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LTC2420CS8#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36
31
LTC2420
TYPICAL APPLICATIO S
U
Single-Chip Instrumentation Amplifier
for the LTC2420
The circuit in Figure 37 is a simple solution for processing
differential signals in pressure transducer, weigh scale or
strain gauge applications that can operate on a supply
voltage range of ±5V to ±15V. The circuit uses an LT
®
1920
single-chip instrumentation amplifier to perform a differ-
ential to single-ended conversion. The amplifier’s output
voltage is applied to the LTC2420’s input and converted to
a digital value with an overall accuracy exceeding 17 bits
(0.0008%). Key circuit performance results are shown in
Table 5.
The practical gain range for this topology as shown is from
5 to 100 because the LTC2420’s wide dynamic range
makes gains below 5 virtually unnecessary, whereas gains
up to 100 significantly reduce the input referred noise.
The optional passive RC lowpass filter between the
amplifier’s output and the LTC2420’s input attenuates
high frequency noise and its effects. Typically, the filter
reduces the magnitude of averaged noise by 30% and
improves resolution by 0.5 bit without compromising
linearity. Resistor R2 performs two functions: it isolates
C1 from the LTC2420’s input and limits the LTC2420’s
input current should its input voltage drop below –300mV
or swing above V
CC
+ 300mV.
The LT1920 is the choice for applications where low cost
is important. For applications where more precision is
required, the LT1167 is a pin-to-pin alternative choice with
a lower offset voltage, lower input bias current and higher
gain accuracy than the LT1920. The LT1920’s maximum
total input-referred offset (V
OST
) is 135µV for a gain of
100. At the same gain, the LT1167’s V
OST
is 63µV. At gains
of 10 or 100, the LT1920’s maximum gain error is 0.3%
and its maximum gain nonlinearity is 30ppm. At the same
gains, the LT1167’s maximum gain error is 0.1% and its
maximum gain nonlinearity is 15ppm. Table 6 summa-
rizes the performance of Figure 37’s circuit using the
LT1167.
Figure 37. The LT1920 is a Simple Solution That Converts a Differential Input
to a Ground Referred Single-Ended Signal for the LTC2420
V
S
+
V
S
–
7
2
1
8
3
4
6
V
IN
+
V
IN
–
R
G
R
G
**
DIFFERENTIAL
INPUT
R
G
3
2
1
††
*OPTIONAL—SEE TEXT
**R
G
= 49.4k/(A
V
– 1): USE 5.49k FOR A
V
= 10; 499Ω FOR A
V
= 100
†
USE SHORT LEAD LENGTHS
†
48
2420 F37
5
6
7
CHIP SELECT
SERIAL DATA OUT
SERIAL CLOCK
CS
SDO
SCK
0.1µF
0.1µF5V
R1*
47Ω
C1*
1µF
SINGLE POINT
“STAR” GROUND
R2*
10k
0.1µF
V
IN
V
REF
V
REFIN
V
CC
GND
LTC2420
F
O
LT1920
32
LTC2420
Table 6. Typical Performance of the LTC2420 ADC When Used with the
LT1167 Instrumentation Amplifiers in Figure 34’s Differential Digitizing Circuit
V
S
= ±5V V
S
= ±15V
PARAMETER A
V
= 10 A
V
= 100 A
V
= 10 A
V
= 100 TOTAL (UNITS)
Differential Input Voltage Range – 30 to 400 –3 to 40 –30 to 500 –3 to 50 mV
Zero Error –94 –1590 –110 –1470 µV
Maximum Input Current 0.5 nA
Nonlinearity ±4.1 ±4.4 ±4.1 ±3.7 ppm
Noise (Without Averaging) 1.4* 0.19* 1.5* 0.18* µV
RMS
Noise (Averaged 64 Readings) 0.18* 0.02* 0.19* 0.02* µV
RMS
Resolution (with Averaged Readings) 21.4 21.0 21.3 21.1 Bits
Overall Accuracy (Uncalibrated) 18.2 18.1 18.2 19.4 Bits
Common Mode Rejection Ratio ≥120 dB
Common Mode Range 2/–1.5** 2.2/–1.7** 11.5/–11** 11.7/–11.2** V
*Input referred noise for the respective gain. **Typical values based on single lab tested sample of each amplifier.
TYPICAL APPLICATIO S
U
Table 5. Typical Performance of the LTC2420 ADC When Used with the
LT1920 Instrumentation Amplifiers in Figure 34’s Differential Digitizing Circuit
V
S
= ±5V V
S
= ±15V
PARAMETER A
V
= 10 A
V
= 100 A
V
= 10 A
V
= 100 TOTAL (UNITS)
Differential Input Voltage Range – 30 to 400 –3 to 40 –30 to 500 –3 to 50 mV
Zero Error –160 –2650 –213 –2625 µV
Maximum Input Current 2.0 nA
Nonlinearity ±8.2 ±7.4 ±6.5 ±6.1 ppm
Noise (Without Averaging) 1.8* 0.25* 1.5* 0.27* µV
RMS
Noise (Averaged 64 Readings) 0.2* 0.03* 0.19* 0.03* µV
RMS
Resolution (with Averaged Readings) 21 20.6 21.3 20.5 Bits
Overall Accuracy (Uncalibrated) 17.2 17.3 17.5 18.2 Bits
Common Mode Rejection Ratio ≥120 dB
Common Mode Range 2/–1.5** 2.2/–1.7** 11.5/–11** 11.7/–11.2** V
*Input referred noise for the respective gain. **Typical values based on single lab tested sample of each amplifier.
33
LTC2420
TYPICAL APPLICATIO S
U
Using a Low Power Precision Reference
The circuit in Figure 38 shows the connections and by-
passing for an LT1461-2.5 as a 2.5V reference. The
LT1461 is a bandgap reference capable of 3ppm/°C tem-
perature stability yet consumes only 45µA of current. The
1k resistor between the reference and the ADC reduces the
transient load changes associated with sampling and
produces optimal results. This reference will not impact
the noise level of the LTC2420 if signals are less than 60%
full scale, and only marginally increases noise approach-
ing full scale. Even lower power references can be used if
only the lower end of the LTC2420 input range is required.
A Differential to Single-Ended Analog Front End
Figure 39 shows the LT1167 as a means of sensing
differential signals. The noise performance of the LT1167
is such that for gains less than 200, the noise floor of the
LTC2420 remains the dominant noise source. At the point
where the noise of the amplifier begins to dominate, the
input referred noise is essentially that of the instrumenta-
tion amplifier. The linearity of the instrumentation ampli-
fier does, however, degrade at higher gains. As a result, if
the full linearity of the LTC2420 is desired, gain in the
instrumentation amplifier should be limited to less than
100, possibly requiring averaging multiple samples to
extend the resolution below the noise floor. The noise level
of the LT1167 at gains greater than 100 is on the order of
50nV
RMS
, although, 1/f noise and temperature effects may
degrade this below 0.1Hz. The introduction of a filter
between the amplifier and the LTC2420 may improve
noise levels under some circumstances by reducing noise
bandwidth. Note that temperature offset drift effects enve-
lope detection in the input of the LT1167 if exposed to RFI,
thermocouple voltages in connectors, resistors and sol-
dered junctions can all compromise results, appearing as
drift or noise. Turbulent airflow over this circuitry should
be avoided.
+
LT1461-2.5
GND
IN
2420 F38
10µF
16V
TANT
TO
LTC2420
REF
OUT5V
1k
0.1µF
CER
+
–
+
LT1167
3.5k
×4
1
2
R
G
5V
8
3
5
1µF
22Ω
5k
1
10µF
2
3
A
V
=
RECOMMENDED R
G
: 500Ω, 0.1%
5ppm/°C
+ 1
49.4kΩ
R
G
4
OPTIONAL
6
2428 F39
–5V
5V
LTC2420
Figure 38. Low Power Reference
Figure 39. A Differential to Single-Ended Analog Front End
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LTC2420CS8#TRPBF

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Manufacturer:
Analog Devices Inc.
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Analog to Digital Converters - ADC 20-B Pwr No Lat Delta-Sigma ADC in SO-
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