LTC2755
18
2755f
Op Amp Selection
Because of the extremely high accuracy of the 16-bit
LTC2755-16, careful thought should be given to op amp
selection in order to achieve the exceptional performance
of which the part is capable. Fortunately, the sensitivity of
INL and DNL to op amp offset has been greatly reduced
compared to previous generations of multiplying DACs.
Tables 4 and 5 contain equations for evaluating the effects
of op amp parameters on the LTC2755’s accuracy when
APPLICATIONS INFORMATION
programmed in a unipolar or bipolar output range. These
are the changes the op amp can cause to the INL, DNL,
unipolar offset, unipolar gain error, bipolar zero and bipolar
gain error. Tables 4 and 5 can also be used to determine
the effects of op amp parameters on the LTC2755-14
and the LTC2755-12. However, the results obtained from
Tables 4 and 5 are in 16-bit LSBs. Divide these results
by 4 (LTC2755-14) and 16 (LTC2755-12) to obtain the
correct LSB sizing.
Table 6 contains a partial list of LTC precision op amps
recommended for use with the LTC2755. The easy-to-use
design equations simplify the selection of op amps to meet
the system’s specifi ed error budget. Select the amplifi er
from Table 6 and insert the specifi ed op amp parameters
in Table 5. Add up all the errors for each category to de-
termine the effect the op amp has on the accuracy of the
part. Arithmetic summation gives an (unlikely) worst-case
effect. A root-sum-square (RMS) summation produces a
more realistic estimate.
A3 • V
OS1
• 19.8 •
I
B1
• 0.13 •
0
A4 • V
OS2
• 13.1 •
A4 • I
B2
• 0.13 •
A4 •
()
5V
V
REF
()
5V
V
REF
()
16.5k
A
VOL1
OP AMP
V
OS1
(mV)
I
B1
(nA)
A
VOL1
(V/V)
V
OS2
(mV)
I
B2
(mV)
A
VOL2
(V/V)
V
OS1
• 3.2 •
I
B1
• 0.0003 •
A1 •
0
0
0
INL (LSB)
()
5V
V
REF
()
5V
V
REF
()
1.5k
A
VOL1
()
66k
A
VOL2
()
131k
A
VOL1
()
131k
A
VOL1
()
131k
A
VOL2
()
131k
A
VOL2
V
OS1
• 0.82 •
I
B1
• 0.00008 •
A2 •
0
0
0
DNL (LSB)
()
5V
V
REF
()
5V
V
REF
A3 • V
OS1
• 13.2 •
I
B1
• 0.13 •
0
0
0
0
UNIPOLAR
OFFSET (LSB)
()
5V
V
REF
()
5V
V
REF
()
5V
V
REF
V
OS1
• 13.2 •
I
B1
• 0.0018 •
A5 •
V
OS2
• 26.2 •
I
B2
• 0.26 •
BIPOLAR GAIN
ERROR (LSB)
()
5V
V
REF
()
5V
V
REF
()
()
()
5V
V
REF
()
5V
V
REF
BIPOLAR ZERO
ERROR (LSB)
UNIPOLAR GAIN
ERROR (LSB)
()
5V
V
REF
()
5V
V
REF
()
5V
V
REF
()
5V
V
REF
()
5V
V
REF
V
OS1
• 13.2 •
I
B1
• 0.0018 •
A5 •
V
OS2
• 26.2 •
I
B2
• 0.26 •
Table 4. Variables for Each Output Range that Adjust the
Equations in Table 5
OUTPUT RANGE A1 A2 A3 A4 A5
5V 1.1 2 1 1
10V 2.2 3 0.5 1.5
±5V 22111.5
±10V 4 4 0.83 1 2.5
±2.5V 1 1 1.4 1 1
–2.5V to 7.5V 1.9 3 0.7 0.5 1.5
Table 6. Partial List of LTC Precision Amplifi ers Recommended for Use with the LTC2755 with Relevant Specifi cations
AMPLIFIER
AMPLIFIER SPECIFICATIONS
V
OS
μV
I
B
nA
A
VOL
V/mV
VOLTAGE
NOISE
nV/√
⎯
H
⎯
z
CURRENT
NOISE
pA/√
⎯
H
⎯
z
SLEW
RATE
V/μs
GAIN BANDWIDTH
PRODUCT
MHz
t
SETTLING
with LTC2755
μs
POWER
DISSIPATION
mW
LT1001 25 2 800 10 0.12 0.25 0.8 120 46
LT1097 50 0.35 1000 14 0.008 0.2 0.7 120 11
LT1112 (Dual) 60 0.25 1500 14 0.008 0.16 0.75 115 10.5/Op Amp
LT1124 (Dual) 70 20 4000 2.7 0.3 4.5 12.5 19 69/Op Amp
LT1468 75 10 5000 5 0.6 22 90 2 117
LT1469 (Dual) 125 10 2000 5 0.6 22 90 2 123/Op Amp
Table 5. Easy-to-Use Equations Determine Op Amp Effects on DAC Accuracy in All Output Ranges (Circuit of Page 1). Subscript 1
Refers to Output Amp, Subscript 2 Refers to Reference Inverting Amp.
