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FEATURES True Single Supply Operation Output Swings Rail-to-Rail Input Voltage Range Extends Below Ground Single Supply Capability from 5 V to 36 V Dual Supply Capability from 2.5 V to 18 V Excellent Load Drive Capacitive Load Drive Up to 350 pF Minimum Output Current of 15 mA Excellent AC Performance for Low Power 800 A Max Quiescent Current Unity Gain Bandwidth: 1.8 MHz Slew Rate of 3.0 V/ s Excellent DC Performance 800 V Max Input Offset Voltage 1 V/ C Typ Offset Voltage Drift 25 pA Max Input Bias Current Low Noise 13 nV/Hz @ 10 kHz APPLICATIONS Battery-Powered Precision Instrumentation Photodiode Preamps Active Filters 12- to 14-Bit Data Acquisition Systems Medical Instrumentation Low Power References and Regulators PRODUCT DESCRIPTION
NULL -IN +IN -VS
Single Supply, Rail to Rail Low Power FET-Input Op Amp AD820
FUNCTIONAL BLOCK DIAGRAM
1 2 3 4
AD820
8 NC 7 +VS 6 VOUT
NC -IN +IN -VS
1 2 3 4
AD820
8 NC 7 +VS 6 VOUT
TOP VIEW (Not to Scale)
5 NULL
TOP VIEW (Not to Scale)
5 NC
NC = NO CONNECT
allowing the AD820 to accommodate input signals below ground in the single supply mode. Output voltage swing extends to within 10 mV of each rail providing the maximum output dynamic range. Offset voltage of 800 V max, offset voltage drift of 1 V/C, typical input bias currents below 25 pA and low input voltage noise provide dc precision with source impedances up to a Gigaohm. 1.8 MHz unity gain bandwidth, -93 dB THD at 10 kHz and 3 V/s slew rate are provided for a low supply current of 800 A. The AD820 drives up to 350 pF of direct capacitive load and provides a minimum output current of 15 mA. This allows the amplifier to handle a wide range of load conditions. This combination of ac and dc performance, plus the outstanding load drive capability, results in an exceptionally versatile amplifier for the single supply user. The AD820 is available in two performance grades. The A and B grades are rated over the industrial temperature range of -40C to +85C. The AD820 is offered in two varieties of 8-lead package: plastic DIP, and surface mount (SOIC).
The AD820 is a precision, low power FET input op amp that can operate from a single supply of 5.0 V to 36 V, or dual supplies of 2.5 V to 18 V. It has true single supply capability with an input voltage range extending below the negative rail,
50 45 40
NUMBER OF UNITS
35 30 25 20 15 10 5 0 0 1 2 3 4 5 6 7 INPUT BIAS CURRENT - pA 8 9 10
Figure 2. Gain of 2 Amplifier; VS = 5, 0, VIN = 2.5V Sine Centered at 1.25 Volts
Figure 1. Typical Distribution of Input Bias Current
REV. C
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 that 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: 781/329-4700 www.analog.com Fax: 781/326-8703 (c) Analog Devices, Inc., 2002
AD820-SPECIFICATIONS (V = 0, 5 V @ T = 25 C, V
S A
CM
= 0 V, VOUT = 0.2 V unless otherwise noted.)
Max 0.8 1.2 25 5 20 500 400 80 80 15 10 Min AD820B Typ 0.1 0.5 2 2 0.5 2 0.5 1000 150 30 Max 0.4 0.9 10 2.5 10 Unit mV mV V/C pA nA pA nA V/mV V/mV V/mV V/mV V/mV V/mV
Parameter DC PERFORMANCE Initial Offset Max Offset over Temperature Offset Drift Input Bias Current at TMAX Input Offset Current at TMAX Open-Loop Gain TMIN to TMAX TMIN to TMAX TMIN to TMAX NOISE/HARMONIC PERFORMANCE Input Voltage Noise 0.1 Hz to 10 Hz f = 10 Hz f = 100 Hz f = 1 kHz f = 10 kHz Input Current Noise 0.1 Hz to 10 Hz f = 1 kHz Harmonic Distortion f = 10 kHz DYNAMIC PERFORMANCE Unity Gain Frequency Full Power Response Slew Rate Settling Time to 0.1% to 0.01% INPUT CHARACTERISTICS Common-Mode Voltage Range1 TMIN to TMAX CMRR TMIN to TMAX Input Impedance Differential Common Mode OUTPUT CHARACTERISTICS Output Saturation Voltage2 VOL-VEE TMIN to TMAX VCC-VOH TMIN to TMAX VOL-VEE TMIN to TMAX VCC-VOH TMIN to TMAX VOL-VEE TMIN to TMAX VCC-VOH TMIN to TMAX Operating Output Current TMIN to TMAX Short-Circuit Current Capacitive Load Drive POWER SUPPLY Quiescent Current Power Supply Rejection TMIN to TMAX
Conditions
Min
AD820A Typ 0.1 0.5 2 2 0.5 2 0.5
VO = 0 V to 4 V
VO = 0.2 V to 4 V RL = 100 k RL = 10 k RL = 1 k
400 400 80 80 15 10
1000 150 30
2 25 21 16 13 18 0.8 RL = 10 k to 2.5 V VO = 0.25 V to 4.75 V -93 1.8 210 3 1.4 1.8 -0.2 -0.2 66 66 +4 +4 80 -0.2 -0.2 72 66
2 25 21 16 13 18 0.8 -93 1.8 210 3 1.4 1.8 +4 +4 80
V p-p nV/Hz nV/Hz nV/Hz nV/Hz fA p-p fA/Hz dB MHz kHz V/s s s V V dB dB pF pF
VO p-p = 4.5 V VO = 0.2 V to 4.5 V
VCM = 0 V to 2 V
1013 0.5 1013 2.8
1013 0.5 1013 2.8
ISINK = 20 A ISOURCE = 20 A ISINK = 2 mA ISOURCE = 2 mA ISINK = 15 mA ISOURCE = 15 mA 15 12
5 10 40 80 300 800
7 10 14 20 55 80 110 160 500 1000 1500 1900 15 12
5 10 40 80 300 800
7 10 14 20 55 80 110 160 500 1000 1500 1900
25 350 TMIN to TMAX VS+ = 5 V to 15 V 620 80 800 66 66
25 350 620 80 800
mV mV mV mV mV mV mV mV mV mV mV mV mA mA mA pF A dB dB
70 70
-2-
REV. C
SPECIFICATIONS (V = 0, 5 V @ T = 25 C, V
S A
CM
= 0 V, VOUT = 0.2 V unless otherwise noted.)
Min AD820A Typ 0.1 0.5 2 2 0.5 2 0.5 400 400 80 80 20 10 1000 150 30 Max 0.8 1.5 25 5 20 400 400 80 80 20 10 Min AD820B Typ 0.3 0.5 2 2 0.5 2 0.5 1000 150 30
AD820
Max 0.4 1 10 2.5 10 Unit mV mV V/C pA nA pA nA V/mV V/mV V/mV V/mV V/mV V/mV
Parameter DC PERFORMANCE Initial Offset Max Offset over Temperature Offset Drift Input Bias Current at TMAX Input Offset Current at TMAX Open-Loop Gain TMIN to TMAX
Conditions
VO = -5 V to 4 V
VO = 4 V to -4 V RL = 100 k RL = 10 k
TMIN to TMAX RL = 1 k TMIN to TMAX NOISE/HARMONIC PERFORMANCE Input Voltage Noise 0.1 Hz to 10 Hz f = 10 Hz f = 100 Hz f = 1 kHz f = 10 kHz Input Current Noise 0.1 Hz to 10 Hz f = 1 kHz Harmonic Distortion f = 10 kHz DYNAMIC PERFORMANCE Unity Gain Frequency Full Power Response Slew Rate Settling Time to 0.1% to 0.01% INPUT CHARACTERISTICS Common-Mode Voltage Range 1 TMIN to TMAX CMRR TMIN to TMAX Input Impedance Differential Common Mode OUTPUT CHARACTERISTICS Output Saturation Voltage 2 VOL-VEE TMIN to TMAX VCC-VOH TMIN to TMAX VOL-VEE TMIN to TMAX VCC-VOH TMIN to TMAX VOL-VEE TMIN to TMAX VCC-VOH TMIN to TMAX Operating Output Current TMIN to TMAX Short-Circuit Current Capacitive Load Drive POWER SUPPLY Quiescent Current Power Supply Rejection TMIN to TMAX
2 25 21 16 13 18 0.8 RL = 10 k VO = 4.5 V -93 1.9 105 3 1.4 1.8 -5.2 -5.2 66 66 +4 +4 80 1013 0.5 1013 2.8 -5.2 -5.2 72 66
2 25 21 16 13 18 0.8 -93 1.8 105 3 1.4 1.8 +4 +4 80 1013 0.5 1013 2.8
V p-p nV/Hz nV/Hz nV/Hz nV/Hz fA p-p fA/Hz dB MHz kHz V/s s s V V dB dB pF pF
VO p-p = 9 V VO = 0 V to 4.5 V
VCM = -5 V to +2 V
ISINK = 20 A ISOURCE = 20 A ISINK = 2 mA ISOURCE = 2 mA ISINK = 15 mA ISOURCE = 15 mA 15 12
5 10 40 80 300 800
7 10 14 20 55 80 110 160 500 1000 1500 1900 15 12
5 10 40 80 300 800
7 10 14 20 55 80 110 160 500 1000 1500 1900
30 350 TMIN to TMAX VS+ = 5 V to 15 V 650 80 800 70 70
30 350 620 80 800
mV mV mV mV mV mV mV mV mV mV mV mV mA mA mA pF A dB dB
70 70
REV. C
-3-
AD820-SPECIFICATIONS
Parameter DC PERFORMANCE Initial Offset Max Offset over Temperature Offset Drift Input Bias Current at TMAX Input Offset Current at TMAX Open-Loop Gain TMIN to TMAX
(VS =
15 V @ TA = 25 C, VCM = 0 V, VOUT = 0 V unless otherwise noted.)
Min AD820A Typ 0.4 0.5 2 2 40 0.5 2 0.5 500 500 100 100 30 20 2000 500 45 Max 2 3 25 5 20 500 500 100 100 30 20 Min AD820B Typ Max 0.3 0.5 2 2 40 0.5 2 0.5 2000 500 45 1.0 2 10 2.5 10 Unit mV mV V/C pA pA nA pA nA V/mV V/mV V/mV V/mV V/mV V/mV V p-p nV/Hz nV/Hz nV/Hz nV/Hz fA p-p fA/Hz dB MHz kHz V/s s s +14 +14 90 V V dB dB pF pF
Conditions
VCM = 0 V VCM = -10 V VCM = 0 V VO = +10 V to -10 V RL = 100 k RL = 10 k
TMIN to TMAX RL = 1 k TMIN to TMAX NOISE/HARMONIC PERFORMANCE Input Voltage Noise 0.1 Hz to 10 Hz f = 10 Hz f = 100 Hz f = 1 kHz f = 10 kHz Input Current Noise 0.1 Hz to 10 Hz f = 1 kHz Harmonic Distortion f = 10 kHz DYNAMIC PERFORMANCE Unity Gain Frequency Full Power Response Slew Rate Settling Time to 0.1% to 0.01% INPUT CHARACTERISTICS Common-Mode Voltage Range 1 TMIN to TMAX CMRR TMIN to TMAX Input Impedance Differential Common Mode OUTPUT CHARACTERISTICS Output Saturation Voltage2 VOL-VEE TMIN to TMAX VCC-VOH TMIN to TMAX VOL-VEE TMIN to TMAX VCC-VOH TMIN to TMAX VOL-VEE TMIN to TMAX VCC-VOH TMIN to TMAX Operating Output Current TMIN to TMAX Short-Circuit Current Capacitive Load Drive POWER SUPPLY Quiescent Current Power Supply Rejection TMIN to TMAX
2 25 21 16 13 18 0.8 RL = 10 k VO = 10 V -85 1.9 45 3 4.1 4.5 -15.2 -15.2 70 70 +14 +14 80 -15.2 -15.2 74 74
2 25 21 16 13 18 0.8 -85 1.9 45 3 4.1 4.5
VO p-p = 20 V VO = 0 V to 10 V
VCM = -15 V to 12 V
1013 0.5 1013 2.8 ISINK = 20 A ISOURCE = 20 A ISINK = 2 mA ISOURCE = 2 mA ISINK = 15 mA ISOURCE = 15 mA 20 15 45 350 TMIN to TMAX VS+ = 5 V to 15 V 700 80 900 70 70
1013 0.5 1013 2.8
5 10 40 80 300 800
7 10 14 20 55 80 110 160 500 1000 1500 1900 20 15
5 10 40 80 300 800
7 10 14 20 55 80 110 160 500 1000 1500 1900
45 350 700 80 900
mV mV mV mV mV mV mV mV mV mV mV mV mA mA mA
70 70
A dB dB
-4-
REV. C
AD820
NOTES 1 This is a functional specification. Amplifier bandwidth decreases when the input common-mode voltage is driven in the range (+ VS - 1 V) to +VS. Common-mode error voltage is typically less than 5 mV with the common-mode voltage set at 1 volt below the positive supply. 2 VOL-VEE is defined as the difference between the lowest possible output voltage (V OL) and the minus voltage supply rail (V EE). VCC-VOH is defined as the difference between the highest possible output voltage (V OH) and the positive supply voltage (V CC). Specifications subject to change without notice.
ABSOLUTE MAXIMUM RATINGS 1
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 V Internal Power Dissipation2 Plastic DIP (N) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 W SOIC (R) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.0 W Input Voltage . . . . . . . . . . . . . . (+VS + 0.2 V) to - (20 V + VS) Output Short Circuit Duration . . . . . . . . . . . . . . . . Indefinite Differential Input Voltage . . . . . . . . . . . . . . . . . . . . . . . 30 V Storage Temperature Range (N) . . . . . . . . . -65C to +125C Storage Temperature Range (R) . . . . . . . . . -65C to +150C Operating Temperature Range AD820A/B . . . . . . . . . . . . . . . . . . . . . . . . . -40C to +85C Lead Temperature Range (Soldering 60 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . . 260C
NOTES 1 Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. 2 8-Lead Plastic DIP Package: JA = 90C/W 8-Lead SOIC Package: JA = 160C/W
ORDERING GUIDE
Model AD820AN AD820BN* AD820AR AD820BR
Temperature Range -40C to +85C -40C to +85C -40C to +85C -40C to +85C
Package Description 8-Lead Plastic Mini-DIP 8-Lead Plastic Mini-DIP 8-Lead SOIC 8-Lead SOIC
Package Options N-8 N-8 R-8 R-8
*Not for new design, obsolete April 2002.
CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD820 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
WARNING!
ESD SENSITIVE DEVICE
REV. C
-5-
AD820-Typical Performance Characteristics
50 VS = 0V, 5V 40
INPUT BIAS CURRENT - pA
5
NUMBER OF UNITS
30
0 VS = 0V, +5V AND VS = 5V 5V
20
10
0 -0.5
-0.4
-0.3
-0.2 -0.1 0 0.1 0.2 OFFSET VOLTAGE - mV
0.3
0.4
0.5
-5 -5 -4 -3 -2 -1 0 1 2 COMMON-MODE VOLTAGE - V 3 4 5
TPC 1. Typical Distribution of Offset Voltage (248 Units)
TPC 4. Input Bias Current vs. Common-Mode Voltage; VS = +5 V, 0 V and VS = 5 V
48 VS = VS = 5V 15V
1k
40
INPUT BIAS CURRENT - pA
-8 -6 -4 -2 0 2 4 6 8 10
100
32
% IN BIN
24
10
16
1
8
0 -10
0.1 -16
-12
OFFSET VOLTAGE DRIFT - V/ C
-8 -4 0 4 8 COMMON-MODE VOLTAGE - V
12
16
TPC 2. Typical Distribution of Offset Voltage Drift (120 Units)
TPC 5. Input Bias Current vs. Common-Mode Voltage; VS = 15 V
50 45
INPUT BIAS CURRENT - pA
100k
40
NUMBER OF UNITS
10k
35 30 25 20 15 10 5 0 0 1 2 3 4 5 6 7 INPUT BIAS CURRENT - pA 8 9 10
1k
100
10
1
0.1 20
40
60 80 100 TEMPERATURE - C
120
140
TPC 3. Typical Distribution of Input Bias Current (213 Units)
TPC 6. Input Bias Current vs. Temperature; VS = 5 V, VCM = 0
-6-
REV. C
AD820
10M 40 POS RAIL 20 OPEN-LOOP GAIN -V/V 1M VS = 0V, 5V VS = 15V
INPUT VOLTAGE - V
RL = 2k RL = 20k NEG RAIL POS RAIL
0
POS RAIL
100k
-20
NEG RAIL RL = 100k NEG RAIL
10k 100
-40 1k 10k LOAD RESISTANCE - 100k 0 60 120 180 240 300 OUTPUT VOLTAGE FROM VOLTAGE RAILS - mV
TPC 7. Open-Loop Gain vs. Load Resistance
TPC 10. Input Error Voltage with Output Voltage within 300 mV of Either Supply Rail for Various Resistive Loads; VS = 5 V
10M
1k
OPEN-LOOP GAIN - V/V
RL = 100k 1M
VS =
15V
INPUT VOLTAGE NOISE - nV/ Hz
VS = 0V, 5V RL = 10k VS = 15V
100
100k RL = 600
VS = 0V, 5V VS = 15V
10
VS = 0V, 5V 10k -60
1
-40
-20
0
20
40
60
80
100
120
140
1
10
TEMPERATURE - C
100 FREQUENCY - Hz
1k
10k
TPC 8. Open-Loop Gain vs. Temperature
TPC 11. Input Voltage Noise vs. Frequency
300
-40 RL = 10k ACL = -1
200
V
-50 -60
INPUT VOLTAGE -
100
RL = 10k
THD - dB
RL = 100k
-70 -80 VS = -90 VS = 15V; VOUT = 20V p-p
0
-100
5V; VOUT = 9V p-p
-200
RL = 600
-100 VS = 0V, 5V; VOUT = 4.5V p-p -110 100
-300 -16
-12
-8
-4 0 4 OUTPUT VOLTAGE - V
8
12
16
1k
10k FREQUENCY - Hz
100k
TPC 9. Input Error Voltage vs. Output Voltage for Resistive Loads
TPC 12. Total Harmonic Distortion vs. Frequency
REV. C
-7-
AD820
100 100
100 90
80 PHASE
80
COMMON-MODE REJECTION - dB
PHASE MARGIN IN DEGREES
80 70 60 50 40 30 20 10 VS = 0V, 5V VS = 15V
OPEN-LOOP GAIN - dB
60 GAIN 40
60
40
20 RL = 2k CL = 100pF
20
0
0
-20 10 100 1k 10k 100k FREQUENCY - Hz 1M
-20 10M
0 10 100 1k 10k 100k FREQUENCY - Hz 1M 10M
TPC 13. Open-Loop Gain and Phase Margin vs. Frequency
TPC 16. Common-Mode Rejection vs. Frequency
1k
5
COMMON-MODE ERROR VOLTAGE - mV
ACL = +1 VS = 15V 100
4
OUTPUT IMPEDANCE -
10
3
NEGATIVE RAIL
POSITIVE RAIL
1
2
+25 C
+125 C 1 -55 C -55 C
+125 C
0.1
0.01 100
0 1k 10k 100k FREQUENCY - Hz 1M 10M -1 0 1 2 3 COMMON-MODE VOLTAGE FROM SUPPLY RAILS - V
TPC 14. Output Impedance vs. Frequency
TPC 17. Absolute Common-Mode Error vs. Common-Mode Voltage from Supply Rails (VS - VCM)
16 12 1% 8 4 0 -4 -8 -12 -16 0.0 0.1% 0.01% ERROR
1000
OUTPUT SATURATION VOLTAGE - mV
OUTPUT SWING FROM 0 TO
Volts
100 VS - VOH VOL - VS 10
1%
1.0
2.0 3.0 SETTLING TIME - s
4.0
5.0
0 0.001
0.01
0.1 1 LOAD CURRENT - mA
10
100
TPC 15. Output Swing and Error vs. Settling Time
TPC 18. Output Saturation Voltage vs. Load Current
-8-
REV. C
AD820
1000 ISOURCE = 10mA
OUTPUT SATURATION VOLTAGE - mV
120 110
POWER SUPPLY REJECTION - dB
ISINK = 10mA 100
100 90 80 70 60 50 40 30 20 10 +PSRR -PSRR
ISOURCE = 1mA ISINK = 1mA ISOURCE = 10 A
10
ISINK = 10 A
1 -60
-40
-20
0
20 40 60 80 TEMPERATURE - C
100
120
140
0 10
100
1k
10k 100k FREQUENCY - Hz
1M
10M
TPC 19. Output Saturation Voltage vs. Temperature
TPC 22. Power Supply Rejection vs. Frequency
80
SHORT CIRCUIT CURRENT LIMIT - mA
30 R1 = 2k
VS = 15V
OUTPUT VOLTAGE - V
70
25 VS = 20 15V
60 50 40 VS = 0V, 5V 30 20 10 0 -60 VS = 0V, 5V VS = 15V + - -OUT
15
10 VS = 0V, 5V
+
5
-40
-20
0
20 40 60 80 TEMPERATURE - C
100
120
140
0 10k
100k 1M FREQUENCY - Hz
10M
TPC 20. Short Circuit Current Limit vs. Temperature
TPC 23. Large Signal Frequency Response
800 T = +125 C 700 T = +25 C
A
600 500 400 300 200 100 0
QUIESCENT CURRENT -
T = -55 C
0
4
8 12 16 20 24 TOTAL SUPPLY VOLTAGE - V
28
30
36
TPC 21. Quiescent Current vs. Supply Voltage vs. Temperature
REV. C
-9-
AD820
+VS 0.01 F 3 VIN 7
AD820
2 4
6 0.01 F RL 100pF VOUT
-VS
Figure 3. Unity Gain Follower
Figure 6. Large Signal Response Unity Gain Follower; VS = 15 V, RL = 10 k
Figure 4. 20 V, 25 kHz Sine Input; Unity Gain Follower; RL = 600 , VS = 15 V
Figure 7. Small Signal Response Unity Gain Follower; VS = 15 V, RL = 10 k
GND
GND
Figure 5. VS = 5 V, 0 V; Unity Gain Follower Response to 0 V to 4 V Step
Figure 8. VS = 5 V, 0 V; Unity Gain Follower Response to 0 V to 5 V Step
-10-
REV. C
AD820
+VS 0.01 F VIN 3 7
AD820
2 4
6 RL 100pF VOUT
GND
Figure 9. Unity Gain Follower
Figure 12. VS = 5 V, 0 V; Unity Gain Follower Response to 40 mV Step Centered 40 mV above Ground
10k VIN +VS
20k VOUT 0.01 F 2 7
100
AD820
3 4
6 RL 100pF
GND
Figure 10. Gain of Two Inverter
Figure 13. VS = 5 V, 0 V; Gain-of-Two Inverter Response to 20 mV Step, Centered 20 mV below Ground
GND
Figure 11. VS = 5 V, 0 V; Gain-of-Two Inverter Response to 2.5 V Step Centered -1.25 V below Ground
REV. C
-11-
AD820
APPLICATION NOTES Input Characteristics
In the AD820, n-channel JFETs are used to provide a low offset, low noise, high impedance input stage. Minimum input commonmode voltage extends from 0.2 V below -VS to 1 V less than +VS. Driving the input voltage closer to the positive rail will cause a loss of amplifier bandwidth (as can be seen by comparing the large signal responses shown in Figures 5 and 8) and increased common-mode voltage error as illustrated in TPC 11. The AD820 does not exhibit phase reversal for input voltages up to and including +VS. Figure 14a shows the response of an AD820 voltage follower to a 0 V to 5 V (+VS) square wave input. The input and output are superimposed. The output polarity tracks the input polarity up to +VS--no phase reversal. The reduced bandwidth above a 4 V input causes the rounding of the output wave form. For input voltages greater than +VS, a resistor in series with the AD820's plus input will prevent phase reversal, at the expense of greater input voltage noise. This is illustrated in Figure 14b. Since the input stage uses n-channel JFETs, input current during normal operation is negative; the current flows out from the input terminals. If the input voltage is driven more positive than +VS - 0.4 V, the input current will reverse direction as internal device junctions become forward biased. This is illustrated in TPC 4.
A current limiting resistor should be used in series with the input of the AD820 if there is a possibility of the input voltage exceeding the positive supply by more than 300 mV, or if an input voltage will be applied to the AD820 when VS = 0. The amplifier will be damaged if left in that condition for more than 10 seconds. A 1 k resistor allows the amplifier to withstand up to 10 volts of continuous overvoltage, and increases the input voltage noise by a negligible amount. Input voltages less than -VS are a completely different story. The amplifier can safely withstand input voltages 20 V below the minus supply voltage as long as the total voltage from the positive supply to the input terminal is less than 36 V. In addition, the input stage typically maintains picoamp level input currents across that input voltage range. The AD820 is designed for 13 nV/Hz wideband input voltage noise and maintains low noise performance to low frequencies (refer to TPC 11). This noise performance, along with the AD820's low input current and current noise means that the AD820 contributes negligible noise for applications with source resistances greater than 10 k and signal bandwidths greater than 1 kHz. This is illustrated in Figure 15.
100k WHENEVER JOHNSON NOISE IS GREATER THAN AMPLIFIER NOISE, AMPLIFIER NOISE CAN BE CONSIDERED NEGLIGIBLE FOR APPLICATION. 1kHz 1k RESISTOR JOHNSON NOISE 100
INPUT VOLTAGE NOISE - VRMS
10k
10 10Hz 1 AMPLIFIER-GENERATED NOISE 0.1 10k 100k 1M 10M 100M SOURCE IMPEDANCE - 1G 10G
GND
(a)
Figure 15. Total Noise vs. Source Impedance
+VS
Output Characteristics
GND
The AD820's unique bipolar rail-to-rail output stage swings within 5 mV of the minus supply and 10 mV of the positive supply with no external resistive load. The AD820's approximate output saturation resistance is 40 sourcing and 20 sinking. This can be used to estimate output saturation voltage when driving heavier current loads. For instance, when sourcing 5 mA, the saturation voltage to the positive supply rail will be 200 mV, when sinking 5 mA, the saturation voltage to the minus rail will be 100 mV.
(b) +5V RP
VIN
AD820
VOUT
The amplifier's open-loop gain characteristic will change as a function of resistive load, as shown in TPCs 7 through 10. For load resistances over 20 k, the AD820's input error voltage is virtually unchanged until the output voltage is driven to 180 mV of either supply. If the AD820's output is driven hard against the output saturation voltage, it will recover within 2 s of the input returning to the amplifier's linear operating region.
Figure 14. (a) Response with RP = 0; VIN from 0 to +VS Figure 36. (b) VIN = 0 to +VS + 200 mV VOUT = 0 to +VS RP = 49.9 k
-12-
REV. C
AD820
Direct capacitive load will interact with the amplifier's effective output impedance to form an additional pole in the amplifier's feedback loop, which can cause excessive peaking on the pulse response or loss of stability. Worst case is when the amplifier is used as a unity gain follower. Figure 16 shows the AD820's pulse response as a unity gain follower driving 350 pF. This amount of overshoot indicates approximately 20 degrees of phase margin--the system is stable, but is nearing the edge. Configurations with less loop gain, and as a result less loop bandwidth, will be much less sensitive to capacitance load effects. Figure 17 is a plot of capacitive load that will result in a 20 degree phase margin versus noise gain for the AD820. Noise gain is the inverse of the feedback attenu- ation factor provided by the feedback network in use. Figure 18 shows a possible configuration for extending capacitance load drive capability for a unity gain follower. With these component values, the circuit will drive 5,000 pF with a 10% overshoot.
+VS 3 VIN 7 0.01 F
AD820
2 4 -VS
6 0.01 F
100 VOUT
20pF 20k
Figure 18. Extending Unity Gain Follower Capacitive Load Capability Beyond 350 pF
OFFSET VOLTAGE ADJUSTMENT
The AD820's offset voltage is low, so external offset voltage nulling is not usually required. Figure 19 shows the recommended technique for AD820's packaged in plastic DIPs. Adjusting offset voltage in this manner will change the offset voltage temperature drift by 4 V/C for every millivolt of induced offset. The null pins are not functional for AD820s in the SO-8 "R" package.
Figure 16. Small Signal Response of AD820 as Unity Gain Follower Driving 350 pF Capacitive Load
5
+VS 3 7
AD820
2 4 5 20k
6 1
NOISE GAIN - 1+
RF RI
4
-VS
Figure 19. Offset Null
3
APPLICATIONS Single Supply Half-Wave and Full-Wave Rectifiers
2
1 300
1k 3k 10k CAPACITIVE LOAD FOR 20 PHASE MARGIN - pF
30k
An AD820 configured as a unity gain follower and operated with a single supply can be used as a simple half-wave rectifier. The AD820's inputs maintain picoamp level input currents even when driven well below the minus supply. The rectifier puts that behavior to good use, maintaining an input impedance of over 1011 for input voltages from 1 volt from the positive supply to 20 volts below the negative supply. The full and half-wave rectifier shown in Figure 20 operates as follows: when VIN is above ground, R1 is bootstrapped through the unity gain follower A1 and the loop of amplifier A2. This forces the inputs of A2 to be equal, thus no current flows through R1 or R2, and the circuit output tracks the input. When VIN is below ground, the output of A1 is forced to ground. The noninverting input of amplifier A2 sees the ground level output of A1, therefore, A2 operates as a unity gain inverter. The output at node C is then a full-wave rectified version of the input. Node B is a buffered half-wave rectified version of the input. Input voltages up to 18 volts can be rectified, depending on the voltage supply used.
RF RI
Figure 17. Capacitive Load Tolerance vs. Noise Gain
REV. C
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AD820
R1 100k R2 100k +VS +VS 0.01 F 0.01 F 3 VIN 2 7 6 4 3 2 7 6 4
Low Power Three-Pole Sallen Key Low-Pass Filter
A
The AD820's high input impedance makes it a good selection for active filters. High value resistors can be used to construct low frequency filters with capacitors much less than 1 F. The AD820's picoamp level input currents contribute minimal dc errors.
C
FULL-WAVE RECTIFIED OUTPUT
A2
A1
AD820
AD820
B
HALF-WAVE RECTIFIED OUTPUT
Figure 22 shows an example, a 10 Hz three-pole Sallen Key Filter. The high value used for R1 minimizes interaction with signal source resistance. Pole placement in this version of the filter minimizes the Q associated with the two-pole section of the filter. This eliminates any peaking of the noise contribution of resistors R1, R2, and R3, thus minimizing the inherent output voltage noise of the filter.
C2 0.022 F
A
R1 243k VIN R2 243k R3 243k C3 0.022 F 2 3
+VS 7
0.01 F
B
C1 0.022 F
AD820
4
6 VOUT 0.01 F
C
0 -10
FILTER GAIN RESPONSE - dB
-VS
Figure 20. Single Supply Half- and Full-Wave Rectifier
4.5 V Low Dropout, Low Power Reference
-20 -30 -40 -50 -60 -70 -80 -90
The rail-to-rail performance of the AD820 can be used to provide low dropout performance for low power reference circuits powered with a single low voltage supply. Figure 21 shows a 4.5 V reference using the AD820 and the AD680, a low power 2.5 V bandgap reference. R2 and R3 set up the required gain of 1.8 to develop the 4.5 V output. R1 and C2 form a low-pass RC filter to reduce the noise contribution of the AD680.
2.5V OUTPUT 4.5V OUTPUT R2 80k (20k ) C3 10 F/25V R3 100k (25k ) REF COMMON
-100 0.1
1
10 FREQUENCY - Hz
100
1k
Figure 22. 10 Hz Sallen Key Low-Pass Filter
5V
U2 AD820
7 2 2.5V 10mV
6 4
3
U1 AD680
4
6
3
2
R1 100k
C1 0.1 F
C2 0.1 F FILM
Figure 21. Single Supply 4.5 V Low Dropout Reference
With a 1 mA load, this reference maintains the 4.5 V output with a supply voltage down to 4.7 V. The amplitude of the recovery transient for a 1 mA to 10 mA step change in load current is under 20 mV, and settles out in a few microseconds. Output voltage noise is less than 10 V rms in a 25 kHz noise bandwidth.
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REV. C
AD820
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
Mini-DIP Package (N-8)
0.39 (9.91) MAX
8 5
0.25 0.31 (6.35) (7.87)
1 4
PIN 1 0.10 (2.54) BSC 0.165 0.01 (4.19 0.25) 0.125 (3.18) MIN 0.018 (0.46 0.003 0.08) 0.033 (0.84) NOM 0.035 (0.89 0.01 0.25) 0.18 0.03 (4.57 0.75) SEATING PLANE
0.30 (7.62) REF
0.011 (0.28 15 0
0.003 0.08)
SOIC Package (R-8)
0.150 (3.81)
8
5 4
0.157 (3.99) 0.150 (3.81) PIN 1
1
0.244 (6.20) 0.228 (5.79) 0.020 (0.051) 45 CHAMF 0.190 (4.82) 0.170 (4.32) 8 0.102 (2.59) 0 0.094 (2.39) 0.098 (0.2482) 0.075 (0.1905)
0.197 (5.01) 0.189 (4.80) 0.010 (0.25) 0.004 (0.10)
0.090 (2.29)
0.050 0.019 (0.48) SEATING (1.27) 0.014 (0.36) PLANE BSC
10 0 0.030 (0.76) 0.018 (0.46)
REV. C
-15-
AD820 Revision History
Location Data Sheet changed from REV. B to REV. C. Page
Edits to FEATURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Edits to PRODUCT DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Delete SPECIFICATIONS for AD820A-3 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Edits to ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Edits to TYPICAL PERFORMANCE CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
C00873-0-1/02(C) PRINTED IN U.S.A.
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