LTC2413 LINER [Linear Technology], LTC2413 Datasheet - Page 37

• Manufacturer Part Number: LTC2413
• Description: 24-Bit No Latency ADC, with Simultaneous 50Hz/60Hz Rejection
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
has common mode rejection far beyond that of most
amplifiers. The LTC1051 is a dual autozero amplifier that
can be used to produce a gain of 15 before its input
referred noise dominates the LTC2413 noise. This ex-
ample shows a gain of 34, that is determined by a feedback
network built using a resistor array containing 8 individual
resistors. The resistors are organized to optimize tem-
perature tracking in the presence of thermal gradients. The
second LTC1051 buffers the low noise input stage from
the transient load steps produced during conversion.
The gain stability and accuracy of this approach is very
good, due to a statistical improvement in resistor match-
ing due to individual error contribution being reduced. A
gain of 34 may seem low, when compared to common
practice in earlier generations of load-cell interfaces, how-
ever the accuracy of the LTC2413 changes the rationale.
Achieving high gain accuracy and linearity at higher gains
may prove difficult, while providing little benefit in terms
of noise reduction.
At a gain of 100, the gain error that could result from
typical open-loop gain of 160dB is –1ppm, however,
worst-case is at the minimum gain of 116dB, giving a gain
error of –158ppm. Worst-case gain error at a gain of 34,
is –54ppm. The use of the LTC1051A reduces the worst-
case gain error to –33ppm. The advantage of gain higher
than 34, then becomes dubious, as the input referred
noise sees little improvement
tially compromised.
Note that this 4-amplifier topology has advantages over
the typical integrated 3-amplifier instrumentation ampli-
fier in that it does not have the high noise level common in
the output stage that usually dominates when an instru-
mentation amplifier is used at low gain. If this amplifier is
used at a gain of 10, the gain error is only 10ppm and input
referred noise is reduced to 0.1 V
can also be configured to provide gain of up to 50 with high
gain stability and linearity.
U
U
1
and gain accuracy is poten-
RMS
W
. The buffer stages
U
Figure 45 shows an example of a single amplifier used to
produce single-ended gain. This topology is best used in
applications where the gain setting resistor can be made
to match the temperature coefficient of the strain gauges.
If the bridge is composed of precision resistors, with only
one or two variable elements, the reference arm of the
bridge can be made to act in conjunction with the feedback
resistor to determine the gain. If the feedback resistor is
incorporated into the design of the load cell, using resis-
tors which match the temperature coefficient of the load-
cell elements, good results can be achieved without the
need for resistors with a high degree of absolute accuracy.
The common mode voltage in this case, is again a function
of the bridge output. Differential gain as used with a 350
bridge is A
is half the differential gain. The maximum differential
signal that can be used is 1/4 V
in the 2-amplifier topology above.
Remote Half Bridge Interface
As opposed to full bridge applications, typical half bridge
applications must contend with nonlinearity in the bridge
output, as signal swing is often much greater. Applications
include RTD’s, thermistors and other resistive elements
that undergo significant changes over their span. For
single variable element bridges, the nonlinearity of the half
bridge output can be eliminated completely; if the refer-
ence arm of the bridge is used as the reference to the ADC,
as shown in Figure 46. The LTC2413 can accept inputs up
to 1/2 V
least 2x the highest value of the variable resistor.
In the case of 100 platinum RTD’s, this would suggest a
value of 800
advisable due to self-heating effects. A value of 25.5k is
shown for R1, reducing self-heating effects to acceptable
levels for most sensors.
1
0.048 V
Input referred noise for A
RMS
.
REF
V
. Hence, the reference resistor R1 must be at
= (R1+ R2)/(R1+175 ). Common mode gain
V
for R1. Such a low value for R1 is not
= 34 for approximately 0.05 V
REF
, as opposed to 1/2 V
RMS
, whereas at a gain of 50, it would be
LTC2413
sn2413 2413fs
37
REF