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AI 400 SERIES PROBES Page 53

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380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

The AI 403 picoammeter probe is applicable to a broad range of physiological and other measurement

tasks, such as:

Scanning tunneling microscope (STM)

Surface corrosion studies

Photomultiplier tube (PMT) amplifier

Infrared (IR) detector amplifier

Photodiode amplifier

Dielectric breakdown studies

Secondary electrode emission

Current transients in switches

Automatic IC test equipment

The 10 GΩ feedback resistor is suitable for the measurement of current ranging from a few

femtoamps up to 1000 picoamps.

While the AI 403 picoammeter probe is usually configured to measure current into a virtual ground,

provision is made for the user to set the virtual potential, so that current may be measured to any

potential chosen by the user, in the range ±2 V.

AI 405, x10, Differential Amplifier

This is the least expensive differential amplifier in the AI 400 series. It has a good common-mode

rejection ratio, low noise and minimum drift. Other than the slightly higher noise, it resembles the

AI 401 in its performance and range of applications.

Transducer Adapter

AI 417 Passive 2 mm Adapter

The AI 417 adapter directly connects EMG, ECG, EEG and other electrodes to the positive and

negative inputs of the CyberAmp through a shielded cable. Compared to the active amplifier probes,

its main advantages are low cost and fully automatic measurement of electrode resistances.

The AI 417 adapter does not provide buffering circuitry and therefore is restricted to electrodes that

have impedances no greater than a few tens of kΩ. Electrode impedances greater than a few tens of

kΩ create at least three problems. First, the 1 MΩ input impedance of the CyberAmp in series with

the electrode impedance acts as a voltage divider, attenuating the signal. Second, the bandwidth of

the signal is limited by the low-pass filter formed by the electrode impedance and the cable

capacitance (360 pF). Third, interference from line-frequency pickup (hum) may increase. The

maximum electrode impedance that does not create these problems is 50 kΩ. In this case, there is a

5% error in the amplitude and the bandwidth is limited to about 10 kHz.

In addition, the resistance of the two electrodes should differ by less than a few kΩ, or the system

common-mode rejection ratio is degraded, increasing the common-mode artifacts and noise.

If electrode resistances greater than 50 kΩ are used, consider using one of the active differential

amplifier probes, such as the AI 401, the AI 402, or the AI 405.

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Microsoft LPF-00004 Betreibershandbuch Seite 71