) IB • 0.065 • (10V/VREF) 0 10k/AVOL 3k/AVOL 0 131k/AVOL 15V 0.1μF 5V 2 LT1460-10 0.1μF 6 2 3 R1 4 RCOM R1 1 REF 23 VCC 4 5 ROFS RFB ROFS R2 20pF 15V 0.1μF RFB IOUT1 16 DATA INPUTS 2 6 16-BIT DAC AGND LTC1597 10 TO 21, 24 TO 27 DGND 22 7 3 – + 7 6 LT1468 4 VOUT = 0V TO –10V 0.1μF –15V WR LD CLR WR LD CLR 9 8 DN214 F01 28 Figure 1. Unipolar VOUT DAC/Op Amp Circuit Swings 10VFS (0V to –10V) and Settles to 16 Bits in < 2μs. For 0V to 10V Swings, the Reference Can Be Inverted with R1, R2 and Another Op Amp 10/99/214_conv just as easily for unipolar and bipolar applications. First, consult an op amp’s data sheet to ﬁnd the worst-case VOS and IB over temperature. Then, plug these numbers in the VOS and IB equations from Table 1 or Table 3 and calculate the temperature induced effects. does the LT1468 effect, for example, the DAC’s DNL? From Table 2, the LT1468 adds approximately 0.01LSB of additional DNL to the output. This, along with the LTC1597’s typical 0.2LSB DNL, give a total DAC/op amp DNL of just 0.21LSB (typ). This is well under the –1LSB needed to ensure monotonic operation. Table 4. Changes to Figure 2’s Accuracy in Terms of INL, DNL, Bipolar Zero and Bipolar Gain Error When Using an LT1112, a Precision Dual Op Amp Figure 2 shows a bipolar application that combines the LTC1597 and a dual LT1112 op amp. The equations for evaluating the effects of two ampliﬁers in bipolar operation are just as easy to use and are shown in Table 3. A quick application of Table 3’s equations gives the accuracy changes shown in Table 4. Again, each effect is substantially less than 1LSB. This shows that with proper op amp selection, the LTC1597’s excellent linearity and precision are not degraded even in bipolar applications. LT1112 TYPICAL VOS1 (mV) 0.02 IB1 (nA) 0.07 0.024 0.006 0.00004 0.00001 0.198 0.138 0.0046 0 5000k 0.002 0.0006 0 0.039 0.02 0 0 0.134 0.264 IB2 (nA) 0.07 0 0 0.0046 0.009 5000k AVOL2 TOTALS While not directly addressed by the simple equations in Tables 1 and 3, temperature effects can be handled DNL (LSB) VOS2 (mV) AVOL1 The totals in Tables 2 and 4 are algebraic sums of the absolute values, producing a worst-case error. INL (LSB) BIPOLAR BIPOLAR ZERO GAIN ERROR ERROR (LSB) (LSB) 0 0 0.013 0.026 0.026 0.007 0.354 0.476 Table 3. Easy-to-Use Equations Determine Op Amp Effects on DAC Accuracy in Bipolar Applications OP AMP INL (LSB) DNL (LSB) VOS1 • 1.2 • (10V/VREF) VOS1 • 0.3 • (10V/VREF) VOS1 • 9.9 • (10V/VREF) IB1 • 0.00055 • (10V/VREF) IB1 • 0.00015 • (10V/VREF) IB1 • 0.065 • (10V/VREF) 0 10k/AVOL 3k/AVOL1 0 196k/AVOL1 0 0 VOS2 • 6.7 • (10V/VREF) VOS2 • 13.2 • (10V/VREF) VOS1 (mV) IB1 (nA) AVOL1 VOS2 (mV) BIPOLAR ZERO ERROR (LSB) BIPOLAR GAIN ERROR (LSB) VOS1 • 6.9 • (10V/VREF) IB2 (nA) 0 0 IB2 • 0.065 • (10V/VREF) IB2 • 0.13 • (10V/VREF) AVOL2 0 0 65k/AVOL2 131k/AVOL2 15V 0.1μF 2 LT1460-10 15V 6 4 + 3 5V 8 A2 1/2 LT1112 1 0.1μF – 2 15pF 3 R1 2 RCOM R1 1 REF 4 ROFS ROFS R2 23 VCC 5 RFB 30pF RFB IOUT1 16 DATA INPUTS 16-BIT DAC AGND LTC1597 10 TO 21, 24 TO 27 DGND WR LD CLR WR LD CLR 6 6 9 8 22 7 5 – A1 1/2 LT1112 7 + 4 VOUT = –10V TO 10V 0.1μF –15V DN214 F02 28 Figure 2. Bipolar Circuit Conﬁguration for a 20V (VOUT = – 10V to 10V) Output Swing Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn214f_convLT/TP 1099 340K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1999

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