Complete Sensor-to-Bits

Complete Sensor-to-Bits
Complete Sensor-to-Bits
Solution Simplifies Industrial
Data-Acquisition System Design
±20 mA ranges. When long cables with substantial electromagnetic
interference (EMI) are encountered, current loops are often used
due to their inherently high noise immunity.
Analog output modules typically control actuators, such as relays,
solenoids, and valves, to complete the automated-control system.
They typically provide output voltages with 5 V, 10 V, ±5 V, and
±10 V full-scale ranges and 4 mA to 20 mA current-loop outputs.
By Maithil Pachchigar
Introduction
Typical analog I/O modules include 2, 4, 8, or 16 channels.
To meet stringent industry standards, these modules require
protection against overvoltage, overcurrent, and EMI surges.
Most PLCs include digital isolation between the ADC and the
CPU and between the CPU and the DAC. High-end PLCs may
also incorporate channel-to-channel isolation, as specified by
the International Electrotechnical Commission (IEC) standards.
Many I/O modules include per-channel software programmable
single-ended or differential input ranges, bandwidth, and
throughput rate.
Thus, designers of industrial equipment and critical infrastructure
encounter significant challenges in meeting their customers’
stringent accuracy, noise, drift, speed, and safety requirements.
Using a PLC as an example, this article shows how the versatile,
low cost, highly integrated ADAS3022 reduces complexity, solving
many challenges encountered in the design of multichannel dataacquisition systems by replacing the analog front-end (AFE) stages.
Ideally suited for precision industrial, instrumentation, power line,
and medical data-acquisition cards with multiple input ranges, this
high-performance device reduces cost and time to market while
offering a small, easy-to-use footprint and true 16-bit precision
at 1 MSPS.
In modern PLCs, the CPU performs numerous control tasks in
an automated manner, employing real-time access to information
to make intelligent decisions. The CPU may embody advanced
software and algorithms, and web connectivity for diagnostic error
checking and fault detection. Commonly used communication
interfaces include RS-232, RS-485, industrial Ethernet, SPI,
and UART.
At the heart of many industrial automation and process-control
systems, programmable logic controllers (PLCs) monitor and
control complex system variables. Employing multiple sensors and
actuators, PLC-based systems measure and control analog process
variables such as pressure, temperature, and flow. PLCs are found
in diverse applications—such as factories, oil refineries, medical
equipment, and aerospace systems—that require high accuracy
and robust, long-term operation. In addition, the competitive
marketplace demands lower cost and shorter design times.
Discrete Implementation of Data-Acquisition System
Industrial designers can build analog modules for PLCs or
similar data-acquisition systems with discrete high-performance
components, as shown in Figure 2. Key design considerations
include input signal configuration and overall system speed,
accuracy, and precision. The signal chain presented here utilizes
the ADG1208/ADG1209 low-leakage multiplexer, AD8251 fastsettling programmable-gain instrumentation amplifier (PGIA),
AD8475 high-speed funnel amplifier, AD7982 differential-input
18-bit PulSAR ® ADC, and ADR4550 ultralow-noise voltage
reference. This solution provides four different gain ranges, but
with maximum input signals of ±10 V, designers will have to worry
about the multiplexer’s switching and settling times, as well as
other analog signal conditioning challenges. In addition, achieving
true 16-bit performance at 1 MSPS can be a major challenge, even
when using these high-performance components.
PLC Application Example
Figure 1 shows a simplified signal chain for a PLC used in
industrial automation and process-control systems. The PLC
typically comprises analog and digital input/output (I/O) modules,
a central processing unit (CPU), and power-management circuitry.
In industrial applications, analog input modules acquire and
monitor signals from remote sensors located in harsh environments
characterized by extreme temperature and humidity, vibration,
and explosive chemicals. Typical signals include single-ended or
differential voltages with 5 V, 10 V, ±5 V, and ±10 V full-scale
ranges, or current loops with 0 mA to 20 mA, 4 mA to 20 mA, and
POWER
MANAGEMENT
COMMON INDUSTRIAL
V/I INPUTS
0mA TO 20mA
4mA TO 20mA
±20mA
DIGITAL
ISOLATOR
IN-AMP/
PGA
MUX
ADC
BUFFER
ANALOG INPUT MODULE
DIGITAL
ISOLATOR
CPU
REF
OP AMP
DAC
BUFFER
PROTECTION
CURRENT LOOPS
TO ALL COMPONENTS
BUFFER
PROTECTION
0V TO 5V
0V TO 10V
±5V
±10V
TO INDUSTRIAL
ACTUATORS
V/I OUTPUTS
0V TO 5V, 0V TO 10V, ±5V, ±10V
0mA TO 20mA, 4mA TO 20mA
REF
ANALOG OUTPUT MODULE
Figure 1. Typical PLC signal chain.
Analog Dialogue 47-04, April (2013)
www.analog.com/analogdialogue
1
The AD7982 specifies a 290-ns transient response from a fullscale step. Thus, to guarantee the specified performance while
converting at 1 MSPS, the PGIA and funnel amp must settle
in less than 710 ns. The AD8251 specifies 785-ns settling time
to 16 bits (0.001%) for a 10-V step, however, so the maximum
throughput that can be guaranteed for this signal chain will be
less than 1 MSPS.
+2.5V
+0.5V
GAIN = 0.16, 0.2, 0.4, 0.8, 1.6
INL MAX = 0.649
INL MIN = –0.592
1.5
1.0
0.5
INL (LSB)
+4.5V
4V
2.0
0
–0.5
±10V RANGE
±5V RANGE
±2.5V RANGE
±1.25V RANGE
MUX
IN AMP
ADG1208/
ADG1209
±10V FUNNEL
TRUE-DIFFERENTIAL
ADC
AMP
AD8251
AD8475
G = +1
G = +2
G = +4
G = +8
G = +0.4
–1.0
AD7982
LPF
+4.5V
4V
+2.5V
–1.5
REF
ADR455x
+0.5V
MUX
CONTROL
–2.0
0
8192
16384 24576 32768 40960 49152 57344 65536
Figure 2. Analog input signal chain using discrete components.
CODE
0
Integrated Solution Simplifies Data-Acquisition System Design
iCMOS ®,
VDDH
DIFF
PAIR
IN0/IN1
DIFF
COM
AVDD
DVDD
VIO
RESET
PD
ADAS3022
CNV
LOGIC/
INTERFACE
IN0
IN4
PulSAR
ADC
PGIA
MUX
IN5
IN6/IN7
COM
–100
–120
SCK
DIN
REFIN
BUF
REF
AUX–
VSSH
AGND
–160
–180
0
100
200
300
400
500
FREQUENCY (kHz)
TEMP
SENSOR
AUX+
DGND
REFx
Figure 3. Functional block diagram of ADAS3022.
This complete sensor-to-bits solution utilizes only one-third of
the board space of discrete implementations, helping engineers to
simplify their designs while reducing the size, time to market, and
cost of advanced industrial data-acquisition systems. Eliminating
the necessity to buffer, level shift, amplify, attenuate, or otherwise
condition the input signal, and the concerns regarding commonmode rejection, noise, and settling time, it alleviates many of the
challenges associated with designing a precision 16-bit, 1-MSPS
data-acquisition system. It delivers the best-in-class 16-bit
accuracy (±0.6-LSB typical INL), low offset voltage, low drift
overtemperature, and optimized noise performance at 1 MSPS
(91-dB typical SNR), as shown in Figure 4. The device is specified
over the –40°C to +85°C industrial temperature range.
2
–80
–140
SDO
IN6
IN7
–60
CS
IN2
IN3
IN4/IN5
–40
BUSY
IN1
IN2/IN3
GAIN = 0.8
fS = 1000kSPS
fIN = 10.1kHz
SNR = 90.7dB
SINAD = 90.6dB
THD = –107dB
SFDR = 106dB
–20
AMPLITUDE (dBFS)
Manufactured in
a proprietary, high-voltage industrial
process technology, the 16-bit, 1-MSPS ADAS3022 dataacquisition IC integrates an 8-channel, low-leakage multiplexer;
a high-impedance PGIA with high common-mode rejection; a
precision, low-drift 4.096-V reference and buffer; and a 16-bit
successive-approximation ADC, as shown in Figure 3.
Figure 4. INL and FFT performance of the ADAS3022.
The PGIA has a large common-mode input range, true highimpedance inputs (>500 MΩ), and a wide dynamic range, allowing
it to accommodate 4-mA to 20-mA current loops, accurately
measure small sensor signals, and reject interference from ac power
lines, electric motors, and other sources (90-dB minimum CMR).
An auxiliary differential input channel can accommodate ±4.096 V
input signals. It bypasses the multiplexer and PGIA stages, allowing
direct interface to the 16-bit SAR ADC. An on-chip temperature
sensor can monitor the local temperature.
This high level of integration saves board space and lowers the
overall parts’ cost, making the ADAS3022 ideal for space-constrained
applications, such as automatic test equipment, power-line monitoring, industrial automation, process control, patient monitoring, and
other industrial and instrumentation systems that operate with ±10-V
industrial signal levels.
Analog Dialogue 47-04, April (2013)
D2
L2
47 H
COUT3
4.7 F
+
C2
1 F
+
1.78
RFILT
L1
47 H
+5V
+
D1
VIN = +5V
CIN +
1 F
C1
1 F
RB0
1
COUT1 +
1 F
L3
1 F
+15V
+ COUT2
2.2 F
VDDH
REN
ENABLE
50k
ADP1613
CC1 +
12nF
+
COMP
CC2
10pF
RC1
100k
FREQ
EN
VIN
GND
SW
CSS +
1 F
CNV
MUX
PulSAR
ADC
PGIA
BUSY
SCK
DIN
SDO
BUF
TEMP
SENSOR
REFIN
REF
AUX–
Z1
DNI
PD
CS
COM
AUX+
CV5 + RS2
1 F
DNI
RESET
LOGIC/
INTERFACE
IN0/IN1
SS
FB
RF2
4.22k
IN0
IN1
IN2
IN2/IN3
IN3
IN4
IN4/IN5
IN5
IN6
IN6/IN7
IN7
VIO
ADAS3022
DIFF DIFF
PAIR COM
RS1
0
AVDD DVDD
VSSH
AGND DGND
REFx
–15V
RF1B
47.5k
Figure 5. Complete 5-V, single-supply, 8-channel data-acquisition solution with integrated PGA.
Figure 5 shows a complete 8-channel data acquisition system (DAS). The ADAS3022 operates with ±15-V and +5-V analog and digital
supplies, and a 1.8-V to 5-V logic I/O supply. The ADP1613 high-efficiency, low-ripple dc-to-dc boost converter allows the DAS to operate
with a single 5-V supply. Configured as a single-ended, primary inductance Ćuk (SEPIC) topology using the ADIsimPower ™ design
tool, the ADP1613 furnishes the ±15-V bipolar supplies required for the multiplexer and PGIA without compromising performance.
The noise performance of the ADAS3022 and the discrete signal chain are compared in Table 1, which uses the input signal amplitude,
gain, equivalent noise bandwidth (ENBW), and input-referred (RTI) noise of each component to calculate the total noise of the
complete signal chain.
Table 1. Noise Performance for the ADAS3022 and the Discrete Signal Chain
ADG1209
AD8251
AD8475
AD7982
Total Noise
ADAS3022
RTI
RTI
RTI
RTI
SNR
RTI Total
SNR
SNR
(μV rms)
(μV rms)
(μV rms)
(μV rms)
(dB)
(μV rms)
(dB)
(dB)
(V rms)
Gain = 1 (±10 V)
6.56
124
77.5
148
95.5
208
90.6
91.5
7.07
Gain = 2 (±5 V)
6.56
83.7
38.8
74.2
95.5
119
89.5
91.0
3.54
Gain = 4 (±2.5 V)
6.56
68.2
19.4
37.1
95.5
80.3
86.8
89.7
1.77
Gain = 8 (±1.25 V)
6.56
55.8
9.69
18.5
95.5
60.0
83.4
86.8
0.88
Input
Signal
Noise
Analog Dialogue 47-04, April (2013)
3
The single-pole low-pass filter (LPF) between the AD8475
and AD7982 (Figure 2) attenuates the kick coming from the
switched-capacitor input of the AD7982 and limits the amount
of high-frequency noise. The –3-dB bandwidth (f –3dB) of the
LPF is 6.1 MHz (R = 20 Ω, C = 1.3 nF), allowing fast settling
of the input signals while converting at 1 MSPS. The ENBW of
the LPF can be calculated as
ENBW = π/2 × f–3dB = 9.6 MHz.
Note that this calculation ignores the noise from the voltage
reference and LPF as it does not significantly affect the total noise,
which is dominated by the PGIA.
Consider an example using the ±5-V input range. In this case,
the AD8251 is set for a gain of 2. The funnel amplifier is set to
a fixed gain of 0.4 for all four input ranges, so a 0.5-V to 4.5-V
differential signal (4 V p-p) will be applied to the AD7982. The
RTI noise of the ADG1208 is derived from the Johnson/Nyquist
noise equation (en2 = 4K BTRON, where K B = 1.38 × 10 –23 J/K,
T = 300K, and RON = 270 Ω). The RTI noise of the AD8251 is
derived from its 27-nV/√Hz noise density as specified in the data
sheet for a gain of 2. Similarly, the RTI noise of the AD8475
is derived from its 10-nV/√Hz noise density using a gain of 0.8
(2 × 0.4). In each calculation, ENBW = 9.6 MHz. The RTI noise
of the AD7982 is calculated from its 95.5-dB SNR as specified in
the data sheet using a gain of 0.8. The total RTI noise of the entire
signal chain is calculated based on the root-sum-square (rss) of the
RTI noise from the discrete components. The total SNR of 89.5 dB
can be computed from the equation SNR = 20 log(VINrms/RTITotal ).
Although the theoretical noise estimate (SNR) and the overall
performance of the discrete signal chain is comparable to that of
the ADAS3022, especially at lower gains (G = 1 and G = 2) and
lower throughput rates (much less than 1 MSPS), it’s not an ideal
solution. The ADAS3022 can reduce cost by about 50% and board
space by about 67%, as compared to the discrete solution, and it
can also accept three additional input ranges (±0.64 V, ±20.48 V,
and ±24.576 V) that the discrete solution cannot offer.
4
Conclusion
The next generation of industrial PLC modules will demand high
accuracy, reliable operation, and functional flexibility, all in a
small, low-cost form factor. The ADAS3022, with industry leading
integration and performance, supports a wide range of voltage
and current inputs to handle a variety of sensors in industrial
automation and process control. An ideal fit for PLC analog
input modules and other data-acquisition cards, the ADAS3022
will enable industrial manufacturers to differentiate their systems
while meeting stringent user requirements.
References
Kessler, Matthew. Synchronous Inverse SEPIC Topology Provides
High Efficiency for Noninverting Buck/Boost Voltage Converters,
Analog Dialogue, Vol. 44, No. 2, 2010.
Slattery, Colm, Derrick Hartmann, and Li Ke. PLC Evaluation
Board Simplifies Design of Industrial Process Control Systems,
Analog Dialogue, Vol. 43, No. 2, 2009.
Circuit Note CN0201. Complete 5 V, Single-Supply, 8-Channel
Multiplexed Data Acquisition System with PGIA for Industrial
Signal Levels.
MT-048 Tutorial. Op Amp Noise Relationships; 1/f Noise, RMS
Noise, and Equivalent Noise Bandwidth.
Author
Maithil Pachchigar [[email protected]]
is an applications engineer in ADI’s Precision
Converters business unit in Wilmington, MA. He
joined ADI in 2010 and supports the precision ADC
product portfolio and customers in the industrial,
instrumentation, medical, and energy segments.
Having worked in the semiconductor industry since 2005, Maithil
has published several technical articles. He received an MSEE from
San Jose State University in 2006 and an MBA from Silicon Valley
University in 2010.
Analog Dialogue 47-04, April (2013)
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