Auxiliary A/D Input Characteristics
96
Input Impedance Selection
Two jumpers are associated with the input impedance, J902 and J903. With J902 and J903 installed, the Aux
A/D input has a 100 k
input impedance. This is the default setting. With the jumpers installed, the
potentiostat can be calibrated without a cable on the input BNC connector.
With both J902 and J903 removed, the Aux A/D input impedance is 10 G
(typically). The High Impedance
setting is required when measuring a signal. The Low Impedance setting is ideal for lowering noise pickup
when the BNC is not in use. So High Impedance is preferred for making measurements with the BNC, and Low
Impedance when nothing is connected to the BNC.
The CMOS switch shorts the circuit nodes connected by these jumpers. As with the jumpers, in the low-
impedance setting, the input impedance is set to 100 k
.
In the high-impedance setting, the CMOS switches
are opened, increasing the input impedance to about 10 G
.
Bandwidth Selection
J900 (or on some boards J901) controls the bandwidth of the Aux A/D input. With J900 (or J901) removed,
there is no filtering on this input, and the –3 dB bandwidth is greater than 300 kHz.
With J900 (or J901) in place (the default setting), a single-pole RC filter is used to limit noise into the A/D. The
nominal cutoff frequency of this filter is 20 Hz.
Note that source impedances greater than 1 k
appreciably lower this cutoff frequency. When the Aux A/D
input is driven by a source with an output impedance of 1 M
, the frequency cutoff is less than 0.25 Hz.
The CMOS switch performs the same function as the jumper.
Aux A/D Specifications
Range
3.276 V
Input Impedance
100 k
(approx)
Low Impedance Selected
10 G
(typical)
High Impedance Selected
Input Bias Current
< 10 nA
High Impedance selected
Filter Cutoff
20 Hz
20%
With Low Bandwidth selected
Function Call to Set the Aux A/D BNC Characteristics
The CMOS switch is set using an Explain function call. The syntax of this call is:
Pstat.SetAuxBNCImpedance (Impedance)
The variable passed to the function is a Boolean (with a value of TRUE or FALSE).
The following definitions can be used to make a script that calls this function more readable.
#define
SETAUXBNCSETTINGS_IMPEDANCE_HIGH TRUE
#define
SETAUXBNCSETTINGS_IMPEDANCE_LOW FALSE
A TRUE in the Impedance variable opens the switches across J902 and J903, selecting high input impedance. A
FALSE selects 100 kΩ input impedance.
If you have removed jumpers J902 and J903,
do not
calibrate the potentiostat unless you
have a cable connecting both Aux A/D inputs to floating ground.
