LTC2410 Linear Technology, LTC2410 Datasheet - Page 34

• Manufacturer Part Number: LTC2410
• Description: 24-Bit No Latency ADC with Differential Input and Differential Reference
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC2410
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.
Figure 48 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
half the differential gain. The maximum differential signal
that can be used is 1/4 V
2-amplifier topology above.
34
BRIDGE
350
V
= 1+ R2/(R1+175 ). Common mode gain is
U
REF
U
RN1
RN1 = 5k 8 RESISTOR ARRAY
U1A, U1B, U2A, U2B = 1/2 LTC1051
, as opposed to 1/2 V
16
Figure 47. Using Autozero Amplifiers to Reduce Input Referred Noise
1
6
RMS
W
3
2
6
5
11
+
–
–
+
. The buffer stages
15
2
U1A
U1B
5V
4
7
8
10
14
U
0.1 F
REF
3
1
7
8
in the
9
13
4
5
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 49. The LTC2410 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.
The basic circuit shown in Figure 49 shows connections
for a full 4-wire connection to the sensor, which may be
located remotely. The differential input connections will
reject induced or coupled 60Hz interference, however, the
1
0.048 V
Input referred noise for A
12
RMS
.
REF
3
2
6
5
. Hence, the reference resistor R1 must be at
+
–
+
–
U2A
U2B
V
5V
for R1. Such a low value for R1 is not
= 34 for approximately 0.05 V
4
8
0.1 F
1
7
2410 F47
3
4
5
6
REF
REF
IN
IN
+
–
LTC2410
+
–
RMS
GND
V
CC
, whereas at a gain of 50, it would be
1, 7, 8, 9,
10, 15, 16
2
SDO
SCK
CS
F
0.1 F
5V
O
REF
11
14
12
13