COMPONENTS

New, Integrated
Interface ASICs
for Capacitive Measurement Technology

Introducing the CAN 404 and CAV 414 analog ASICs, designed to measure small swings in capacitance across a wide range of values.

Jörg Stecker, Analog Microelectronics GmbH

Pressure, together with temperature, is one of the most often measured physical quantities in process control operations. The range of technologies used is accordingly wide and includes piezoresistive, inductive, and capacitive.

In distance and level measurement, problems often arise when the object or substance of interest has a low relative permittivity (e.g., plastics and organic materials). In such cases, capacitive sensors and optical devices are often selected.

It is not unusual in sensor technology for a problem with one measurement technique to form the basis of another, different principle of detection. The disruptive effect of moisture on the capacitive measurement of distance, for example, is now one of the basics of moisture sensors. Today, the majority of these are made of polymer materials that are capable of reacting to changes in moisture by absorbing or releasing water. Here, too, the change in the capacitance of the micromechanical structure to which the polymer material is attached is taken as the determinant measurable variable.

It is worth noting that whatever the technology used for data acquisition, i.e., generates the change in capacitance, the measurement requirement is always the same: to measure and convert extremely small swings in capacitance (a few picofarads) into a s‘andard industrial signal. In response, Analog Microelectronics GmbH in Mainz, Germany, has developed two analog ASICs, the CAN 404 and CAV 414.

Both ASICs have a complete analog signal acquisition function for capacitive sensors, the requisite evaluation electronics, interfacing circuitry, and internal references for use in level detection. The principal circuitry is the same for both, as is the linear transfer characteristic at the front end (area of detection). Both can also be used for a wide range of capacitances.

CAN 404 has a NAMUR output designed primarily for use in the chemical industry. CAV 414 includes an integrated voltage output stage that enables any voltage within the supply voltage range to be set by an external network of resistors. A special feature of the CAV 414 is that the standard industrial norms of 0–5 V and 0–10 V can be set externally. Both devices operate according to the principle illustrated in Figure 1.

Figure 1. This block diagram illustrates the required discrete functions to properly signal-condition a capacitive sensor.

A variable reference oscillator, whose frequency is set by means of capacitance C_OSC, drives two symmetrical integrators that are phase-locked and clock-synchronized (see Figure 2).

Figure 2. Combining relatively few external components in the CAV 414 creates an entire signal conditioning strategy that is both inexpensive and sparing of real estate.

The amplitudes of the two driven integrators are determined by capacitances C_X1 and C_X2. With high common-mode rejection and a high resolution, comparison of the two amplitudes produces a signal that corresponds to the change in capacitance of C_X1 and C_X2 relative to each other. This difference signal is then conditioned in an ensuing active low-pass (mean value generation) filter. The filtered DC signal passes through an amplifier circuit to the output stage, by which the output voltage can be set.

It is possible to set individual circuit variables, such as filter constants and amplification, with the aid of the integrated reference voltage, using only a few external components.

By using the two integrators driven by the reference oscillator, whose capacitances are designated here as the measurement reference capacitance C_X1 and the measurement signal capacitance C_X2, it is possible to measure swings in capacitance of 5%–100% in relation to the measurement reference capacitance. As C_X1 can be varied in a range of 10 pF to 1 nF, the range of measurement for the measurement signal capacitance is 10.5 pF to 2 nF.

For example, a capacitive measuring head has a basic capacitance of 20 pF. The measurement reference capacitance can be realized by a discrete capacitor with a value of 20 pF. Changes in signal from 0 to 1 pF (5%) or a maximum of 0 to 20 pF (100%) can ýhen be measured. With smaller changes in signal (<1 pF), only a percentage of the full-scale output signal is available. In this example, an output signal of 5 V is obtainable using a signal capacitance of 20.5 pF and an output setting of 10 V. Should the capacitances not be completely identical, the resulting offset can be compensated with an external resistor.

To avoid a temperature drift of the IC offset, the temperature coefficient of the two capacitances should, as far as possible, be the same.

Figure 3. The CAV 414 is appropriate for a variety of measurement cells.

Given that the relative amplitude difference is fixed by the measurement reference capacitance and the measurement signal capacitance, differential capacitors can be used as well because both capacitors change value, as opposed to having one fixed capacitor (see Figure 3).

The CAV 414 and CAN 404 provide protection against reverse polarity and incorporate a current limit on the output side. In addition to single-chip signal conditioning, designers of transducers can make a complete sensor system with the fewest possible components.

The system of controlled integrators enables a wide capacitance range to be covered. In addition, the components’ great flexibility of adjustment provides solutions to a wide variety of capacitive measurement problems. Should a current loop signal of 4–20 mA or any other current output be required (e.g., 0–20 mA)—which is often the case—CAV 414 can be easily combined with the AM 422 IC to produce a compact industrial sensor (see Figure 4).

Figure 4. With a very inexpensive voltage-to-current converter such as the AM 422, designs can offer voltage or current outputs, or both.

The advantage of using two ASICs is that the power dissipation (700 mW maximum) caused by the 20 mA current output, for example, occurs only in the second IC, which can be thermally decoupled from the signal acquisition function. This largely prevents the inevitable warming of the IC from having any effect on the measuring capacitance.

One particular benefit of the two-chip model is that the CAV 414 provides an adjustable voltage signal (e.g., 0–5 or 0–10 V) and AM 422 provides an adjustable current signal (e.g., 0–4 or 4–20 mA). The solution suggested here is thus suitable for parallel use by both standard industrial norms, with settings that can be adjusted separately as the application requires.

The CAN 404 and CAV 414 are suitable for the linear detection of small swings in capacitance across a wide range of values. They can thus be used in level indicators, in distance and moisture sensors, and in inclinometers and accelerometers. The availability of the ICs in die form allows them to be incorporated into a wide variety of miniaturized capacitive sensors for use in industrial applications.

For more information, contact David Ezekiel, Servoflo Corp. (distributors), 75 Allen St., Lexington, MA 02421; 781-862-9572, fax 781-862-9244, david@servoflo.com.

Home | About Us | Site Index | Privacy Policy © 2006 Questex Media Group, Inc.. All rights reserved. Reproduction in whole or in part is prohibited. Please send any technical comments or questions to our webmaster.