White Matter NanoZ Manual do Utilizador descarregar pdf (Página 34)

nanoZ User Manual

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For measuring impedance, the nanoZ utilizes a voltage divider circuit:

According to Ohm's law, the ratio of voltages V1 and V2 in the circuit is:

V 2

= 1 +

ref

This formula generalizes to AC sinusoidal signals where V1, V2 and Z

are

complex numbers whose angles represent phase relations in the circuit. When a

known voltage V1 is applied, and V2 is measured, it is possible to solve the

above equation for Z

, which is exactly how the nanoZ measures impedance.

During impedance measurement test currents flow through the circuit. The nanoZ

uses a 4mV peak-to-peak sinusoidal waveform for V1, which yields a maximum

test current through Z

of 1.4nA RMS when Z

is approaching zero, and 0.7nA

RMS when Z

is 1MOhm.

The nanoZ has a single measurement circuit, including the generator voltage V1,

the amplifier for V2, and the reference resistor R

ref

. Different channels, having

different electrode impedances Z

, are connected to this circuit via an on-board

64-to-1 analog multiplexer. Here is a simplified schematic of the overall circuit:

Either the impedance measurement circuit or the electroplating constant current

source can be connected to a channel via switches SW1 and SW2.

The electroplating current source is programmed by a voltage coming from an 8-

bit DAC, yielding 256 current steps between -12uA (electrode negative) and

+12uA (electrode positive). The DAC can produce both DC and alternating

waveforms from the nanoZ's on-board memory.

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White Matter NanoZ Manual do Utilizador Página 34