TUNNEL CURRENT PRE-AMP
This circuit is based on the electrometer op-amp AD549J, manufactured by Burr-Brown and available from RS. An electrometer amplifier is sensitive to very small currents and can in fact be used to measure charge. In this case it is used to translate a current of a few nanoamperes to a voltage of a few hundred millivolts.
At first sight this circuit may seem unconventional. I have replaced the 1k resistor that would normally be found on the inverting input, by a potential divider on the other side of the gap. Electrically this would be equivalent to having an input resistor and having the other side of the gap connected to zero volts. This alternative has two advantages. Firstly, it restricts the tunnel current loop to a tight circle in close proximity to the op-amp. The current circulating in this loop is very small and needs to be shielded from electromagnetic noise. Secondly, it eliminates the need for a separate zero volts rail from the main board. This reduces the number of mechanical paths for background vibration. The slight disadvantage is that the effective tunnel bias voltage Vb cannot be measured directly from the I/O board and can only be calculated with reference to the zero volts rail on the I/O board.
In this circuit the gap voltage is provided by the op-amp itself, because the feedback causes the -ve input to follow the +ve input. The value of Vb is derived from the fifth DAC on the I/O board and is therefore controlled by software.
The capacitor across the feedback resistor reduces high frequency noise.
The gap resistance during controlled tunneling is in the range of 1 giga-ohm.