Tunneling Amplifier

Table of Contents

Description

The tunneling current in a scanning tunneling microscope (STM) arises from the quantum mechanical tunneling of electrons between a conductive sample and a sharp metallic tip under an applied bias voltage. This current is typically in the picoampere (pA) to nanoampere (nA) range, often below the detection threshold of standard analog-to-digital converters (ADCs). Accurate measurement of this current necessitates a high-impedance, low-noise preamplifier with ultra-high gain and minimal input bias current. We therefore implemented a transimpedance amplifier (TIA) using the OPA928DT from Texas Instruments, optimized for ultra-low input bias current and low voltage noise density.

Amplifier Operation

The transimpedance amplifier converts an input current $I_{\text{tip}}$ into a voltage $V_{\text{out}}$ via a feedback resistor $R_f$, according to the ideal equation:

In our design, $R_f = 100\mathrm{M\Omega}$, resulting in a voltage gain sufficient to resolve currents as small as 10pA. The tip is connected to the inverting input of the op-amp, while the non-inverting input is held at analog ground (0V). The op-amp output feeds back through the large $R_f$, and an additional small series resistor (220$\Omega$) at the output node helps isolate the amplifier from capacitive loading due to long cables, improving phase margin and overall stability.

Noise Considerations

The total noise in a transimpedance amplifier includes contributions from:

• The input current noise of the op-amp ($i_n$),
• The input voltage noise ($e_n$) interacting with the source impedance,
• Thermal (Johnson) noise from the feedback resistor: $$v_n^2 = 4kTR_f \Delta f,$$ where $k$ is Boltzmann’s constant, $T$ is temperature, and $\Delta f$ is the bandwidth.

Given the very large $R_f$, thermal noise is significant and sets a practical limit on sensitivity. The OPA928, however, offers extremely low $i_n$ and $e_n$, and its unity-gain bandwidth of 10~MHz ensures sufficient open-loop gain for our application.

Power Rails and Decoupling

Power to the amplifier is supplied through a two-pin header (J1) that delivers regulated $\pm15$V analog rails. Each rail is decoupled locally with a 4.7$\mu$F electrolytic capacitor in parallel with a 100~nF ceramic capacitor. This local bypass network helps reduce high-frequency power supply noise and prevents oscillations by stabilizing the op-amp power inputs.

Guarding and Layout Strats

Due to the extremely high input impedance and sub-nanoampere current levels, special layout techniques are essential to prevent signal corruption due to leakage currents. We employed a comprehensive guarding strategy:

• A driven guard pour surrounds all high-impedance input traces. This guard is actively driven to the same potential as the inverting input by the op-amp’s output, effectively eliminating potential differences that would drive leakage currents through PCB substrates.
• A continuous internal guard plane exists on an inner PCB layer directly beneath the signal trace. This shields against vertical leakage through the dielectric material.
• A via fence connects the guard pour across all PCB layers, forming a complete three-dimensional electromagnetic and leakage barrier.
• The solder mask was intentionally removed over the guard region to prevent capacitive charge accumulation on the surface, which could inject transient currents into the signal path.
• Ground pours are placed near—but electrically isolated from—the guard pour to minimize electromagnetic interference (EMI) and provide low-impedance return paths for surrounding circuitry.

Output and Interface

The amplifier’s output, labeled Preamp OUT, is routed to a test pad (TP1) and a shielded connector for delivery to the ADC on the STM mainboard. This output signal, a voltage proportional to tunneling current, is digitized and used in both imaging and feedback control. The output stage of the amplifier is configured to drive a capacitive load and is verified to remain stable under expected cable parasitics.