EVAL-AD5933EB Datasheet, PDF (33/40 Pages) Analog Devices – 1 MSPS, 12-Bit Impedance Converter, Network Analyzer

Data Sheet
ELECTRO-IMPEDANCE SPECTROSCOPY
The AD5933 has found use in the area of corrosion monitoring.
Corrosion of metals, such as aluminum and steel, can damage
industrial infrastructures and vehicles such as aircraft, ships,
and cars. This damage, if left unattended, may lead to premature
failure requiring expensive repairs and/or replacement. In
many cases, if the onset of corrosion can be detected, it can
be arrested or slowed, negating the requirement for repairs or
replacement. At present, visual inspection is employed to detect
corrosion; however, this is time consuming, expensive, and
cannot be employed in hard-to-access areas.
An alternative to visual inspection is automated monitoring
using corrosion sensors. Monitoring is cheaper, less time
consuming, and can be deployed where visual inspections are
impossible. Electrochemical impedance spectroscopy (EIS) has
been used to interrogate corrosion sensors, but at present large
laboratory test instruments are required. The AD5933 offers an
accurate and compact solution for this type of measurement,
enabling the development of field deployable sensor systems
that can measure corrosion rates autonomously.
Mathematically, the corrosion of aluminum is modeled using an
RC network that typically consists of a resistance, RS, in series
with a parallel resistor and capacitor, RP and CP. A system metal
would typically have values as follows: RS is 10 Ω to 10 kΩ,
RP 1 is kΩ to 1 MΩ, and CP is 5 µF to 70 µF. Figure 38 shows
a typical Bode plot, impedance modulus, and phase angle vs.
frequency, for an aluminum corrosion sensor.
AD5933
100k
–75
10k
–50
1k
–25
100
10
0.1
1
0
10
100
1k
10k
100k
FREQUENCY (Hz)
Figure 38. Bode Plot for Aluminum Corrosion Sensor
To make accurate measurements of these values, the impedance
needs to be measured over a frequency range of 0.1 Hz to 100 kHz.
To ensure that the measurement itself does not introduce a
corrosive effect, the metal needs to be excited with minimal
voltage, typically in the ±20 mV range. A nearby processor
or control unit such as the ADuC702x would log a single
impedance sweep from 0.1 kHz to 100 kHz every 10 minutes
and download the results back to a control unit. To achieve
system accuracy from the 0.1 kHz to 1 kHz range, the system
clock needs to be scaled down from the 16.776 MHz nominal
clock frequency to 500 kHz, typically. The clock scaling can be
achieved digitally using an external direct digital synthesizer
like the AD9834 as a programmable divider, which supplies a
clock signal to MCLK and which can be controlled digitally by
the nearby microprocessor.
Rev. E | Page 33 of 40