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Title:
FILTER CIRCUITS COMPOSED OF AN INTEGRATED CIRCUIT CHIP AND AN EXTERNAL CAPACITANCE
Document Type and Number:
WIPO Patent Application WO/1997/004524
Kind Code:
A1
Abstract:
A voltage controlled filter circuit with a voltage controlled resistor as a control element having a D.C. voltage applied to an ADC (analog to digital converter) compared to a supplied reference voltage, and as bits are set to HIGH or LOW level, then N Channel enhancement mode MOSFETs are turned on or off, bypassing the resistors in the ON state and placing the resistor in the current path when in the OFF state, providing control of the filter cutoff or critical frequency, critical frequency range being user selectable with external capacitor(s).

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Inventors:
MAXWELL SCOTT D
Application Number:
PCT/US1996/011517
Publication Date:
February 06, 1997
Filing Date:
July 10, 1996
Export Citation:
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Assignee:
BOEING CO (US)
International Classes:
H03H11/12; (IPC1-7): H03H11/12
Foreign References:
DE9216205U11993-03-04
EP0511536A11992-11-04
Other References:
PATENT ABSTRACTS OF JAPAN vol. 012, no. 221 (E - 625) 23 June 1988 (1988-06-23)
Download PDF:
Claims:
What is claim
1. ed: A filter circuit integrated into a single chip comprising in combination: an analog to digital converter capable of freerun and stable latching outputs; a plurality of low "on" resistance N channel enhancement mode MOSFETs; a plurality of resistors forming a resistor ladder network; a D.C. control voltage input; means for applying said D.C. control voltage to said analog to digital converter, comparing to said reference voltage, as bits are set to HIGH or LOW, turning said N channel enhancement mode MOSFETs ON or OFF, thereby bypassing resistors in said resistor ladder network in the ON state and placing the resistor in the current path in the OFF state, thereby providing a voltage controlled resistor circuit; contacts on said integrated circuit package providing means for extemal capacitor attachment, thereby providing frequency range selection capability; and an operational amplifier circuit in combination with said voltage controlled resistor, and said capacitor(s) to provide a voltage controlled filter circuit.
2. A filter circuit integrated into a single chip comprising in combination: an analog to digital converter capable of freerun and stable latching outputs; a plurality of low "on" resistance N channel enhancement mode MOSFETs; a plurality of resistors forming a resistor ladder network; a D.C. control voltage input; a D.C. reference voltage input; means for applying said D.C. control voltage to said analog to digital converter, comparing to said reference voltage, as bits are set to HIGH or LOW, turning said N channel enhancement mode MOSFETs ON or OFF, thereby bypassing resistors in said resistor ladder network in the ON state and placing the resistor in the current path in the OFF state, thereby providing a voltage controlled resistor circuit; contacts on said integrated circuit package providing means for extemal capacitor attachment, thereby providing frequency range selection capability; and an operational amplifier circuit in series combination with said voltage controlled resistor(s), and in parallel combination with said capacitor(s) to provide a low pass voltage controlled filter circuit. 3) A filter circuit integrated into a single chip comprising in combination: an analog to digital converter capable of freerun and stable latching outputs; a plurality of low "on" resistance N channel enhancement mode MOSFETs; a plurality of resistors forming a resistor ladder network; a D.C. control voltage input; a D.C. reference voltage input; means for applying said D.C. control voltage to said analog to digital converter, comparing to said reference voltage, as bits are set to HIGH or LOW, turning said N channel enhancement mode MOSFETs ON or OFF, thereby bypassing resistors in said resistor ladder network in the ON state and placing the resistor in the current path in the OFF state, thereby providing a voltage controlled resistor circuit; contacts on said integrated circuit package providing means for extemal capacitor attachment, thereby providing frequency range selection capability; and an operational amplifier circuit in parallel combination with said voltage controlled resistor(s), and in series combination with said capacitor(s) to provide a high pass voltage controlled filter circuit.
Description:
FILTER CIRCUITS COMPOSED OF AN INTEGRATED CIRCUIT CHIP AND AN EXTERNAL CAPACITANCE.

REFERENCE TO RELATED APPLICATIONS

This is a continuation in-part of application Serial No. 08/348,283 file November 30, 1994 which is a continuation-in-part of application Serial No.07/498,862, filed March 23, 1990.

FIELD OF THE INVENTION

This invention relates to a voltage controlled resistor, and more particularly to

DC voltage controlled circuits for accurately, linearly, and repeatably simulating a variable resistor. The described variable resistor in combination with an op amp and external capacitors thereby providing a voltage controlled filter circuit.

BACKGROUND OF THE INVENTION

Resistor simulation circuits are known as exemplified by U.S. Patent No. 4,354,250 to Lee. Also ADC (analog to digital converters) have been shown in the patent literature, see e.g. U.S. Patent No. 4,366,470 to Takanashi et al which shows a voltage selector which employs IGFETs as voltage switching elements. Also, electric resistors including field effect transistors are known as shown e.g. in U.S. Patent No. 4,667,216 to Bigall et al. An electronically controlled resistor bank is shown in which selected resistors in a series chain are electronically shunted by PFETs to delete their values from the overall resistance of the chain (See, in the literature, Page 30 of NASA Tech Briefs, July/August 1987).

SUMMARY OF THE INVENTION

It is therefore the object ofthe present invention to provide a voltage controlled resistor which is power tolerant with respect to the required voltage across and current therethrough, and to incorporate this circuit as a control element within a voltage controlled filter circuit.

Accordingly, within this variable resistor control element, a voltage comparison technique is utilized wherein a D.C. voltage is applied to an analog to digital converter (which is capable of "Free-Run" made, viz. start running on power up, have output latches so outputs are not transient), compared to a supplied reference voltage, and as bits are set to HIGH or LOW "N Channel enhancement mode MOSFETs" are turned ON or OFF bypassing resistors in a resistor ladder network in the ON state placing the resistor in the current path when in the OFF state. As the resistance ofthe voltage controlled resistor changes, the cutoff or critical frequency ofthe voltage controlled filter changes correspondingly.

A filter circuit integrated into a single chip comprises in combination: an analog to digital converter capable of free-run and stable latching outputs; a plurality of low "on" resistance N channel enhancement mode MOSFETs; a plurality of resistors forming a resistor ladder network; a D.C. control voltage input; a D.C. reference voltage input; means for applying D.C. control voltage to the analog to digital converter, comparing to reference voltage, as bits are set to HIGH or LOW, turning N channel enhancement mode MOSFETs ON or OFF, thereby bypassing resistors in said resistor ladder network in the ON state and placing the resistor in the current path in the OFF state, thereby providing a voltage controlled resistor circuit, as shown in figures 1 and 2.

Contacts on the integrated circuit package provide means for external capacitor attachment, thereby providing frequency range selection capability.

An operational amplifier circuit in combination with the voltage controlled resistor, and capacitor(s) provides several voltage controlled filter circuits as shown in figures 3, 4, 5 and 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

VOLTAGE CONTROLLED RESISTOR CIRCUITS OF FIGS. 1 AND 2

Three common circuit components, the ADC, the MOSFET and the Resistor utilized in combination to provide the present Voltage Controlled Resistor which will have a predictable, repeatable, power tolerant output. Basically, a variable resistive output is achieved by applying selected D.C. voltages to the input of an ADC which will set predictable bit combinations as an output from the ADC, these bit ouφuts tried to the Gate of MOSFETs, one to each bit output, in the "hi" state turn on the MOSFET (with enhancement mode MOSFETs) yielding a Drain to Source resistance close to "0" ohms and thus bypass the resistor which is in parallel with the Drain and Source ofthe given MOSFET. When a given bit is "lo" the MOSFET will be off yielding close to an infinitely high Drain to Source resistance so as to put the resistor in series with other resistors not being bypassed due to a "hi" bit state. The total output resistance then is the sum ofthe resistors in the resistor ladder not being bypassed by MOSFETs.

Referring now to the drawings, and more particularly to FIGS. 1 and 2 illustrative respectively of the present 8 and 12 bit analog to digital converter embodiments of the voltage controlled resistor circuits it should be noted that the respective ADC (analog to digital converter) circuits are capable of the "free-run" mode. However, the 12 bit ADC embodiment of FIG. 2 required latching circuit, and the addition of timing circuit 2 to stabilize the outputs.

The first embodiment ofthe present voltage controlled resistor circuit of FIG. 1 utilizing 8 bit ADC circuit while capable of the aforementioned "free-run" mode has output latches but required a start-up circuit 3.

Regardless of the respective ADCs used in the first and second embodiments of FIGS. 1 or 2, the present voltage controlled resistor circuits require the application of D.C. control voltage inputs 4 and 5 to the respective ADC circuits and which is compared to respective supplied reference voltages 6 and 7, and as bits are set HIGH or LOW, respective N channel enhancement mode MOSFETs 8 and 9 are turned ON or OFF bypassing the resistors in the respective resiεtor ladder networks 10 and 11 to

the ON state placing the resistor in the current path when in the OFF state. A very low ON state resistance is required in each FET so that the FF τ s do not add appreciably to the overall resistance of respective resistor ladder networks 10 and 11. On an IC (integrated circuit), this is accomplished by placing many individual FETs in parallel with one another. The individual FETs should also have very high (infinite) input impedance as with a MOSFET so that biasing and current requirements are not of significance. In the exemplary voltage controlled resistor circuits of FIGs 1 and 2 there is a requirement for bleed resistors 12 and 13 respectively in these voltage controlled resistor circuits on the inputs ofthe FETs due to the high input capacitance.

On the output of the FETs various combinations of resistor values may be utilized, however the present application circuits require a substantially linear progression of resistance values. Accordingly, the resistors were utilized in a 2 n R (resistance) configuration with R m ax associated with the most significant bit and R associated with the least significant bit which configuration avoids having large changes in resistance due to noise acting on the least significant bit (LSB).

When the input voltage is at zero ("0"), all bits are Lo so that maximum resistance is seen on the output. As the voltage is increased, successive binary bit combinations are set so as to subtract resistance in steps ofthe smallest resistor "R" until Vin = Vref, at which point the resistance is zero or the sum ofthe resistance of the FETs, since all are in an ON condition at this time. Thus, 4096 resistance steps for the second embodiments utilizing 12 bit ADC circuit and 256 resistance steps for the first embodiment utilizing 8 bit ADC circuit.

PARTS LIST FOR 8 BIT VOLTAGE CONTROLLED RESISTOR CIRCUIT EMBODIMENT OF FIG. 1

NOMENCLATURE MANUFACTURER PART NO. USED

1. 8 BIT ADC NATIONAL SEMICOND ADC 800 PCD 1

2. POSITIVE VOLTAGE REG NATIONAL SEMICOND LM 317 1

3. NEGATIVE VOLTAGE NATIONAL SEMICOND LM 337 1 REG

4. QUAD FET IC NATIONAL SEMICOND VQ1006P 1

5. TIMER IC NATIONAL SEMICOND LM 555 1

6. N-CHANNEL RCA RFP 15N05 1 ENHANCEMENT MODE- MOSFET

7. ZENER DIOODE 8.2V TEXAS INSTR. IN756A

8. RESISTORS

20 Ω CORNING C4 1

40.2 Ω CORNING C4 1

80.6 Ω CORNING C4 1

162 Ω CORNING C4 1

324 Ω CORNING C4 1

649 Ω CORNING C4 1

1300 Ω CORNING C4 1

2610 Ω CORNING C4 1

120 Ω CORNING C4 1

243 Ω CORNING C4 1

1K Ω CORNING C4 1

2K Ω CORNING C4 1

10K Ω CORNING C4 9

50K Ω CORNING C4 1

100K Ω CORNING C4 2

9. POTENTIOMETERS :

1K Ω BECKMAN 58 PR IK 1

2K Ω BECKMAN 58 PR 2K 2

200K Ω BECKMAN 58 PR 200K 1

10. CAPACITORS:

10 PF ARCO DM-15-100J 1

.01 UF TRW X463UW 1

.022 UF TRW X463UW 1

1 UF KEMET T4322BW5K035AS 2

4.7 UF METUCHEN DR50BBΝ475Ν 2

10 UF SPRAGUE TE-1055 1

PARTS LIST FOR 12 BIT VOLTAGE CONTROLLED RESISTOR CIRCUIT EMBODIMENT OF FIG. 2

NOMENCLATURE MANUFACTURER PART NO. USED

1. 12 BIT ADC NATIONAL SEMICOND ADC 1210 HCD 1

2. POS. VOLTAGE REG NATIONAL SEMICOND LM 317 1

3. NEG. VOLTAGE REG NATIONAL SEMICOND LM 337 1

4. TIMER IC NATIONAL SEMICOND LM 555 1

5. QUAD D-LATCH RCA CD4042BE 3

6. HEX CMDS BUFFER RCA CD401OB 1

7. N-CHANNEL RCA RFP15N05 12 ENHANCEMENT

8. POTENTTOMETERS:

2K Ω BECKMAN 58PR2K 2

100K Ω BECKMAN 58PR10OK 3

9. RESISTORS:

5.6 Ω 2W INTL KES. BWH

12.1 Ω 1/4W CORNING C4

24.3 Ω 1/4W CORNING C4

47.5 Ω 1/4W CORNING C4

95.3 Ω 1/4W CORNING C4

191 Ω 1/4W CORNING C4

383 Ω 1/4W CORNING C4

768 Ω 1/4W CORNING C4

1540 Ω 1/4W CORNING C4

3090 Ω 1/4W CORNING C4

6190 Ω 1/4W CORNING C4

12288 Ω 1/4W CORNING C4

120 Ω CORNING C4

243 Ω CORNING C4

1K Ω CORNING C4

100K Ω CORNING C4

1M Ω CORNING C4

10. CAPACITORS:

100 PF CORNING C4

.001 UF CORNING MZY020-102H-10

.01 UF TRW X463UW

.1 UF KEMET T310A154K035AS

1 UF KEMET T4322BW5K035AS

4.7 UF METUCHEN DR50BBN475 2

Turning now to FIGs 3 and 4 wherein voltage controlled filter circuits are shown utilizing the aforementioned and described voltage controlled resistor circuit. It should be noted that FIG. 3 shows a single pole low-pass filter circuit and FIG. 4 shows a multipoled filter circuit viz. a 2 pole Butterworth filter circuit.

VOLTAGE CONTROLLED FILTER CIRCUITS OF FIGS. 3 AND 4

The herein before described Voltage Controlled Resistor and Operational Amplifier on a single IC chip 100, in combination with an extemal user supplied capacitor are utilized to provide the present Voltage Controlled Filter circuits which, will provide a linear (if desired), predictable and repeatable (from VCF to VCF) output. Filter cutoff frequencies are selected by inputting selected D.C. voltages to the input ofthe Voltage Controlled Resistor in a single pole filter or Resistors in multipoled filters, thereby changing the RC time constraints as hereinafter described so as to change the filter cutoff frequency. The range of frequencies the Voltage Controlled Filter circuits will control is user selectable by changing the value ofthe capacitor extemal to IC chip 100.

The frequency control of an analog filter active or passive is done by controlling the relationship between either a series resistor and a parallel capacitor (low-pass) or a series capacitor and a parallel resistor (high-pass).

The design philosophy is to make the resistive value controllable as a function of a voltage input. This is accomplished with the herein before described voltage controlled resistor circuit.

The present voltage controlled filter circuits of FIGs. 3 and 4 utilize the present voltage controlled resistor circuit and the op amp stages for the given filter type on a single IC chip. The capacitor(s) remain as an extemal part selectable by the V.C.F. user to achieve various frequency ranges based on the transfer function calculating "critical frequency" of the filter type. The basic configurations shown in FIGs. 3 and 4 are the single pole (first order) low-pass and high-pass active filters. With respect to the single pole low-pass filter of FIG. 3 where VCR represents the aforementioned described voltage controlled resistor circuit incorporated therein by reference, the critical frequency "fc" occurs when x c = R or fc = 1/2 RC. IF: C = . l uf;

R = 1000 fc = l/2π(1000) (1x10-6) = 1591.55 Hz. WHERE R = 2000 Ω, fc = 795.78 Hz. WHERE R = 3000 Ω, fc = 530.52 Hz. WHERE R = 4000 Ω, fc = 397.90 Hz.

In the embodiment of FIG. 3 a varying one resistive element causes a linear change in frequency response.

For multipoled filters each active resistive element must be changed to obtain a linear response. FIG. 4 shows a 2 pole Butterworth configuration in which the aforementioned described voltage controlled resistor circuit VCR circuit is incorporated by reference and in which fc = 1/2 π Ri R2 Ci C2 If: C 1 = C 2

R 1 = = R 2 Then: fc = 1

2πVR 2 C 2

= __1

2π RC

Where: C = . l uf

R = 1000 Ω fc = 1591.55 Hz. Where: C = . l uf

R = 2000 Ω fc = 795.78 Hz.

FIG. 5 is illustrative of a filter circuit ofthe single-pole, high-pass type utilizin an operational amplifier and the present VCR circuit and a single integrated circuit chip 100. A capacitor C, since extemal to integrated circuit chip 100, remains selectable by the user so that the filter frequency range is selectable by the user.

FIG. 6 shows an integrated circuit chip 100 composed of an operational amplifier and the present VCR circuits for providing filter frequency cutoff control through VCR circuits, and having extemally coupled capacitance selectable by the user for providing predetermined filter frequency ranges.

Other embodiments of the present VCR circuit may include: high-pass, low¬ pass, band-pass, and band-stop filter applications. Also the present (VCR) voltage controlled resistor circuit may be utilized in: 1) Data system selective filtering for instrumentation amplifiers;

2) Automated tuning for band-pass circuits (i.e. radio);

3) Closed loop control systems (i.e. servo controllers) frequency compensation (i.e. lead, lag) based on the result of another parameter which produces a voltage that will drive the V.C.F. to a desired filter setting, thereby improving the dynamic system parameter (i.e. load) control;

4) Circuits in which an analog filter would be utilized and automation desired.