Guide to STM Amplifier Sensitivity
The pre-amp converts the tunnelling current to a voltage
that is used by the feedback circuit.
Pre-amp sensitivity settings are awkward but are
important to the proper function of the STM.
There are three things you need to be sure of:
1. The pre-amp sensitivity.
2. The position of the gain switch on the scanner.
3. The software Conversion Coefficient (nA/V)
(found in View  Setup).
Hardware and software need to be consistent for the STM to work
properly or the system can become unstable or simply output data
that is not the same as the settings in the software.
1. Pre-amp Sensitivity
There are three pre-amp types that are
differentiated by their maximum sensitivity:
•0.1 nA/V
•1 nA/V
•10 nA/V
The fool-proof way to find out which you have:
1. Remove the four screws on the tip
holder piece.
2. Carefully pull out the pre-amp
module. Do not pull on the cable.
Each sensitivity is identified by the colour of
the pre-amp circuit board:
0.1 nA/V -> Green
1 nA/V -> Blue
10 nA/V -> Red
2.
1.
2. Gain Switch
×10
×1
The switch on the top of the STM scanner
determines the conversion ratio of the current to
a maximum voltage of 10V.
There are two gain settings, ×1 and ×10.
V
nA
10
9
If we use a 1 nA/V pre-amp (blue board) and the gain switch is set to ×1 (to
the right), the output sensitivity will be 1 nA/V. The pre-amp will convert 1 nA 8
7
to 1 V.
6
If the gain switch is set to the left (current to voltage conversion of ×10), every
5
nA that tunnels is converted to 10V. This means the sensitivity is 0.1 nA/V.
4
A general rule is that the switch in the right side position 3
will give the maximum sensitivity - 1 nA/V as opposed to 2
1
th
0.1 nA/V (1/10 of the right position).
0
10
9
8
7
6
5
4
3
2
1
0
Example:
2. Gain Switch
×10
×1
The switch on the top of the STM scanner
determines the conversion ratio of the current to
a maximum voltage of 10V.
There are two gain settings, ×1 and ×10.
V
10
9
If we use a 1 nA/V pre-amp (blue board) and the gain switch is set to ×1 (to
the right), the output sensitivity will be 1 nA/V. The pre-amp will convert 1 nA 8
7
to 1 V.
6
If the gain switch is set to the left (current to voltage conversion of ×10), every
5
nA that tunnels is converted to 10V. This means the sensitivity is 0.1 nA/V.
4
A general rule is that the switch in the right side position 3
will give the maximum sensitivity - 1 nA/V as opposed to 2
1
th
0.1 nA/V (1/10 of the right position).
0
Example:
nA
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Preamp Summary
×10
×1
The switch on the top of the STM scanner
determines the conversion ratio of the current to
the voltage.
There are two gain settings, ×1 and ×10.
0.01 nA/V ×10
GREEN BOARD
×10
0.1 nA/V
×1
×1
1 nA/V
×10
BLUE BOARD
RED BOARD
10 nA/V
×1
Current to Voltage Conversion
The gain switch simply changes the resistance applied to the
current.
Applying Ohm’s law, R = V/I, to the previous example (blue board):
×10
For a sensitivity of 0.1 nA/V (gain switch on
left), I = 10-8 and V = 1:
V/I = 1/10-8 = 108Ω = 100 MΩ
×1
For a sensitivity 1 nA/V (gain switch on right), I
= 10-9 and V = 1:
V/I = 1/10-9 = 109Ω = 1 GΩ
Higher sensitivity results from higher resistance.
3. Conversion Coefficient
The conversion coefficient (the sensitivity), known from the
pre-amp board colour and the gain switch position, should
be entered into the software in View  Setup:
Blue board, switch set to right. (or red board, switch set to left)
3. Conversion Coefficient
An incorrect conversion coefficient can cause problems:
“…if you have a 1 nA/V amplifier on the right side switch setting, but the software thinks it
is 10 nA/V and your STM setpoint is 0.1 nA, the system will engage when 0.01 nA of
current is actually passing through the amplifier. That’s going to mean your tip is much
farther away from the surface than your really want.”
You’re essentially telling the software the signal is ten times greater than it
actually is so currents are multiplied up.
Particularly important for break junction experiments. To far away and the
junction might not be made, even if the maximum current of the scanner is
reached.
Checks
It is very important then to get this right, and is useful to check the
conversion coefficient when you take over the STM or change the scanner
and calibration file.
I have written the pre-amp board sensitivity on
the two (different) STM3 scanners.
Gain switches are normally on the right so only
the conversion coefficient in software should
change.
Some calibration files are wrong!
Fail to do the checks and...
1. Your tip-sample separation could be very small.
If your current is too high (such as how I had mine) then a 1000-datapoint
plot will not give you the resolution you need to pick out your single
molecule conductance:
0.075 nA  0.75 nA
0.67 nA  6.7 nA
@0.6V  I/V = 1.25 nS
= 1.61 × 10-5 G0
Or...
2. Your tip-sample separation could be very large.
If your current is too low then plateau detection is limited, however,
plateaus at lower conductances come into view (?).
0.5 nA  0.05 nA
@0.6V  I/V = 0.0833 nS
= 1.076 × 10-6 G0
0.05 nA normally crowded
out by low end noise.
New plateaus at lower
SMC?