Multichannel Systems nanoZ manual - Manual (Page 37)

33
Appendix
A:
Principle
of
Operation
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:
!
V1
V 2
= 1+
R
ref
Z
x
This formula generalizes to AC sinusoidal signals where V1, V2 and Z
x
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
x
, 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
x
of 1.4nA RMS when Z
x
is approaching zero, and 0.7nA
RMS when Z
x
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
x
, 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.