Datasheet OP184, OP484, OP284 Datasheet (Analog Devices)

Page 1

Precision Rail-to-Rail Input & Output

FEATURES Single-Supply Operation Wide Bandwidth: 4 MHz Low Offset Voltage: 65 mV Unity-Gain Stable High Slew Rate: 4.0 V/ms Low Noise: 3.9 nV/√

APPLICATIONS Battery Powered Instrumentation Power Supply Control and Protection Telecom DAC Output Amplifier ADC Input Buffer

GENERAL DESCRIPTION

The OP184/OP284/OP484 are single, dual and quad singlesupply, 4 MHz bandwidth amplifiers featuring rail-to-rail inputs and outputs. They are guaranteed to operate from +3 to +36 (or ±1.5 to ±18) volts and will function with a single supply as low as +1.5 volts.

These amplifiers are superb for single supply applications requiring both ac and precision dc performance. The combination of bandwidth, low noise and precision makes the OP184/OP284/ OP484 useful in a wide variety of applications, including filters and instrumentation.

Other applications for these amplifiers include portable telecom equipment, power supply control and protection, and as amplifiers or buffers for transducers with wide output ranges. Sensors requiring a rail-to-rail input amplifier include Hall effect, piezo electric, and resistive transducers.

The ability to swing rail-to-rail at both the input and output enables designers to build multistage filters in single-supply systems and to maintain high signal-to-noise ratios.

The OP184/OP284/OP484 are specified over the HOT extended industrial (–40°C to +125°C) temperature range. The single and dual are available in 8-pin plastic DIP plus SO surface mount packages. The quad OP484 is available in 14-pin plastic DIPs and 14-lead narrow-body SO packages.

Operational Amplifiers

OP184/OP284/OP484

PIN CONFIGURATIONS

8-Lead Epoxy DIP

(P Suffix)

8-Lead SO

(S Suffix)

NULL

–IN A +IN A

V–

OP184

2 3 4

NC = NO CONNECT

8-Lead Epoxy DIP

(P Suffix)

8-Lead SO

(S Suffix)

OP284

OUT A

–IN A

+IN A

V–

14-Lead Epoxy DIP

(P Suffix)

14-Lead Narrow-Body SO

(S Suffix)

OUT A

–IN A

+IN A

V+ +IN B –IN B

OUT B

OP484

4 5 6 7

8 7 6 5

14 13 12 11 10

8 7 6 5

9 8

NC V+ OUT A NULL

V+ OUT B –IN B +IN B

OUT D –IN D +IN D V– +IN C –IN C OUT C

REV. 0

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 617/329-4700 World Wide Web Site: http://www.analog.com Fax: 617/326-8703 © Analog Devices, Inc., 1996

Page 2

OP184/OP284/OP484–SPECIFICATIONS

ELECTRICAL CHARACTERISTICS

(@ VS = +5.0 V, VCM = 2.5 V, TA = +258C unless otherwise noted)

Parameter Symbol Conditions Min Typ Max Units

INPUT CHARACTERISTICS

Offset Voltage “OP184/284E” Grade V

Offset Voltage “OP184/284F” Grade V

Offset Voltage “OP484E” Grade V

Offset Voltage “OP484F” Grade V

Input Bias Current I

Input Offset Current I

(Note 1) 65 µV –40°C ≤ T

≤ +125°C 165 µV

125 µV

–40°C ≤ T

≤ +125°C 350 µV

75 µV

–40°C ≤ T

≤ +125°C 175 µV

150 µV

–40°C ≤ T

≤ +125°C 450 µV

60 350 nA

–40°C ≤ T

≤ +125°C 575 nA

250nA

–40°C ≤ T

≤ +125°C50nA

Input Voltage Range 0+5V Common-Mode Rejection Ratio CMRR V Common-Mode Rejection Ratio CMRR V Large Signal Voltage Gain A

= 0 V to 5 V 60 dB

= 1.0 V to 4.0 V, –40°C ≤ TA ≤ +125°C86 dB

= 2 kΩ, 1 V ≤ VO ≤ 4 V 50 240 V/mV

= 2 kΩ, –40°C ≤ TA ≤ +125°C 25 V/mV

Bias Current Drift ∆IB/∆T 150 pA/°C

OUTPUT CHARACTERISTICS

Output Voltage High V Output Voltage Low V Output Current I

OH OL

OUT

IL = 1.0 mA +4.85 V IL = 1.0 mA 125 mV

±6.5 mA

POWER SUPPLY

Power Supply Rejection Ratio PSRR V Supply Current/Amplifier I Supply Voltage Range V

= +2.0 V to +10 V, –40°C ≤ TA ≤ +125°C76 dB

= 2.5 V, –40°C ≤ TA ≤ +125°C 1.45 mA

+3 +36 V

DYNAMIC PERFORMANCE

Slew Rate SR R Settling Time t

= 2 kΩ 1.65 2.4 V/µs

To 0.01%, 1.0 V Step 2.5 µs

Gain Bandwidth Product GBP 3.25 MHz Phase Margin Øo 45 Degrees

NOISE PERFORMANCE

Voltage Noise e Voltage Noise Density e Current Noise Density i

NOTES

Input Offset Voltage measurements are performed by automated test equipment approximately 0.5 seconds after application of power.

Specifications subject to change without notice.

p-p 0.1 Hz to 10 Hz 0.3 µV p-p

n n

f = 1 kHz 3.9 nV/√Hz

0.4 pA/√Hz

–2–

REV. 0

Page 3

OP184/OP284/OP484

ELECTRICAL CHARACTERISTICS

(@ VS = +3.0 V, VCM = 1.5 V, TA = +258C unless otherwise noted)

Parameter Symbol Conditions Min Typ Max Units

INPUT CHARACTERISTICS

Offset Voltage “OP184/284E” Grade V

Offset Voltage “OP184/284F” Grade V

Offset Voltage “OP484E” Grade V

Offset Voltage “OP484F” Grade V

Input Bias Current I

Input Offset Current I

(Note 1) 65 µV –40°C ≤ T

≤ +125°C 165 µV

125 µV

–40°C ≤ T

≤ +125°C 350 µV

100 µV

–40°C ≤ T

≤ +125°C 200 µV

150 µV

–40°C ≤ T

≤ +125°C 450 µV

60 350 nA

–40°C ≤ T

≤ +125°C 575 nA

–40°C ≤ TA ≤ +125°C50nA

Input Voltage Range 0+3V Common-Mode Rejection Ratio CMRR V Common-Mode Rejection Ratio CMRR V

= 0 V to 3 V 60 dB

= 0 V to 3 V, –40°C ≤ TA ≤ +125°C56 dB

OUTPUT CHARACTERISTICS

Output Voltage High V Output Voltage Low V

OH OL

IL = 1.0 mA +2.85 V IL = 1.0 mA 125 mV

POWER SUPPLY

Power Supply Rejection Ratio PSRR V Supply Current/Amplifier I

= ±1.25 V to ±1.75 V 76 dB

= 1.5 V, –40°C ≤ TA ≤ +125°C 1.35 mA

DYNAMIC PERFORMANCE

Gain Bandwidth Product GBP 3 MHz

NOISE PERFORMANCE

Voltage Noise Density e

NOTES

Input Offset Voltage measurements are performed by automated test equipment approximately 0.5 seconds after application of power.

Specifications subject to change without notice.

f = 1 kHz 3.9 nV/√Hz

REV. 0

–3–

Page 4

OP184/OP284/OP484 ELECTRICAL CHARACTERISTICS

(@ VS = 615.0 V, VCM = 0 V, TA = +258C unless otherwise noted)

Parameter Symbol Conditions Min Typ Max Units

INPUT CHARACTERISTICS

Offset Voltage “OP184/284E” Grade V

Offset Voltage “OP284F” Grade V

Offset Voltage “OP484E” Grade V

Offset Voltage “OP484F” Grade V

Input Bias Current I

Input Offset Current I

(Note 1) 100 µV –40°C ≤ T

≤ +125°C 200 µV

175 µV

–40°C ≤ T

≤ +125°C 375 µV

150 µV

–40°C ≤ T

≤ +125°C 300 µV

250 µV

–40°C ≤ T

≤ +125°C 500 µV

80 350 nA

–40°C ≤ T

≤ +125°C 575 nA

–40°C ≤ TA ≤ +125°C50nA

Input Voltage Range –15 +15 V Common-Mode Rejection Ratio CMRR V Common-Mode Rejection Ratio CMRR V Large Signal Voltage Gain A

Offset Voltage Drift “E” Grade ∆V

/∆T 0.2 2.00 µV/°C

= –14.0 V to +14.0 V, –40°C ≤ TA ≤ +125°C86 90 dB

= –15.0 V to +15.0 V 80 dB

= 2 kΩ, –10 V ≤ VO ≤ 10 V 150 1000 V/mV

= 2 kΩ, –40°C ≤ TA ≤ +125°C 75 V/mV

Bias Current Drift ∆IB/∆T 150 pA/°C

OUTPUT CHARACTERISTICS

Output Voltage High V Output Voltage Low V Output Current I

OH OL

OUT

IL = 1.0 mA +14.8 V IL = 1.0 mA –14.875 V

±10 mA

POWER SUPPLY

Power Supply Rejection Ratio PSRR V Supply Current/Amplifier I Supply Current/Amplifier I

SY SY

= ±2.0 V to ±18 V, –40°C ≤ TA ≤ +125°C90 dB

= 0 V, –40°C ≤ TA ≤ +125°C 2.0 mA

= ± 18 V, –40°C ≤ TA ≤ +125°C 2.25 mA

DYNAMIC PERFORMANCE

Slew Rate SR R Full-Power Bandwidth BW Settling Time t

= 2 kΩ 2.4 4.0 V/µs

1% Distortion, R

= 2 kΩ, V

= 29 V p-p 35 kHz

To 0.01%, 10 V Step 4 µs

Gain Bandwidth Product GBP 4.25 MHz Phase Margin Øo 50 Degrees

NOISE PERFORMANCE

Voltage Noise e Voltage Noise Density e Current Noise Density i

NOTES

Input Offset Voltage measurements are performed by automated test equipment approximately 0.5 seconds after application of power.

Specifications subject to change without notice.

p-p 0.1 Hz to 10 Hz 0.3 µV p-p

n n

f = 1 kHz 3.9 nV/√Hz

0.4 pA/√Hz

WAFER TEST LIMITS

(@ VS = +5.0 V, VCM = 2.5 V, TA = +258C unless otherwise noted)

Parameter Symbol Conditions Limit Units

Offset Voltage OP284 V Offset Voltage OP484 V Input Bias Current I Input Offset Current I Input Voltage Range V Common-Mode Rejection Ratio CMRR V Power Supply Rejection Ratio PSRR V Large Signal Voltage Gain A Output Voltage High V Output Voltage Low V Supply Current/Amplifier I

NOTE Electrical tests and wafer probe to the limits shown. Due to variations in assembly methods and normal yield loss, yield after packaging is not guaranteed for standard product dice. Consult factory to negotiate specifications based on dice lot qualifications through sample lot assembly and testing.

B OS

OS OS

VO OH OL

= +1 V to +4 V 86 dB min

= ±2 V to ±18 V 90 dB min

= 2 kΩ 50 V/mV min

IL = 1.0 mA 4.85 V min IL = 1.0 mA 125 mV max VO = 0 V, R

= ∞ 1.45 mA max

–4–

65 µV max 75 µV max 350 nA max 50 nA max V– to V+ V min

REV. 0

Page 5

OP184/OP284/OP484

ABSOLUTE MAXIMUM RATINGS

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±18 V

Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±18 V

Differential Input Voltage Output Short-Circuit Duration to GND

. . . . . . . . . . . . . . . . . . . . . ±0.6 V

. . . . . . . . Indefinite

Storage Temperature Range

P, S Packages . . . . . . . . . . . . . . . . . . . . . . –65°C to +150°C

Operating Temperature Range

OP184/OP284/OP484E, F . . . . . . . . . . . . –40°C to +125°C

Junction Temperature Range

P, S Packages . . . . . . . . . . . . . . . . . . . . . . –65°C to +150°C

Lead Temperature Range (Soldering 60 sec) . . . . . . . . +300°C

Package Type u

Units

8-Pin Plastic DIP (P) 103 43 °C/W 8-Pin SOIC (S) 158 43 °C/W 14-Pin Plastic DIP (P) 83 39 °C/W 14-Pin SOIC (S) 92 27 °C/W

NOTES

Absolute maximum ratings apply to both DICE and packaged parts unless otherwise noted.

For input voltages greater than 0.6 volts, the input current should be limited to less than 5 mA to prevent degradation or destruction of the input devices.

θJA is specified for the worst case conditions; i.e., θ

for cerdip and P-DIP packages; θ for SOIC package.

is specified for device soldered in circuit board

is specified for device in socket

OP284 Die Size 0.065 × 0.092 Inch, 5,980 Sq. Mils Substrate (Die Backside) Is Connected to V–. Transistor Count, 62.

ORDERING GUIDE

Temperature Package Package

Model Range Description Option

OP184EP –40°C to +125°C 8-Pin Plastic DIP N-8 OP184ES –40°C to +125°C 8-Pin SOIC SO-8 OP184FP –40°C to +125°C 8-Pin Plastic DIP N-8 OP184FS –40°C to +125°C 8-Pin SOIC SO-8

OP284EP –40°C to +125°C 8-Pin Plastic DIP N-8 OP284ES –40°C to +125°C 8-Pin SOIC SO-8 OP284FP –40°C to +125°C 8-Pin Plastic DIP N-8 OP284FS –40°C to +125°C 8-Pin SOIC SO-8

OP484EP –40°C to +125°C 14-Pin Plastic DIP N-14 OP484ES –40°C to +125°C 14-Pin SOIC SO-14 OP484FP –40°C to +125°C 14-Pin Plastic DIP N-14 OP484FS –40°C to +125°C 14-Pin SOIC SO-14

QB5QB6

QB4

CB1 N+

JB1

P+M

QB1

RB1

–IN +IN

QB2

JB2

RB2

QL1

QL2

QB3

CC1

OP484 Die Size 0.080 × 0.110 Inch, 8,800 Sq. Mils Substrate (Die Backside) Is Connected to V–. Transistor Count, 120.

RB4

Q11

RB3

Q12

QB7

Q10

QB9

QB8

CFF

QB10

Q13

CC2

Q14

R11

Q17Q16

Q18

OUT

Q15

R10

REV. 0

Figure 1. Simplified Schematic

–5–

Page 6

COMMON MODE VOLTAGE – Volts

INPUT BIAS CURRENT – nA

500

–500

–15 –10 15–5 5 100

400 300 200 100

0 –100 –200 –300 –400

VS = ±15V

LOAD CURRENT – mA

OUTPUT VOLTAGE – mV

1,000

100

0.01 0.1 10

SOURCE

SINK

VS = ±15V

TEMPERATURE – °C

SUPPLY CURRENT/AMPLIFIER – mA

1.2

0.5

1.1

0.8

0.7

0.6

1.0

0.9

–40 12525 85

VS = ±15V

VS = +5V

VS = +3V

OP184/OP284/OP484–Typical Performance Characteristics

300

VS = +3V

270

= +25°C

240

VCM = 1.5V

210 180 150 120

QUANTITY

90 60 30

–100 –75 100

–50 –25 0 25 50 75

INPUT OFFSET VOLTAGE – µV

Figure 2. Input Offset Voltage Distribution

300

VS = +5V

270

TA = +25°C

240

VCM = 2.5V

210 180 150 120

QUANTITY

90 60 30

–100 –75 100

–50 –25 0 25 50 75

INPUT OFFSET VOLTAGE – µV

300

250

200

150

QUANTITY

100

0 0.25 1.50.50 0.75 1.0 1.25

OFFSET VOLTAGE DRIFT, TCVOS – µV/°C

VS = +5V –40°C ≤ T

≤ +125°C

Figure 5. Input Offset Voltage Drift Distribution

300

250

200

150

QUANTITY

100

0 0.25 1.50.50 0.75 1.0 1.25

OFFSET VOLTAGE DRIFT, TCVOS – µV/°C

VS = ±15V –40°C ≤ T

≤ +125°C

Figure 8. Input Bias Current vs. Common-Mode Voltage

Figure 3. Input Offset Voltage Distribution

200

VS = ±15V

175

= +25°C

150

125

100

QUANTITY

–75 –50 0 50 75 100

–125 –100 125

–25 25

INPUT OFFSET VOLTAGE – µV

Figure 4. Input Offset Voltage Distribution

Figure 6. Input Offset Voltage Drift Distribution

–40

–45

–50

–55

–60

–65

–70

INPUT BIAS CURRENT – nA

–75

–80

–40 12525 85

TEMPERATURE – °C

VCM = VS/2

VS = +5V

VS = ±15V

Figure 7. Bias Current vs. Temperature

–6–

Figure 9. Output Voltage to Supply Rail vs. Load Current

Figure 10. Supply Current vs. Temperature

REV. 0

Page 7

OP184/OP284/OP484

FREQUENCY – Hz

CLOSED-LOOP GAIN – dB

–40

100 10k

1M 10M

–10

VS = +5V R

= 2kΩ

= +25°C

–20 –30

100k

FREQUENCY – Hz

CLOSED-LOOP GAIN – dB

–40

100 10k

1M 10M

–10

VS = ±15V R

= 2kΩ

= +25°C

–20 –30

100k

FREQUENCY – Hz

CLOSED-LOOP GAIN – dB

–40

100 10k

1M 10M

–10

VS = +3V R

= 2kΩ

= +25°C

–20 –30

100k

1.50

1.25

1.0

0.75

0.5

0.25

SUPPLY CURRENT (PER AMPLIFIER) – mA

0 ±2.5 ±20±5.0 ±10 ±12.5 ±15

SUPPLY VOLTAGE – Volts

TA = +25°C

±17.5±7.5

Figure 11. Supply Current vs. Supply Voltage

–I

VS = ±15V

SHORT CIRCUIT CURRENT – mA

0 –50 125

–I

VS = +5V, VCM = +2.5V

–25 0 25 7550 100

TEMPERATURE – °C

80 60 50 40 30 20 10

OPEN-LOOP GAIN – dB

–10 –20 –30

10k 100k 10M

FREQUENCY – Hz

VS = +3V T

NO LOAD

= +25°C

0 45 90 135 180 225 270

Figure 14. Open-Loop Gain and Phase vs. Frequency (No Load)

80 60 50 40 30 20 10

OPEN-LOOP GAIN – dB

–10 –20 –30

10k 100k 10M

FREQUENCY – Hz

VS = ±15V T

NO LOAD

= +25°C

0 45 90 135 180 225 270

PHASE SHIFT – Degrees

Figure 17. Closed-Loop Gain vs. Frequency (2 k

PHASE SHIFT – Degrees

Ω

Load)

Figure 12. Short Circuit Current vs. Temperature

80 60 50 40 30 20 10

OPEN-LOOP GAIN – dB

–10 –20 –30

10k 100k 10M

Figure 13. Open-Loop Gain and Phase vs. Frequency (No Load)

REV. 0

FREQUENCY – Hz

VS = +5V T

NO LOAD

= +25°C

Figure 15. Open-Loop Gain and Phase vs. Frequency (No Load)

2.5k

0 45 90 135 180 225

PHASE SHIFT – Degrees

270

1.5k

OPEN-LOOP GAIN – V/mV

500

–25 0 25 7550 100

–50 125

VS = ±15V –10V < V R

= 2kΩ

VS = +5V 1V < V

< 4V

= 2kΩ

TEMPERATURE – °C

Figure 16. Open-Loop Gain vs. Temperature

–7–

< 10V

Figure 18. Closed-Loop Gain vs.

Ω

Frequency (2 k

Load)

Figure 19. Closed-Loop Gain vs.

Ω

Frequency (2 k

Load)

Page 8

FREQUENCY – Hz

100

PSRR – dB

120

160

–40

1k 100k

10M

TA = +25°C

–20

140

10k 1M

VS = ±15V

VS = +5V

VS = +3V

CAPACITIVE LOAD – pF

OVERSHOOT – %

10 100 1000

–OS

+OS

VS = ±2.5V T

= +25°C, A

VCL

= 1

= ±50mV

OP184/OP284/OP484–Typical Performance Characteristics

300

VS = +5V

270

= +25°C

240 210 180 150 120

OUTPUT IMPEDANCE – Ω

60 30

0 100

AV = 100

10k 1M

1k 100k

FREQUENCY – Hz

AV = 10

AV = 1

Figure 20. Output Impedance vs. Frequency

300

VS = ±15V

270

= +25°C

240 210 180 150 120

OUTPUT IMPEDANCE – Ω

60 30

1k 100k

100

AV = 100

10k 1M

FREQUENCY – Hz

AV = 10

AV = 1

10M

VS = +5V V

= 0.5–4.5V

= 2kΩ

= +25°C

MAXIMUM OUTPUT SWING – Vp-p

10k 100k 1M 10M

FREQUENCY – Hz

Figure 23. Maximum Output Swing vs. Frequency

MAXIMUM OUTPUT SWING – Vp-p

10k 100k 1M 10M

FREQUENCY – Hz

VS = ±15V

= ±14V

= 2kΩ

= +25°C

Figure 26. PSRR vs. Frequency

Figure 21. Output Impedance vs. Frequency

300

VS = +3V

270

= +25°C

240 210 180 150 120

OUTPUT IMPEDANCE – Ω

60 30

0 100

1k 100k

AV = 100

10k 1M

FREQUENCY – Hz

Figure 22. Output Impedance vs. Frequency

AV = 10

AV = 1

10M

Figure 24. Maximum Output Swing vs. Frequency

180

TA = +25°C

160 140 120 100

80 60

CMRR – dB

40 20

–20

100

10k 1M

1k 100k

FREQUENCY – Hz

VS = ±15V

VS = +3V

VS = +5V

10M

Figure 25. CMRR vs. Frequency

–8–

Figure 27. Small Signal Overshoot vs. Capacitive Load

+SLEW RATE

–SLEW RATE

+SLEW RATE

SLEW RATE – V/µs

–SLEW RATE

–50 125

–25 0 75 100

25 50

TEMPERATURE – °C

VS = ±15V R

= 2kΩ

VS = +5V R

= 2kΩ

Figure 28. Slew Rate vs. Temperature

REV. 0

Page 9

OP184/OP284/OP484

100

10mV

VS = ±15V

= 100k

= 0.3µVp-p

FREQUENCY – Hz

100

120

160

–40

1k 100k 10M

–20

140

10k 1M

VS = ±15V

VS = +3V

TA = +25°C

CHANNEL SEPARATION – dB

100

1µs

100mV

VS = +5V A

= 1

= 2kΩ

= 300pF

- +25°C

400mV

√

NOISE DENSITY – nV/

FREQUENCY – Hz

±2.5V ≤ VS ≤ ±15V T

= +25°C

10010

1000

Figure 29. Voltage Noise Density vs. Frequency

±2.5V ≤ VS ≤ ±15V T

= +25°C

CURRENT NOISE DENSITY – pA/√Hz

FREQUENCY – Hz

10010

1000

8 6 4 2 0

0.1% –2 –4

STEP SIZE – Volts

–6 –8

–10

01 624

0.01%

SETTLING TIME – µs

VS = ±15V T

= +25°C

Figure 32. Settling Time vs. Step Size

Figure 35. Channel Separation vs. Frequency

100

+400mV

100mV

VS = +5V A

= 1

= OPEN

= 300pF

= +25°C

1µs

Figure 30. Current Noise Density vs. Frequency

Figure 31. Settling Time vs. Step Size

REV. 0

5 4 3 2 1

0.1% 0.01%

–1

STEP SIZE – Volts

–2 –3 –4 –5

01 624

SETTLING TIME – µs

VS = +5V T

= +25°C

Figure 33. 0.1 Hz to 10 Hz Noise

100

VS = +5V, 0V

= 100k

= 0.3µVp-p

10mV

Figure 34. 0.1 Hz to 10 Hz Noise

–9–

Figure 36. Small Signal Transient Response

Figure 37. Small Signal Transient Response

Page 10

OP184/OP284/OP484

POS

NEG

OUT

INPUT FROM

SECOND GAIN

STAGE

0.1

VS = ±1.5V A

200mV

–200mV

100

100mV

NO LOAD T

= +25°C

500ns

Figure 38. Small Signal Transient Response

= 1

200mV

–200mV

VS = ±0.75V

= 1

100

NO LOAD

= +25°C

100mV

Figure 39. Small Signal Transient Response

APPLICATIONS Functional Description

The OP284 and OP484 are precision single-supply, rail-to-rail operational amplifiers. Intended for the portable instrumentation marketplace, the OP184/OP284/OP484 combines the attributes of precision, wide bandwidth, and low noise to make it a superb choice in those single supply applications that require both ac and precision dc performance. Other low supply voltage applications for which the OP284 is well suited are active filters, audio microphone preamplifiers, power supply control, and telecom. To combine all of these attributes with rail-to-rail input/ output operation, novel circuit design techniques are used.

POS

+IN

Q3Q1 Q2

R2 4k

–IN

R4 3k

VO = ±0.75V

AV = 1000

= ±2.5V

= 2kΩ

20k

1µs

0.010

THD+N – %

0.001

0.0005

VO = ±2.5V

VO = ±1.5V

20 100 10k

FREQUENCY – Hz

Figure 40. Total Harmonic Distortion vs. Frequency

stage. A key issue in the input stage is the behavior of the input bias currents over the input common-mode voltage range. Input bias currents in the OP284 are the arithmetic sum of the base currents in Q1-Q3 and in Q2-Q4. As a result of this design approach, the input bias currents in the OP284 not only exhibit different amplitudes, but also exhibit different polarities. This effect is best illustrated in Figure 8. It is, therefore, of paramount importance that the effective source impedances connected to the OP284’s inputs be balanced for optimum dc and ac performance.

To achieve rail-to-rail output, the OP284 output stage design employs a unique topology for both sourcing and sinking current. This circuit topology is illustrated in Figure 42. As previously mentioned, the output stage is voltage-driven from the second gain stage. The signal path through the output stage is inverting; that is, for positive input signals, Q1 provides the base current drive to Q6 so that it conducts (sinks) current. For negative input signals, the signal path via Q1-Q2-D1-Q4-Q3 provides the base current drive for Q5 to conduct (source) current. Both amplifiers provide output current until they are forced into saturation, which occurs at approximately 20 mV from negative rail and 100 mV from the positive supply rail.

For example, Figure 41 illustrates a simplified equivalent circuit for the OP184/OP284/OP484’s input stage. It is comprised of an NPN differential pair, Q1-Q2, and a PNP differential pair, Q3-Q4, operating concurrently. Diode network D1-D2 serves to clamp the applied differential input voltage to the OP284, thereby protecting the input transistors against avalanche damage. Input stage voltage gains are kept low for input rail-to-rail operation. The two pairs of differential output voltages are connected to the OP284’s second stage, which is a compound folded cascode gain stage. It is also in the second gain stage where the two pairs of differential output voltages are combined into a single-ended output signal voltage used to drive the output

NEG

Figure 41. OP284 Equivalent Input Circuit

–10–

Figure 42. OP284 Equivalent Output Circuit

REV. 0

Page 11

Thus, the saturation voltage of the output transistors sets the

OUT

1/2

OP284

limit on the OP284’s maximum output voltage swing. Output short circuit current limiting is determined by the maximum signal current into the base of Q1 from the second gain stage. Under output short circuit conditions, this input current level is approximately 100 µA. With transistor current gains around 200, the short circuit current limits are typically 20 mA. The output stage also exhibits voltage gain. This is accomplished by use of common-emitter amplifiers, and as a result, the voltage gain of the output stage (thus, the open-loop gain of the device) exhibits a dependence to the total load resistance at the output of the OP284.

Input Overvoltage Protection

As with any semiconductor device, if conditions exist where the applied input voltages to the device exceed either supply voltage, the device’s input overvoltage I-V characteristic must be considered. When an overvoltage occurs, the amplifier could be damaged, depending on the magnitude of the applied voltage and the magnitude of the fault current. Figure 43 illustrates the over voltage I-V characteristic of the OP284. This graph was generated with the supply pins connected to GND and a curve tracer’s collector output drive connected to the input.

5 4 3 2 1

0 –1 –2

INPUT CURRENT – mA

–3 –4 –5

–5–4–3–2–1012345

INPUT VOLTAGE – Volts

Figure 43. Input Overvoltage I-V Characteristics of the OP284

As shown in the figure, internal p-n junctions to the OP284 energize and permit current flow from the inputs to the supplies when the input is 1.8 V more positive and 0.6 V more negative than the respective supply rails. As illustrated in the simplified equivalent circuit shown in Figure 41, the OP284 does not have any internal current limiting resistors; thus, fault currents can quickly rise to damaging levels.

This input current is not inherently damaging to the device, provided that it is limited to 5 mA or less. For the OP284, once the input exceeds the negative supply by 0.6 V, the input current quickly exceeds 5 mA. If this condition continues to exist, an external series resistor should be added at the expense of additional thermal noise. Figure 44 illustrates a typical noninverting configuration for an overvoltage protected amplifier where the series resistance, R

, is chosen such that:

OP184/OP284/OP484

Figure 44. A Resistance in Series with an Input Limits Overvoltage Currents to Safe Values

For example, a 1 kΩ resistor will protect the OP284 against input signals up to 5 V above and below the supplies. For other configurations where both inputs are used, then each input should be protected against abuse with a series resistor. Again, in order to ensure optimum dc and ac performance, it is recommended to balance source impedance levels. For more information on the general overvoltage characteristics of amplifiers, please refer to the 1993 System Applications Guide, Section 1, pages 56-69. This reference textbook is available from the Analog Devices Literature Center.

Output Phase Reversal

Some operational amplifiers designed for single-supply operation exhibit an output voltage phase reversal when their inputs are driven beyond their useful common-mode range. Typically for single-supply bipolar op amps, the negative supply determines the lower limit of their common-mode range. With these devices, external clamping diodes, with the anode connected to ground and the cathode to the inputs, prevent input signal excursions from exceeding the device’s negative supply (i.e., GND), preventing a condition that could cause the output voltage to change phase. JFET-input amplifiers may also exhibit phase reversal, and, if so, a series input resistor is usually required to prevent it.

The OP284 is free from reasonable input voltage range restrictions, provided that input voltages no greater than the supply voltages are applied. Although the device’s output will not change phase, large currents can flow through the input protection diodes as was shown in Figure 43. Therefore, the technique recommended in the Input Overvoltage Protection section should be applied to those applications where the likelihood of input voltages exceeding the supply voltages is high.

Designing Low Noise Circuits in Single Supply Applications

In single supply applications, devices like the OP284 extend the dynamic range of the application through the use of rail-to-rail operation. In fact, the OP284 family is the first of its kind to combine single supply, rail-to-rail operation and low noise in one device. It is the first device in the industry to exhibit an input noise voltage spectral density of less than 4 nV/√ 1 kHz. It was also designed specifically for low-noise, singlesupply applications, and as such, some discussion on circuit noise concepts in single supply applications is appropriate.

Hz at

IN ( MAX )–VSUPPLY

RS=

REV. 0

5 mA

–11–

Page 12

OP184/OP284/OP484

TOTAL SOURCE RESISTANCE, RS – Ω

100

NOISE FIGURE – dB

1k 10k 100k

8 7

FREQUENCY = 1kHz T

= +25°C

Referring to the op amp noise model circuit configuration illustrated in Figure 45, the expression for an amplifier’s total equivalent input noise voltage for a source resistance level R

given by:

2 e

()

[]

()

nOA

× R

,units in

()

nOA

where RS = 2R = Effective, or equivalent, circuit source

resistance, (e

)2 = Op amp equivalent input noise voltage spectral

nOA

power (1 Hz BW), (i

)2 = Op amp equivalent input noise current spectral

nOA

power (1 Hz BW), (e

)2 = Source resistance thermal noise voltage power =

(4kTR), k = Boltzmann’s constant = 1.38 × 10

–23

J/K, and

T = Ambient temperature of the circuit, in Kelvin, =

273.15 + T

"NOISELESS"

(°C)

NReNOA

NOA

IDEAL

NOISELESS

OP AMP

= 2R

Figure 45. Op Amp Noise Circuit Model Used to Determine Total Circuit Equivalent Input Noise Voltage and Noise Figure

As a design aid, Figure 46 illustrates the total equivalent input noise of the OP284 and the total thermal noise of a resistor for comparison. Note that for source resistance less than 1 kΩ, the equivalent input noise voltage of the OP284 is dominant.

Since circuit SNR is the critical parameter in the final analysis, the noise behavior of a circuit is often expressed in terms of its noise figure, NF. Noise figure is defined as the ratio of a circuit’s output signal-to-noise to its input signal-to-noise. An expression of a circuit’s NF in dB, and in terms of the operational amplifier’s voltage and current noise parameters defined previously, is given by:

NF (dB) =10 log 1 +

  



 



()

nOA

()

nRS

nOARS



 

 





where NF (dB) = Noise figure of the circuit, expressed in dB,

= Effective, or equivalent, source resistance presented

to amplifier, (e

)2 = OP284 noise voltage spectral power (1 Hz BW),

nOA

)2 = OP284 noise current spectral power (1 Hz BW),

nOA

)2 = Source resistance thermal noise voltage power

nRS

= (4kTR

Circuit noise figure is straightforward to calculate because the signal level in the application is not required to determine it. However, many designers using NF calculations as the basis for achieving optimum SNR believe that low noise figure is equal to low total noise. In fact, the opposite is true, as illustrated in Figure 47. Here, the noise figure of the OP284 is expressed as a function of the source resistance level. Note that the lowest noise figure for the OP284 occurs at a source resistance level of 10 kΩ. However, Figure 46 shows that this source resistance level and the OP284 generate approximately 14 nV/√

Hz of total equivalent circuit noise. Signal levels in the application would invariably be increased to maximize circuit SNR—not an option in low voltage, single supply applications.

Figure 46. OP284 Total Noise vs. Source Resistance

100

FREQUENCY = 1kHz

= +25°C

√

OP284 TOTAL

EQUIVALENT NOISE

RESISTOR THERMAL NOISE ONLY

EQUIVALENT THERMAL NOISE – nV/

100

1k 10k 100k

TOTAL SOURCE RESISTANCE, RS – Ω

Figure 47. OP284 Noise Figure vs. Source Resistance

In single supply applications, therefore, it is recommended for optimum circuit SNR to choose an operational amplifier with the lowest equivalent input noise voltage and to choose source resistance levels consistent in maintaining low total circuit noise.

–12–

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Page 13

Overdrive Recovery

OUT

1.1kΩ

+3V

A1, A2 = 1/2 OP284 GAIN = 1 + –––

R3 SET R2 = R3 R1 + P1 = R4

10kΩ

RP1

1kΩ

RP2 1kΩ

1.1kΩ

9.53kΩ

P1 500Ω

AC CMRR

TRIM

5pF–40pF

+2.5V

REF

5kΩ

+3V

1/2

OP284

100kΩ

+3V

0.1µF

17.4kΩ

AD589

RESISTORS = 1%, 100ppm/°C POTENTIOMETER = 10 TURN, 100ppm/°C

The overdrive recovery time of an operational amplifier is the time required for the output voltage to recover to its linear region from a saturated condition. The recovery time is important in applications where the amplifier must recover quickly after a large transient event. The circuit shown in Figure 48 was used to evaluate the OP284’s overload recovery time. The OP284 takes approximately 2 µs to recover from positive saturation and approximately 1 µs to recover from negative saturation.

OP184/OP284/OP484

10kΩ

+5V

1/2

OP284

–5V

OUT

10V STEP

10kΩ

9kΩ

Figure 48. Output Overload Recovery Test Circuit

A Single-Supply, +3 V Instrumentation Amplifier

The OP284’s low noise, wide bandwidth, and rail-to-rail input/ output operation makes it ideal for low supply voltage applications such as in a two op amp instrumentation amplifier as shown in Figure 49. The circuit uses the classic two op amp instrumentation amplifier topology with four resistors to set the gain. The transfer equation of the circuit is identical to that of a noninverting amplifier. Resistors R2 and R3 should be closely matched to each other as well as to resistors (R1 + P1) and R4 to ensure good common-mode rejection performance. Resistor networks should be used in this circuit for R2 and R3 because they exhibit the necessary relative tolerance matching for good performance. Matched networks also exhibit tight relative resistor temperature coefficients for good circuit temperature stability. Trimming potentiometer P1 is used for optimum dc CMR adjustment, and C1 is used to optimize ac CMR. With the circuit values as shown, circuit CMR is better than 80 dB over the frequency range of 20 Hz to 20 kHz. Circuit RTI (Referred-toInput) noise in the 0.1 Hz to 10 Hz band is an impressively low

0.45 µV p-p. Resistors RP1 and RP2 serve to protect the OP284’s inputs against input overvoltage abuse. Capacitor C2 can be included to the limit circuit bandwidth and, therefore, wide bandwidth noise in sensitive applications. The value of this capacitor should be adjusted depending on the required closed-loop bandwidth of the circuit. The R4-C2 time constant creates a pole at a frequency equal to:

Figure 49. A Single Supply, +3 V Low Noise Instrumentation Amplifier

A +2.5 V Reference from a +3 V Supply

In many single-supply applications, the need for a 2.5 V reference often arises. Many commercially available monolithic

2.5 V references require at least a minimum operating supply of 4 V. The problem is exacerbated when the minimum operating supply voltage is +3 V. The circuit illustrated in Figure 50 is an example of a +2.5 V reference that operates from a single +3 V supply. The circuit takes advantage of the OP284’s rail-to-rail input/output voltage ranges to amplify an AD589’s 1.235 V output to +2.5 V. The OP284’s low TCV

of 1.5 µV/°C helps

maintain an output voltage temperature coefficient that is dominated by the temperature coefficients of R2 and R3. In this circuit with 100 ppm/°C TCR resistors, the output voltage exhibits a temperature coefficient of 200 ppm/°C. Lower tempco resistors are recommended for more accurate performance over temperature.

One measure of the performance of a voltage reference is its capacity to recover from sudden changes in load current. While sourcing a steady-state load current of 1 mA, this circuit recovers to 0.01% of the programmed output voltage in 1.5 µs for a total change in load current of ± 1 mA.

REV. 0

f (3dB) =

2 π R4 C2

Figure 50. A +2.5 V Reference that Operates on a Single +3 V Supply

–13–

Page 14

OP184/OP284/OP484

A +5 V Only, 12-Bit DAC Swings Rail-to-Rail

The OP284 is ideal for use with a CMOS DAC to generate a digitally-controlled voltage with a wide output range. Figure 51 shows a DAC8043 used in conjunction with the AD589 to generate a voltage output from 0 V to 1.23 V. The DAC is actually operating in “voltage switching” mode where the reference is connected to the current output, I taken from the V

pin. This topology is inherently noninvert-

REF

, and the output voltage is

OUT

ing as opposed to the classic current output mode, which is inverting and not usable in single supply applications.

+5V

DAC8043

OUT

GND CLK SR1 LD

4765

232Ω

DIGITAL

CONTROL

REF

32.4kΩ

+5V

1/2

OP284

100kΩ

OUT

= –––– (5V)

4096

17.8kΩ

1.23V

AD589

Figure 51. A +5 V Only, 12-Bit DAC Swings Rail-to-Rail

In this application the OP284 serves two functions. First, it buffers the high output impedance of the DAC’s V

REF

pin,

which is on the order of 10 kΩ. The op amp provides a low impedance output to drive any following circuitry. Second, the op amp amplifies the output signal to provide a rail-to-rail output swing. In this particular case, the gain is set to 4.1 so that the circuit generates a 5 V output when the DAC output is at full scale. If other output voltage ranges are needed, such as 0 V ≤ V

≤ 4.095 V, the gain can be easily changed by adjusting

OUT

the values of R2 and R3.

A High-Side Current Monitor

In the design of power supply control circuits, a great deal of design effort is focused on ensuring a pass transistor’s long-term reliability over a wide range of load current conditions. As a result, monitoring and limiting device power dissipation is of prime importance in these designs. The circuit illustrated in Figure 52 is an example of a +3 V, single-supply high-side current monitor that can be incorporated into the design of a voltage regulator with fold-back current limiting or a high current power supply with crowbar protection. This design uses an OP284’s rail-to-rail input voltage range to sense the voltage drop across a 0.1 Ω current shunt. A p-channel MOSFET used as the feedback element in the circuit converts the op amp’s differential input voltage into a current. This current is applied to R2 to generate a voltage that is a linear representation of the load current. The transfer equation for the current monitor is given by:

Monitor Output = R2 ×

 



SENSE



× I





For the element values shown, the Monitor Output’s transfer characteristic is 2.5 V/A.

MONITOR

OUTPUT

+3V

Si9433

100Ω

SENSE

0.1Ω

2.49kΩ

+3V

1/2

AD284

+3V

0.1µF

Figure 52. A High-Side Load Current Monitor

Capacitive Load Drive Capability

The OP284 exhibits excellent capacitive load driving capabilities. It can drive up to 1 nF as shown in Figure 27. Even though the device is stable, a capacitive load does not come without penalty in bandwidth. The bandwidth is reduced to under 1 MHz for loads greater than 2 nF. A “snubber” network on the output does not increase the bandwidth, but it does significantly reduce the amount of overshoot for a given capacitive load. A snubber consists of a series R-C network (R

, CS), as

shown in Figure 53, connected from the output of the device to ground. This network operates in parallel with the load capacitor, C

, to provide the necessary phase lag compensation. The

value of the resistor and capacitor is best determined empirically.

+5V

0.1µF

100mVp-p

1/2

OP284

50Ω C

100nF

C 1nF

OUT

Figure 53. Snubber Network Compensates for Capacitive Load

The first step is to determine the value of the resistor RS. A good starting value is 100 Ω (typically, the optimum value will be less than 100 Ω). This value is reduced until the small-signal transient response is optimized. Next, C

is determined—10 µF

is a good starting point. This value is reduced to the smallest value for acceptable performance (typically, 1 µF). For the case of a 10 nF load capacitor on the OP284, the optimal snubber network is a 20 Ω in series with 1 µF. The benefit is immedi- ately apparent as shown in the scope photo in Figure 54. The top trace was taken with a 1 nF load, and the bottom trace was taken with the 50 Ω, 100 nF snubber network in place. The amount of overshoot and ringing is dramatically reduced. Table I below illustrates a few sample snubber networks for large load capacitors.

–14–

REV. 0

Page 15

OP184/OP284/OP484

µs

100

ONLY

CIRCUIT

50mv50m

2µs

1nF LOAD

SNUBBER

Figure 54. Overshoot and Ringing Is Reduced by Adding a “Snubber” Network in Parallel with the 1 nF Load

Table I. Snubber Networks for Large Capacitive Loads

Load Capacitance Snubber Network (CL)(R

, CS)

1 nF 50 Ω, 100 nF 10 nF 20 Ω, 1 µF 100 nF 5 Ω, 10 µF

A Low Dropout Regulator with Current Limiting

Many circuits require stable regulated voltages relatively close, in potential to an unregulated input source. This “low dropout” type of regulator is readily implemented with a rail-to-rail output op amp such as the OP284 because the wide output swing allows easy drive to a low saturation voltage pass device. Furthermore, it is particularly useful when the op amp also enjoys a rail-rail input feature, as this factor allows it to perform highside current sensing for positive rail current limiting. Typical examples are voltages developed from 3 V to 9 V range system sources or anywhere where low dropout performance is required for power efficiency. The 4.5 V case here works from 5 V nominal sources with worst-case levels down to 4.6 V or less.

Figure 55 shows such a regulator set up using an OP284 plus a low R

, P-channel MOSFET pass device. Part of the low

DS(ON)

dropout performance of this circuit is provided by Q1, which has a rating of 0.11 Ω with a gate drive voltage of only 2.7 V. This relatively low gate drive threshold allows operation of the regulator on supplies as low as 3 V without compromising overall performance.

The circuit’s main voltage control loop operation is provided by U1B, half of the OP284. This voltage control amplifier amplifies the 2.5 V reference voltage produced by three terminal U2, a REF192. The regulated output voltage V

OUT=VOUT 2

For this example, since V

OUT

()

of 4.5 V with V

R2 R3

OUT

OUT2

is then:

= 2.5 V re-

quires a U1B gain of 1.8 times, R3 and R2 are chosen for a ratio of 1.2:1 or 10.0 kΩ:8.06 kΩ (using closest 1% values). Note that for the lowest V

dc error, R2iR3 should be maintained

OUT

equal to R1 (as here), and the R2-R3 resistors should be stable, close tolerance metal film types. The table in Figure 55 summarizes R1-R3 values for some popular voltages. However, note that, in general, the output can be anywhere between V

and the 12 V maximum rating of Q1.

OUT2

While the low voltage saturation characteristic of Q1 is a key part of the low dropout, another component is a low current sense comparison threshold with good dc accuracy. Here, this is provided by current sense amplifier U1A, which is provided by a 20 mV reference from the 1.235 V AD589 reference diode D2 and the R7-R8 divider. When the product of the output current and the R

value match this voltage threshold, the cur-

rent control loop is activated, and U1A drives Q1’s gate through D1. This causes the overall circuit operation to enter current mode control with a current limit I



LIMIT

R(D2)





defined as:

LIMIT

 

()

R7 + R8



REV. 0

VS > V

+ 0.1V

OUT

OPTIONAL ON/OFF CONTROL INPUT CMOS HI (OR OPEN) = ON

LO = OFF

COMMON

0.1µF

OP284

U1A

4.53kΩ

0.1µF

OUT

2.5V

1N4148

R3 10kΩ

0.01µF

U1B

OP284

OUT

5.0V

4.5V

3.3V

3.0V

OUTPUT TABLE

4.99k

4.53k

2.43k

1.69k

AD589

0.05Ω

4.99kΩ

301kΩ

27.4kΩ

1N4148

R11 1kΩ

REF192

R10

1kΩ

4.99kΩ

3 2

0.01µF

1µF

Figure 55. A Low Dropout Regulator with Current Limiting

–15–

22.1kΩ

2.21kΩ

SI9433DY

8.06kΩ

R2 R3

10.0k

8.06k

3.24k

2.00k

10.0k

OUT

4.5V @ 350mA (SEE TABLE)

C6 10µF

OUT

COMMON

Page 16

OP184/OP284/OP484

+3V

2.67kΩ

1µF

2.67kΩ

2µF

(1µF x 2)

2.67kΩ

1.33kΩ (2.67kΩ ÷ 2)

2.67kΩ

R6 10kΩ

R8 1kΩ

1kΩ

R11

10kΩ

0.03µF

R12

150Ω

R10 20kΩ

1µF

R9 20kΩ

+3V

1.5V C6 1µF

A1, A2, A3 = OP484

Q = 0.75 NOTE: FOR 50Hz APPLICATIONS

CHANGE R1–R4 TO 3.1kΩ AND R5 TO 1.58kΩ (3.16kΩ ÷ 2).

Obviously, it is desirable to keep this comparison voltage small, since it becomes a significant portion of the overall dropout voltage. Here, the 20 mV reference is higher than the typical offset of the OP284 but still reasonably low as a percentage of V

(< 0.5%). In adapting the limiter for other I

OUT

sense resistor R

should be adjusted along with R7-R8, to main-

LIMIT

levels,

physiological signals, such as heart rates, blood pressure readings, EEGs, EKGs, etc. This notch filter effectively squelches 60 Hz pickup at a filter Q of 0.75. Substituting 3.16 kΩ resistors for the 2.67 kΩ in the twin-T section (R1 through R5) configures the active filter to reject 50 Hz interference.

tain this threshold voltage between 20 mV and 50 mV. Performance of the circuit is excellent. For the 4.5 V output

version, the measured dc output change for a 225 mA load change was on the order of a few microvolts while the dropout voltage at this same current level was about 30 mV. The current limit as shown is 400 mA, which allows the circuit to be used at levels up to 300 mA or more. While the Q1 device can actually support currents of several amperes, a practical current rating takes into account the SO-8 device’s 2.5 W, 25°C dissipation. Because a short circuit current of 400 mA at an input level of 5 V will cause a 2 W dissipation in Q1, other input conditions should be considered carefully in terms of Q1’s potential overheating. Of course, if higher powered devices are used for Q1, this circuit can support outputs of tens of amperes as well as the higher V

levels noted above.

OUT

The circuit shown can be used either as a standard low dropout regulator, or it can be used with ON/OFF control. By driving Pin 3 of U1 with the optional logic control signal V

, the

output is switched between ON and OFF. Note that when the output is OFF in this circuit, it is still active (i.e., not an open circuit). This is because the OFF state simply reduces the voltage input to R1, leaving the U1A/B amplifiers and Q1 still active.

When ON/OFF control is used, resistor R10 should be used with U1 to speed ON-OFF switching and to allow the output of the circuit to settle to a nominal zero voltage. Components D3 and R11 also aid in speeding up the ON-OFF transition by providing a dynamic discharge path for C2. OFF-ON transition time is less than 1 ms, while the ON-OFF transition is longer but under 10 ms.

A +3 V, 50 Hz/60 Hz Active Notch Filter with False Ground

To process signals in a single-supply system, it is often best to use a false ground biasing scheme. A circuit that uses this approach is illustrated in Figure 56. In this circuit, a false-ground circuit biases an active notch filter used to reject 50 Hz/60 Hz power line interference in portable patient monitoring equipment. Notch filters are quite commonly used to reject power line frequency interference that often obscures low frequency

Figure 56. A +3 V Single Supply, 50/60 Hz Active Notch Filter with False Ground

Amplifier A3 is the heart of the false-ground bias circuit. It simply buffers the voltage developed at R9 and R10 and is the reference for the active notch filter. Since the OP484 exhibits a rail-to-rail input common-mode range, R9 and R10 are chosen to split the +3 V supply symmetrically. An in-the-loop compensation scheme is used around the OP484 that allows the op amp to drive C6, a 1 µF capacitor, without oscillation. C6 maintains a low impedance ac ground over the operating frequency range of the filter.

The filter section uses a OP484 in a twin-T configuration whose frequency selectivity is very sensitive to the relative matching of the capacitors and resistors in the twin-T section. Mylar is the material of choice for the capacitors, and the relative matching of the capacitors and resistors determines the filter’s pass band symmetry. Using 1% resistors and 5% capacitors produces satisfactory results.

–16–

REV. 0

Page 17

OP184/OP284/OP484

*OP284 SPICE Macro-model 9/94 / Rev. A * ARG/ADI * * Copyright 1995 by Analog Devices * * Refer to “README.DOC” file for License Statement. Use of this model * indicates your acceptance of the terms and provisions in the License * Statement. * * Node assignments * noninverting input * | inverting input * | | positive supply * | | | negative supply * | | | | output * | | | | | .SUBCKT OP284 1 2 99 50 45 * * INPUT STAGE * Q1 5 2 3 QIN 1 Q2 6 11 3 QIN 1 Q3 7 2 4 QIP 1 Q4 8 11 4 QIP 1 DC1 2 11 DC DC2 11 2 DC Q5 4 9 99 QIP 1 Q6 9 9 99 QIP 1 Q7 3 10 50 QIN 1 Q8 10 10 50 QIN 1 R1 99 5 4E3 R2 99 6 4E3 R3 7 50 4E3 R4 8 50 4E3 IREF 9 10 50.5E-6 EOS 1 11 POLY(2) (22,98) (14,98) -25E-6 1E-2 1 IOS 2 1 5E-9 CIN 1 2 2E-12 GN1 98 1 (17,98) 1E-3 GN2 98 2 (23,98) 1E-3 * * VOLTAGE NOISE SOURCE WITH FLICKER NOISE * VN1 13 98 DC 2 VN2 98 15 DC 2 DN1 13 14 DEN DN2 14 15 DEN * * CURRENT NOISE SOURCE WITH FLICKER NOISE * VN3 16 98 DC 2 VN4 98 18 DC 2 DN3 16 17 DIN DN4 17 18 DIN * * 2ND CURRENT NOISE SOURCE WITH FLICKER NOISE * VN5 19 98 DC 2 VN6 98 24 DC 2

DN5 19 23 DIN DN6 23 24 DIN * * GAIN STAGE * EREF 98 0 POLY(2) (99,0) (50,0) 0 0.5 0.5 G1 98 20 POLY(2) (6,5) (8,7) 0 0.5E-3 0.5E-3 R9 20 98 1E3 * * COMMON MODE STAGE WITH ZERO AT 100Hz * ECM 98 21 POLY(2) (1,98) (2,98) 0 0.5 0.5 R10 21 22 1 R11 22 98 100E-6 C4 21 22 1.592E-3 * * NEGATIVE ZERO AT 20MHz * E1 27 98 (20,98) 1E6 R17 27 28 1 R18 28 98 1E-6 C8 25 26 7.958E-9 ENZ 25 98 (27,28) 1 VNZ 26 98 DC 0 FNZ 27 28 VNZ -1 * * POLE AT 40MHz * G4 98 29 (28,98) 1 R19 29 98 1 C9 29 98 3.979E-9 * * POLE AT 40MHz * G5 98 30 (29,98) 1 R20 30 98 1 C10 30 98 3.979E-9 * * OUTPUT STAGE * ISY 99 50 0.276E-3 GIN 50 31 POLY(1) (30,98) .862574E-6 505.879E-6 RIN 31 50 2.75E6 VB 99 32 0.7 Q11 32 31 33 QON 1 R21 33 34 4.5E3 I1 34 50 50E-6 R22 99 35 6E3 Q12 36 36 35 QOP 1 I2 36 50 50E-6 R23 99 37 2.6E3 R24 34 38 5E3 Q13 39 36 37 QOP 1 Q14 39 38 40 QON 1.5 R25 40 50 40 Q15 39 39 41 QON 1 R26 41 42 1E3 R27 99 43 220 Q16 44 44 43 QOP 1.5 Q17 44 39 42 QON 1 R28 42 50 2E3 VSCP 99 97 DC 0

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OP184/OP284/OP484

FSCP 46 99 VSCP 1 RSCP 46 99 40 Q20 44 46 99 QOP 1 Q18 45 44 97 QOP 4.5 Q19 45 34 51 QON 4.5 VSCN 51 50 DC 0 FSCN 50 47 VSCN 1 RSCN 47 50 40 Q21 34 47 50 QON 1 CC2 31 45 20E-12 CF1 31 34 15E-12 CF2 31 42 15E-12 CO1 34 45 15E-12 CO2 42 45 5E-12 D3 45 99 DX D4 50 45 DX .MODEL DC D(IS=130E-21) .MODEL DX D() .MODEL DEN D(RS=100 KF=12E-15 AF=1) .MODEL DIN D(RS=5.358 KF=56E-15 AF=1) .MODEL QIN NPN(BF=200 VA=200 IS=0.5E-16) .MODEL QIP PNP(BF=100 VA=60 IS=0.5E-16) .MODEL QON NPN(BF=200 VA=200 IS=0.5E-16 RC=50) .MODEL QOP PNP(BF=200 VA=200 IS=0.5E-16 RC=160) .ENDS

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0.795 (20.19)

0.725 (18.42)

0.280 (7.11)

0.240 (6.10)

PIN 1

SEATING PLANE

0.022 (0.558)

0.014 (0.356)

0.060 (1.52)

0.015 (0.38)

0.210 (5.33) MAX

0.130 (3.30) MIN

0.070 (1.77)

0.045 (1.15)

0.100 (2.54)

BSC

0.160 (4.06)

0.115 (2.93)

0.325 (8.25)

0.300 (7.62)

0.015 (0.381)

0.008 (0.204)

0.195 (4.95)

0.115 (2.93)

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

OP184/OP284/OP484

0.210 (5.33) MAX

0.160 (4.06)

0.115 (2.93)

0.022 (0.558)

0.014 (0.356)

0.2440 (6.20)

0.2284 (5.80)

0.0098 (0.25)

0.0040 (0.10) SEATING

PLANE

8-Lead Epoxy DIP

(P Suffix)

0.430 (10.92)

0.348 (8.84)

0.280 (7.11)

PIN 1

0.100 (2.54)

0.240 (6.10)

0.060 (1.52)

0.015 (0.38)

0.070 (1.77)

0.045 (1.15)

BSC

8-Lead SO

(S Suffix)

0.1968 (5.00)

0.1890 (4.80)

0.1574 (4.00)

0.1497 (3.80)

0.0688 (1.75)

PIN 1

0.0532 (1.35)

0.0192 (0.49)

0.0500 (1.27)

0.0138 (0.35)

BSC

0.130 (3.30) MIN

SEATING PLANE

0.0098 (0.25)

0.0075 (0.19)

0.325 (8.25)

0.300 (7.62)

0.015 (0.381)

0.008 (0.204)

8° 0°

0.195 (4.95)

0.115 (2.93)

0.0196 (0.50)

0.0099 (0.25)

0.0500 (1.27)

0.0160 (0.41)

x 45°

0.2440 (6.20)

0.2284 (5.80)

0.0098 (0.25)

0.0040 (0.10) SEATING

PLANE

14-Lead Epoxy DIP

(P Suffix)

14-Lead Narrow-Body SO

(S Suffix)

0.3444 (8.75)

0.3367 (8.55)

14 8

0.1574 (4.00)

0.1497 (3.80)

PIN 1

0.0500 (1.27)

BSC

0.0688 (1.75)

0.0532 (1.35)

0.0192 (0.49)

0.0138 (0.35)

0.0098 (0.25)

0.0075 (0.19)

0.0196 (0.50)

0.0099 (0.25)

8° 0°

x 45°

0.0500 (1.27)

0.0160 (0.41)

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C2167–12–10/96

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PRINTED IN U.S.A.

Datasheet OP184, OP484, OP284 Datasheet (Analog Devices)

Specifications and Main Features

Frequently Asked Questions

User Manual