Here is a practical approach to reducing high-voltage-induced noise. A simple relay circuit cancels noise in the actuator stage of a scanning electron microscope.
Most laboratories and industrial environments have various kinds of electrical-noise sources at all frequencies from heavy machinery, instruments, power supplies, and TV stations.
Engineers have used many simple devices and techniques to handle this noise. These techniques include the use of proper grounding methods, shielded and twisted wires, signal averaging, differential-input-voltage amplifiers, and filters.
Although these methods can control and reduce the noise in most measurements, some techniques just prevent noise from entering the system, whereas others remove only extraneous noise from the signal. These methods usually find use only in low-voltage systems; they do not address high-voltage-induced noise.
This Design Idea offers a practical approach to reducing high-voltage-induced noise. The floating input of a scanning electron microscope has high impedance, and it acts as an antenna, picking up noise signals. The microscope's actuators need a high-voltage signal to drive their piezoelectric slip-stick stack motors. The motion mechanism requires a ramping waveform spanning to 800V p-p. The mechanism requires multiple channels because there are three degrees of tip motion. Some microscopes incorporate optical-path-adjustment microsliders for atomic-force microscopy; those scopes need even more channels.
Figure 1: This simple circuit connects unenergized inputs to ground through a resistor.
Traditionally, each channel needs a high-voltage amplifier. So, two degrees of tip motion need two high-voltage amplifiers, three degrees need three amplifiers, and so on. High-voltage amplifiers are expensive and need considerable space on the PCB (printed-circuit board), however. Therefore, controlling multiple degrees of tip motion using only one high-voltage amplifier that switches among multiple channels saves cost and space. The pins of high-voltage connectors have enough space between them to avoid disturbing adjacent signals. But high-voltage connectors are expensive and too large to easily arrange. So, the best choice is to use a commercial RS 232-standard, nine-pin/25-pin connector. The pins of most commercial RS 232 connectors are close enough together to easily pick up induced high-voltage signals. You can solve this problem by connecting a low impedance to the floating pins of the RS 232 connector. In this circuit, three piezoelectric motors, PZ 1, PZ 2, and PZ 3, connect to the T 1, T 5, and T 9 pins of the RS 232 9T connector. The circuit has three relays that switch the high-voltage input to the piezoelectric motors. The normally open node of the relays connects to the high-voltage-amplifier output. The normally closed nodes of the relays connect to three 1 kΩ resistors to bypass high-voltage-induced noise to ground.
*About the author: Jui-I Tsai contributed this article.
This article is a Design Idea selected for re-publication by the editors. It was first published on September 13, 2007 in EDN.com.*