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When I'm examining objects dominated by double lay ...
Though it seems simple, capacities are not at all easy objects for an electrochemical measuring system. One reason for problems may be the instability of the feedback loop of the potentiostat used. The capacity adds phase shift to the loop. So undetected parasitic oscillations of the potentiostat may produce irregular results such as "starry skies" or severe current offsets.
Another problem arises with the measuring method itself used by certain equipment. In the era of computers it seems easy to change from the traditional analog scan technique to its digital approximation by a small steps stairs technique. This task can be done by a software controlled D/A converter. The advantage of such a technique is the possibility to easily produce arbitrary wave forms, for instance stable slow signals (slew rate down to zero). The measurement itself too can be performed with the computer using A/D converters. This is the base for flexible and user-friendly CV equipment - but the stair-sweep-approximation idea is too simple to be good! Signals, created by a DA-converter remain discrete. Even if one tries to increase the resolution dramatically, it will stay finite. On the other hand the theories of CV techniques claim steady sweep signals.
Regarding capacitive objects will help to focus on this important difference. If one applies a voltage signal U(t) to a capacitor, the current I(t)=C·dU/dt will flow. A constant slew rate results in a constant current. Discrete steps will cause current pulses with d-pulse shape - that means in theory infinite height for an infinite short time interval but with a well defined integral charge of Q=C·DU. In practice the d-pulse will be distorted to a short pulse of high amplitude. Its shape is determined by the pulse response of the potentiostat and parasitic effects. If the measurement technique samples the response signal after a short time delay relative to the step, the result must be wrong! The only way to get a correct current result is to measure the charge by integrating the total step interval and calculate the mean current. This calculated current value is identical with the measured one with the continuous method.
How can you find out, if your equipment uses this "clean" integrating CV technique? Take an aluminum electrolyte capacitor of 1 mF and perform a test measurement. Choose, for instance, a triangle scan of ±1 V at ±100 mV/s slew rate. The capacitance of 1 mF should cause a square wave of ±0.1 mA current to flow. Consider the capacity tolerance of -5 % / +25 %. Check the correct capacity using an impedance measurement at low frequency (e.g. 1 Hz): Now the measured current should fit the calculated current exactly.
If not, call the service-hotline of your equipment's manufacturer!
What influence have connection cords between cell ...
One of the most critical elements of an impedance measurement setup are the cell and the connection cords between cell and measurement system. By reason of its impedance (capacitance, inductance and resistance) the cords work as a low-pass filter that will damp high frequencies and cause a small phase shift. Beside this the cables are antennas that can pick up every electromagnetic field such as the 50/60 Hz power line frequency.
So what can you do? You should use relatively thick cables (more than 3 mm in diameter) as short as possible if you examine low impedance objects. The connectors should be high quality and of BNC- or Lemosa-type. You should not lay the cables in parallel with a line or another high voltage or high frequency cord.
How can Gamry design a low-noise Potentiostat that ...
Some may tell you that a Gamry Potentiostat is noisy because it's installed inside a noisy computer.
Here's why they're wrong.
The Bottom Line
Gamry’s specification for “Noise and Ripple” of the applied potential for the PCI4/300, PCI4/750, and FAS2 Potentiostats is 20 µV(microVolts) rms (1 Hz – 10 kHz). We guarantee this specification for our Potentiostats in any Windows-compatible computer. Compare this value to the noise specification for any other commercial potentiostat. Our search found no instrument whose noise specification matched or exceeded Gamry’s!
In the lore of scientific instruments, a computer is considered to be electronically noisy. If that is true, then it seems counter-intuitive to expect a potentiostat that is installed inside a computer to exhibit low noise. How does Gamry do it?
Is a computer noisy? Yes, a computer has very high noise levels inside its cabinet. A computer is a digital device, which generates both magnetic and electric fields as its internal signals switch. These fields can couple noise into analog circuits. However (and this is important) the noise in a computer occurs at very high frequencies, typically greater than 100 MHz. These frequencies are far higher than the frequencies that are important to electrochemistry.
Grounding. The Gamry Potentiostat uses optical couplers to electrically isolate the analog potentiostat from the earth ground of the computer in which it is installed. Large AC currents flowing through the computer’s ground would otherwise create noise in the potentiostat.
Filters. The Gamry Potentiostat contains three analog filters at 5 Hz, 1000 Hz, and 200 kHz. One of these filters is always active, discriminating against the computer’s high frequency noise.
Shielding. If you examine a Gamry Potentiostat, you’ll see a big sheet of metal that completely covers the analog components of the Potentiostat. This is a shield that protects the Potentiostat from electrostatic pickup of computer noise.
What about the other guys? If Gamry’s noise spec is 20 µV rms, shouldn’t other manufacturers have an even lower noise spec? After all, their potentiostat is not inside the computer. It turns out that the greatest source of noise for a potentiostat that is connected to the power grid through a receptacle in the wall is the 50 Hz or 60 Hz signal from the mains. AC power voltage presents two problems: (1) its amplitude is very large, either 110 or 220 volts, and (2) it is at a frequency that is of interest to the electrochemist, so filtering is difficult. Noise may be a real problem at the mains frequency or at some harmonic, e.g., 120 Hz or 180 Hz.
Since we’re on the subject, what does “Noise and Ripple” really mean? For a Gamry Potentiostat, the noise level that will be observed on the applied potential is less than 20 µV rms (root mean square). To convert rms to peak-to-peak, multiply by 1.414 (square root of 2) to convert to a peak signal, then multiply by 2 to convert to peak-to-peak.
Noise in the applied voltage often shows up in the current measurement. A 1 cm2 electrode with 40 µF of capacitance has an impedance of about 30 ohms at 120 Hz. A potentiostat with 50 µV of noise at 120 Hz connected to this electrode will generate a noise current greater than 4 µA peak-to-peak. Good luck measuring a 10 µA current signal that is contaminated with this much noise!
Make a smart decision. If someone tells you something negative about Gamry, they may not have your best interests in mind. Contact us and ask for our comments.