signal conditioning based on programmable analog circuits_popr

8/6/2019 Signal Conditioning Based on Programmable Analog Circuits_popr

1/6

SIGNAL CONDITIONING BASED ON PROGRAMMABLEANALOG CIRCUITS

P. PIETRZAK, W. TYLMAN, Z. K ULESZA, B. PKOSAWSKI,M. MULARCZYK, A. NAPIERALSKI

TECHNICAL UNIVERSITYOF LODZKEYWORDS: Programmable Analog Circuit, Signal Conditioning,

Vibration Measurement Systems

ABSTRACT: Programmable circuits are commonly used class of electronic devices. In practice this term usually refersto microcontrollers with different kind of programmable memories and to programmable logic circuits (PAL, CPLD,FPGA). Most of these devices are digital circuits. Thanks to the huge progress in miniaturization of electroniccircuits, the new class of devices programmable analog circuits was put on the market. These circuits bring newpossibilities to design electronic circuit. One of the most promising fields of application is construction of analogsignal conditioning circuits. This paper describes feature and parameters of sample programmable analog circuits.It depicts internal structure and theory of operation of these devices. One of commercially available programmableanalog circuits was used in development of signal conditioning block for vibration measurement system.It works as programmable gain amplifier and low-pass anti-aliasing filter with programmable cut-off frequency.Amplifier transient characteristics for different gain values and frequency response of the filter were measured.

GENERAL INFORMATION

The common trend among the manufacturers ofelectronic devices is to miniaturize their products. Veryoften this leads to integration of large number offunctional blocks of the device in one small, complexintegrated circuit. Small dimensions and weight of thedevice gives the possibility to use it as a portable

apparatus. However, this is not the main and soleobjective. Using one, specialized integrated circuitinstead of many separate ones gives the opportunity tosignificantly lower the price of the device when themass production methods are employed. Moreover,a single integrated circuit will require much lesselectric energy, therefore it can operate much longerusing battery power. An important advantage of theintegrated measurement systems is the ability toimprove the parameters of the constructed devices bothin the field of precision (e.g. due to minimisation ofthe influence of the environment noise) and ofoperation speed. Such systems may integrate advanced

analog circuits, signal processor cores and variousmeasurement transducers. It is extremely important forexample for the distributed vibrations measurement

system [3, 4], for which it may be possible to mountthe measurement modules inside the generator.Manufacturers of the analog integrated circuits have problems manufacturing capacitors, inductors andresistors in the silicon substrate. They take up largeamount of space and often turn out to be thermallyunstable. Besides, the polysilicon resistors arenonlinear. The solution to these problems are the

switched capacitance circuits [6]. They allow tomanufacture complex analog circuits with arbitraryfrequency response. The switched capacitancetechnique is the basis of the construction of thereprogrammable analog circuits. They are universalcircuits for which the user can determine theconfiguration of the interconnections between internalelements and set the required values of the passiveelements. As a result it is possible to obtain a circuithaving virtually any desired functionality andprogrammatically adjustable parameters (gain, filterfrequencies) without any need for external elements.At the Department of Microelectronics and Computer

Science, Technical University of d, in order to testthe possibility to use the reprogrammable analogcircuits in the industrial measurement systems, one ofthe commercially available circuits of this type has


2/6

been used to construct signal conditioning circuit in thevibration monitoring system for large rotatingmachines [2].

ANALOG REPROGRAMMABLECIRCUITS: STRUCTURE ANDOPERATING PRINCIPLE

The vast progress in the field of semiconductorintegrated circuits production, which can be observedin the last few years, resulted in development of analogintegrated circuits that not only can have theiroperation parameters adjusted programmatically, butalso the interconnections between the built-in analog blocks can be changed in the same manner. Suchdevices are called analog reprogrammable circuits.One of the main blocks used for their construction isa transconductional amplifier, which principle ofoperation can be compared to a voltage controlled,linear current source. Its main elements are two FET or bipolar transistors, working as classical differentialamplifier. An important feature of theses amplifiers istheir low output impedance.Other important elements of the circuit are digitallycontrolled analog switches and multiplexers, whichallow circuit programming. By writing required valuesto the configuration memory they allow to alter thecircuit's internal interconnections and define signalpropagation paths. The analog switches also allow toconstruct electronic potentiometers and programmablecapacitances.The semiconductor switches are the basic elements ofthe abovementioned switched capacity circuits. Thesecircuits can be manufactured using the most popular

CMOS technology, which allow them to be integratedin large scale integration circuits. In the switchedcapacitance technique the capacitors are used to tostore charge that corresponds to samples of the analogcircuits. This type of circuits can be used to constructgenerators, digital-analog converters, modulators, andabove all filters.In comparison to the classic active filters, designed onthe basis of operational amplifiers, the switchedcapacitance filters have small dimensions, steepfrequency response at the band limits, easy realizationof high-level filters, low power requirements, easytuning.By changing the operating frequency of various partsof the circuit (using built-in frequency dividers) it is possible to adjust the steepness of the frequencyresponse outside the band limits. The cut-off frequencyand gain can be adjusted by connecting appropriatenumber of capacitors (using analog switches). Thesignal-to-noise ratio of the switched capacitance filtersis limited by thermal noise of the amplifiers andswitches. As far as this parameter is concerned, classicactive RC filters are often better.

Fig. 1. Internal structure of simple CABof AN10E40 circuit [7]

In the frequency spectrum of the switched capacitancecircuit output signal there are spectral linescorresponding to the sidebands of the base band (bandclose to the filter fundamental frequency). Moreover,the filtered signal contains disturbances with the

frequency equal to the switching signals frequenciesand their multiplicities. In order to eliminate thisbehaviour it is necessary to use additional continuousfilters, for example active RC filters: antialiasing at theinput and smoothing at the output.The main way to construct resistive elements, stillremain the FET transistor configured to work in triodearea [6].The analog reprogrammable circuits utilizing theabovementioned elements can be divided into threegroups. The first one are the circuits similar to thedigital reprogrammable FPGA circuits. A typicalexample is Anadigm AN10EXX family [7]. Thesecircuits consist of analog cells (Fig. 1) basic blocksusually containing an amplifier and resistive andcapacitive elements using switch capacitancetechnology. The configuration of each cell is storedand determined using digital signals that controlanalog switches. The circuit contains signal lines, towhich the inputs and outputs of the selected cells canbe connected using analog switches (Fig. 2).

Fig. 2. Internal structure of AN10E40integrated circuit [7]


3/6

In this way the FPAA (Field Programmable AnalogArray) circuits can be used to implement amplifiers(including measurement amplifiers), filters orcomparators that have desired parameters.The second group are circuits where the analog blocksare designed specifically to perform particularfunctions (amplifying, filtration etc.). They often offeronly limited means of altering configuration of their

elements. An example is the Lattice ispPac family [9].The simplest member of this family ispPac10 (Fig. 3) contains four amplifiers, connected by aprogrammable matrix of analog connections. The gainof each amplifier can be programmed between 0 and20sB. The circuit can serve as a basis for simpleprogrammable gain amplifiers and filters.

Fig. 3. Internal structure of ispPAC10integrated circuit [9]

In the ispPAC20 circuit some of the amplifiers arereplaced with comparators and 8-bit digital-analogconverter. Additionally, one of the amplifiers has a programmable voltage source connected to its input.The ispPAC8X series circuits contain programmablefifth-order filters together with programmable gainamplifiers (Fig. 4).

Fig. 4. Internal structure of ispPAC80integrated circuit [9]

The last, third group is represented by CypressCY8C25XXX family. These circuits containreprogrammable analog subcircuits, reprogrammabledigital subcircuits and modified core of the '51microcontroller with digital input/outputs (Fig. 5).

The circuit contains eight blocks of programmabledigital logic and twelve programmable analog blocks.The connections between these blocks are made usinga network of signal lines (separately for the digital andanalog parts) connected to a complex matrix ofconfigurational multiplexers. A sample structure of theanalog block is presented in Fig. 6.The built-in microcontroller executes most of the

instructions during one clock cycle. A multiplier,capable of multiplying two 8-bit numbers in threeclock cycles, further aids the computations. Themicrocontroller program is stored in ISP FLASHmemory. The user can utilize SRAM memory for thedata.

Fig. 5. . Internal structure of CY8C27 series circuit [8]

Fig. 6. Internal structure of type C analog block in CY8C27integrated circuit family [8]


4/6

The reprogrammable analog blocks of theCY8C25XXX family can be used to constructamplifiers and filters. Its digital blocks to constructcounters, D/A and A/D converters, multiplexers, timersand PWM generators. It should be noted that the built-in microprocessor can supervise the analog and digital blocks, utilizing its interruption system. It can alsoalter parameters of the blocks during circuit operation.

Thanks to the built-in charge pump, the circuit can beused with the supply voltage of 1V. The manufacturermakes available free software that facilitates design process of devices that make use of theabovementioned circuits.

AN APPLICATION OF THE ANALOGREPROGRAMMABLE CIRCUIT IN THESIGNAL PATH OF THE VIBRATIONMEASUREMENT SYSTEM

The reprogrammable analog Cypress CY8C27443circuit, belonging to the last of the groups described inthe previous section, has been used to construct signalconditioning circuitry in the data acquisition and processing module for the acceleration sensors. Itsblock diagram is presented in Fig. 7.

MEMSAccelerometer

EEPROMInputAmplifier

Piezoelectric

Accelerometer

Power

Supply

Input

MultiplexerAnalog

Programmable Circu A/D ConverterReference Voltag

Source SignalProcessorCAN Interface

Fig. 7. Diagram of vibration measurement module [2]

The preamplified and having 2.5V bias signals fromthe piezoelectric and micromachine silicon sensors arerouted to the CY8C27443 inputs. The bias is necessary because the circuits are powered using single 5Vvoltage (the bias is equal to half the supply voltage)[2].The role of the CY8C27443 circuit in the signalconditioning block is twofold: it is a programmable

gain amplifier and low-pass fourth-order filter withprogrammable cut-off frequency. The parameters of both these subcircuits are determined on the basis ofthe signals supplied to the appropriate inputs of theCY8C27443 circuit (Fig. 8).

Fig. 8. Configuration of analog programmable circuit [2]

The amplifier consists of two analog blocks and has thegain of 2, 4, 8 or 16 times. The connections betweenblocks are depicted in Fig. 9.

Fig. 9. Diagram of amplifier made of two CY8C27443analog blocks [8]

Looking at the parameters of the circuit (Table 1) itcan be seen that the circuit has high value of the noisedensity coefficient, high value of the input offsetvoltage and small value of the common mode rejectionratio. In order to minimize the negative influence ofthese parameters the amplitude of the input signalshould be high. This results in the need of preamplifiercircuits (Fig. 7).

TABLE 1. Parameters of the amplifier realized using theanalog programmable circuit

Supply voltage 5VSupply current 4mAMaximum offset voltage 3mV

Maximum offset current 1nAMaximum noise density coefficient 99nV/HzInput voltage range 0V..5VOutput voltage range 0.05V..4,95VFrequency bandwidth 2MHz (G=2)Common mode rejection ratio 59dB

The measured transfer characteristic of the amplifiercircuit are presented in Fig. 10.


5/6

Fig. 10. Amplifier transient characteristicsfor different gain values

The plots show amplifier saturation for the inputvoltages below 0.3V and above 4.6V. The amplifier

has good linearity and (not shown in the plot) constantgain for wide range of input signal frequencies.Converting the analog signal into digital representationrequires antialiasing filters that satisfy the Nyquist-Shannon theorem (stating that the sampling frequencybust be at least two times higher that the maximumfrequency of the sampled signal). In the discussedmodule this function is performed by two identical,constructed using the reprogrammable circuit, second-order filters connected in series. A schematic of one ofthese filters is presented in Figure 11.

Fig. 11. Diagram of programmable 2-nd order filter

In order to obtain good selectivity a Chebyshev filterwas used. The values of the capacitors and theirswitching frequencies were calculated using the MSExcel spreadsheet provided by the manufacturer [8].The implementation of the filter in the circuit wasaided by libraries from the PSoC Designer program.The transfer function of the filters is following:

(2)

The desired amplitude characteristic of the filter withthe cut-off frequency of 1kHz is presented in Figure12.

Fig. 12. Designed frequency response of 1kHzcut-off frequency low-pass filter [2]

As anticipated, the noise density coefficient and input

offset voltage of the filter turned out to be too high forprecise diagnostic measurements (Table 2).

Table 2. Parameters of the filter developed using the analogprogrammable circuit

Supply voltage 5VSupply current 4mAMaximum offset voltage 18mVMaximum noise density coefficient 2000nV/HzInput voltage range 0V..5VOutput voltage range 0.05V..4,95V

In order to change the upper cut-off frequency of thefilter it is necessary to change values of capacitors andoperating frequency of the internal switches.Unfortunately, the configuration of the built-in programmable PWM modulator cannot be changeduring circuit operation. This limitation has beenovercome by constructing an additional, programmablePWM modulator using digital blocks. The measuredamplitude characteristic for different settings of thecircuit are presented in Figure 12.

Fig. 12. Obtained frequency response of the filter [2]

After switching the power supply on the built-inmicrocontroller configures the circuit. The internal

multiplexers are set so that they connect the analogblocks, digital blocks and external terminals accordingto the code stored in the flash non-volatile memory.After the configuration phase is finished, the circuit


6/6

signal conditioning based on programmable analog circuits_popr

Documents