Datasheet

LTC2757
16
2757f
APPLICATIONS INFORMATION
Table 5 contains a partial list of LTC precision op amps
recommended for use with the LTC2757. 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 5 and insert the specifi ed op amp parameters
in Table 4. 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.
Op amp offset contributes mostly to DAC output offset
and gain error, and has minimal effect on INL and DNL.
For example, consider the LTC2757 in unipolar 5V output
range. (Note that for this example, the LSB size is 19µV.)
An op amp offset of 35µV will cause 1.8LSB of output
offset, and 1.8LSB of gain error; but 0.4LSB of INL, and
just 0.1LSB of DNL.
While not directly addressed by the simple equations in
Tables 3 and 4, temperature effects can be handled just as
easily for unipolar and bipolar applications. First, consult
an op amp’s data sheet to fi nd the worst-case V
OS
and I
B
over temperature. Then, plug these numbers in the V
OS
and I
B
equations from Table 4 and calculate the tempera-
ture-induced effects.
For applications where fast settling time is important, Ap-
plication Note 120,
1ppm Settling Time Measurement for
a Monolithic 18-Bit DAC
, offers a thorough discussion of
18-bit DAC settling time and op amp selection.
Recommendations
To achieve the full specifi ed static and dynamic performance
of the LTC2757, the LT1468 amplifi er is recommended;
it offers a unique combination of fast settling and excel-
lent DC precision. When using the LT1468 as an output
amp, however, the offset voltage (75µV max) must be
nulled to avoid degrading the linearity of the LTC2757.
The LT1468 datasheet shows how to do this with a digital
potentiometer.
For DC or low-frequency applications, the LTC1150 is the
simplest 18-bit accurate output amplifi er. An auto-zero
amp, its exceptionally low offset (10µV max) and offset
drift (0.01µV/°C) make nulling unnecessary. Note: for
swings above 8V, use an LT1010 buffer to boost the load
current capability of the LTC1150. The settling of auto-zero
amps is a special case; see Application Note 120,
1ppm
Settling Time Measurement for a Monolithic 18-Bit DAC
,
Appendix E, for details.
The LT1012 and LT1001 are good intermediate output-amp
solutions that achieve moderate speed and good accuracy.
They are also excellent choices for the reference inverting
amplifi er in fi xed-reference applications.
Figure 3 shows a composite output amplifi er that achieves
fast settling (8µs) and very low offset (3µV max) without
offset nulling. This circuit offers high open-loop gain
(1000V/mV min), low input bias current (0.15nA max),
fast slew rate (25V/µs min), and a high gain-bandwidth
product (30MHz typ). The high speed path consists of
an LTC6240, which is an 18MHz ultra-low bias current
amplifi er, followed by an LT1360, a 50MHz fast-slewing
amplifi er which provides additional gain and the ability to
Table 5. Partial List of LTC Precision Amplifi ers Recommended for Use with the LTC2757 with Relevant Specifi cations
AMPLIFIER
AMPLIFIER SPECIFICATIONS
V
OS
μV
I
B
nA
A
VOL
V/mV
VOLTAGE
NOISE
nV/√Hz
CURRENT
NOISE
pA/√Hz
SLEW
RATE
V/μs
GAIN BANDWIDTH
PRODUCT
MHz
t
SETTLING
with LTC2757
μs
POWER
DISSIPATION
mW
LTC1150 10 0.05 5600 90 0.0018 3 2.5 10ms 24
LT1001 25 2 800 10 0.12 0.25 0.8 120 46
LT1012 25 0.1 2000 14 0.02 0.2 1 120 11.4
LT1097 50 0.35 2500 14 0.008 0.2 0.7 120 11
LT1468 75 10 5000 5 0.6 22 90 2.1 117