DN214 - Selecting Op Amps for Precision 16-Bit DACs

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
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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
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