SBAS582C – JULY 2014 – REVISED APRIL 2015
9.2.2.1 Design Requirements
This reference design provides a complete solution for a single-supply industrial control analog input module.
The design is suitable for process control end equipment such as programmable logic controllers (PLCs),
distributed control systems (DCS) and data acquisition systems (DAS) modules that must digitize standard
industrial current inputs, and bipolar or unipolar input voltage ranges up to ±10 V. In an industrial environment,
the analog voltage and current ranges typically include ±2.5 V, ±5 V, ±10 V, 0 V to 5 V, 0 V to 10 V, 4 mA to
20 mA, and 0 mA to 20 mA. This reference design can measure all standard industrial voltage and current
inputs. Eight channels are provided on the module, and each channel can be configured as a current or voltage
input with software configuration.
The key design requirements are given below:
•
Up to eight channels of user-programmable inputs:
–
Voltage inputs (with a typical Z
IN
of 1 M
Ω
): ±10 V, ±5 V, ±2.5 V, 0 V to 10 V and 0 V to 5 V.
–
Current inputs (with a Z
IN
of 300
Ω
): 0 mA to 20 mA, 4 mA to 20 mA, and ±20 mA.
•
A 16-bit SAR ADC with SPI.
•
Accuracy of
≤
0.2% at 25°C over entire input range of voltage and current inputs.
•
Onboard isolated Fly-Buck™ power supply with inrush current protection.
•
Slim-form factor 96 × 50.8 × 10 mm (L × W × H).
•
LabView-based GUI for signal-chain analysis and functional testing.
•
Designed to comply with IEC61000-4 standards for ESD, EFT, and surge.
9.2.2.2 Detailed Design Procedure
The application circuit and system diagram for this design is shown in
The module has eight analog input channels, and each channel can be configured as a current or voltage input
with software configuration. The design uses the ADS8688 (16-bit, 8-channel, single-supply SAR ADC) with an
on-chip PGA and reference. The on-chip PGA provides a high-input impedance (typically 1 M
Ω
) and filters noise
interference. The on-chip, 4.096-V, ultra-low drift voltage reference is used as the reference for the ADC core.
The digital isolation is achieved using an
and
. The host microcontroller communicates
with a
(an 8-bit, I
2
C, I/O expander over an I
2
C bus). The ISO1541D is a bidirectional, I
2
C isolator that
isolates the I
2
C lines for the TCA6408A. The TCA6408A controls the low R
ON
opto-switch (TLP3123), which is
used to switch between voltage-to-current input modes. The input channel configuration is done in
microcontroller firmware.
A low-cost, constant, on-time, synchronous buck regulator in fly-buck configuration with an external transformer
(
) generates the isolated power supply. The LM5017 has a wide input supply range, making this device
ideal for accepting a 24-V industrial supply. This transformer can accept up to 100 V, thereby making reliable
transient protection of the input supply more easily achievable. The fly-buck power supply isolates and steps the
input voltage down to 6 V. The supply then provides that voltage to the
(the low dropout regulator) to
generate 5 V to power the ADS8688 and other circuitry. The LM5017 also features a number of other safety and
reliability functions, such as undervoltage lockout (UVLO), thermal shutdown, and peak current limit protection.
Input analog signals are protected against high-voltage, fast-transient events often expected in an industrial
environment. The protection circuitry makes use of the transient voltage suppressor (TVS) and ESD diodes. The
RC low-pass mode filters are used on each analog input before the input reaches the ADS8688, which eliminates
any high-frequency noise pickups and minimizes aliasing.
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