Single Supply, Rail-to-Rail
Low Power, FET-Input Op Amp
AD824
Data Sheet
PIN CONFIGURATION
Single supply operation: 3 V to 30 V
Very low input bias current: 2 pA
Wide input voltage range
Rail-to-rail output swing
Low supply current per amplifier: 500 µA
Wide bandwidth: 2 MHz
Slew rate: 2 V/µs
No phase reversal
OUT A 1
14
OUT D
13
–IN D
–IN A
2
+IN A
3
V+
4
+IN B
5
10
–IN B
6
9
–IN C
8
OUT C
OUT B 7
AD824
12 +IN D
TOP VIEW
(Not to Scale) 11 V–
+IN C
00875-001
FEATURES
Figure 1. 14-Lead SOIC (R Suffix)
APPLICATIONS
Photo diode preamplifier
Battery powered instrumentation
Power supply control and protection
Medical instrumentation
Remote sensors
Low voltage strain gage amplifiers
DAC output amplifier
GENERAL DESCRIPTION
The AD824 is a quad, FET input, single supply amplifier,
featuring rail-to-rail outputs. The combination of FET inputs
and rail-to-rail outputs makes the AD824 useful in a wide
variety of low voltage applications where low input current is
a primary consideration.
The AD824 is guaranteed to operate from a 3 V single supply
up to ±15 V dual supplies. AD824AR-3V parametric
performance at 3 V is fully guaranteed.
Fabricated on Analog Devices, Inc., complementary bipolar
process, the AD824 has a unique input stage that allows the
input voltage to safely extend beyond the negative supply and
to the positive supply without any phase inversion or latch-up.
The output voltage swings to within 15 mV of the supplies.
Capacitive loads to 350 pF can be handled without oscillation.
Rev. E
The FET input combined with laser trimming provides an input
that has extremely low bias currents with guaranteed offsets
below 1 mV. This enables high accuracy designs even with high
source impedances. Precision is combined with low noise,
making the AD824 ideal for use in battery powered medical
equipment.
Applications for the AD824 include portable medical
equipment, photo diode preamplifiers, and high impedance
transducer amplifiers.
The ability of the output to swing rail-to-rail enables designers
to build multistage filters in single supply systems and maintain
high signal-to-noise ratios.
The AD824 is specified over the extended industrial (−40°C to
+85°C) temperature range and is available in narrow 14-lead
SOIC package.
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Technical Support
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AD824
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
Input Characteristics .................................................................. 12
Applications ....................................................................................... 1
Output Characteristics............................................................... 12
Pin Configuration ............................................................................. 1
Applications Information .............................................................. 13
General Description ......................................................................... 1
Single Supply Voltage-to-Frequency Converter ..................... 13
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Single Supply Programmable Gain Instrumentation
Amplifier ..................................................................................... 13
Electrical Specifications ............................................................... 3
3 V, Single Supply Stereo Headphone Driver ......................... 14
Absolute Maximum Ratings ............................................................ 6
Low Dropout Bipolar Bridge Driver ........................................ 14
Thermal Resistance ...................................................................... 6
A 3.3 V/5 V Precision Sample-and-Hold Amplifier .............. 15
ESD Caution .................................................................................. 6
Outline Dimensions ....................................................................... 16
Typical Performance Characteristics ............................................. 7
Ordering Guide .......................................................................... 16
Theory of Operation ...................................................................... 12
REVISION HISTORY
4/15—Rev. D to Rev. E
Change to Figure 1 Caption ............................................................ 1
5/14—Rev. C to Rev. D
Updated Format .................................................................. Universal
Removed 16-Lead SOIC Package (Throughout) .......................... 1
Deleted Wafer Test Limits Section ................................................. 5
Deleted AD824 SPICE Macro-model Section ............................ 15
Changes to Ordering Guide .......................................................... 16
2/03—Rev. B to Rev. C
Deleted N Package .............................................................. Universal
Edits to General Description........................................................... 1
Edits to Absolute Maximum Ratings ............................................. 5
Edits to Ordering Guide .................................................................. 5
Edits to Figure 4 .............................................................................. 12
Edits to Figure 8 .............................................................................. 13
Updated Outline Dimensions ....................................................... 16
1/02—Rev. A to Rev. B
Edits to Electrical Specifications................................................. 2, 3
Edits to Absolute Maximum Ratings ............................................. 5
Edits to Ordering Guide .................................................................. 5
Deleted Dice Characteristics ........................................................... 5
Rev. E | Page 2 of 16
Data Sheet
AD824
SPECIFICATIONS
ELECTRICAL SPECIFICATIONS
At VS = 5.0 V, VCM = 0 V, VOUT = 0.2 V, TA = 25°C; unless otherwise noted.
Table 1.
Parameter
INPUT CHARACTERISTICS
Offset Voltage (AD824A)
Symbol
Test Conditions/Comments
Min
Typ
Max
Unit
0.1
1.0
1.5
12
4000
10
1013||3.3
mV
mV
pA
pA
pA
pA
V
dB
dB
dB
Ω||pF
20
50
250
180
40
100
1000
400
2
V/mV
V/mV
V/mV
V/mV
µV/°C
ISOURCE = 20 µA
TMIN to TMAX
ISOURCE = 2.5 mA
TMIN to TMAX
ISINK = 20 µA
TMIN to TMAX
ISINK = 2.5 mA
TMIN to TMAX
Sink/source
TMIN to TMAX
f = 1 MHz, AV = 1
4.975
4.97
4.80
4.75
4.988
4.985
4.85
4.82
15
20
120
140
±12
±10
100
V
V
V
V
mV
mV
mV
mV
mA
mA
Ω
VS = 2.7 V to 12 V
TMIN to TMAX
TMIN to TMAX
70
66
VOS
TMIN to TMAX
Input Bias Current
IB
2
300
2
300
TMIN to TMAX
Input Offset Current
IOS
TMIN to TMAX
Input Voltage Range
Common-Mode Rejection Ratio
Input Impedance
Large Signal Voltage Gain
Offset Voltage Drift
OUTPUT CHARACTERISTICS
Output Voltage High
CMRR
VCM = 0 V to 2 V
VCM = 0 V to 3 V
TMIN to TMAX
AVO
VO = 0.2 V to 4.0 V
RL = 2 kΩ
RL = 10 kΩ
RL = 100 kΩ
TMIN to TMAX, RL = 100 kΩ
−0.2
66
60
60
ΔVOS/ΔT
VOH
Output Voltage Low
VOL
Short Circuit Limit
ISC
Open-Loop Impedance
POWER SUPPLY
Power Supply Rejection Ratio
ZOUT
Supply Current/Amplifier
DYNAMIC PERFORMANCE
Slew Rate
Full-Power Bandwidth
Settling Time
Gain Bandwidth Product
Phase Margin
Channel Separation
NOISE PERFORMANCE
Voltage Noise
Voltage Noise Density
Current Noise Density
Total Harmonic Distortion
ISY
PSRR
SR
BWP
tS
GBP
φo
CS
RL = 10 kΩ, AV = 1
1% distortion, VO = 4 V p-p
VOUT = 0.2 V to 4.5 V, to 0.01%
en p-p
en
in
THD
+3.0
80
74
25
30
150
200
80
500
600
dB
dB
µA
No load
f = 1 kHz, RL = 2 kΩ
2
150
2.5
2
50
–123
V/µs
kHz
µs
MHz
Degrees
dB
0.1 Hz to 10 Hz
f = 1 kHz
f = 1 kHz
f = 10 kHz, RL = ∞, AV = +1
2
16
0.8
0.005
µV p-p
nV/√Hz
fA/√Hz
%
Rev. E | Page 3 of 16
AD824
Data Sheet
At VS = ±15.0 V, VOUT = 0 V, TA = 25°C; unless otherwise noted.
Table 2.
Parameter
INPUT CHARACTERISTICS
Offset Voltage (AD824A)
Symbol
Test Conditions/Comments
Min
VOS
Input Bias Current
IB
Input Offset Current
IB
IOS
TMIN to TMAX
VCM = 0 V
TMIN to TMAX
VCM = −10 V
TMIN to TMAX
Max
Unit
0.5
0.6
4
500
25
3
500
2.5
4.0
35
4000
1013||3.3
mV
mV
pA
pA
pA
pA
pA
V
dB
dB
Ω||pF
20
Input Voltage Range
Common-Mode Rejection Ratio
CMRR
VCM = −15 V to 13 V
TMIN to TMAX
Input Impedance
Large Signal Voltage Gain
AVO
VO = −10 V to +10 V;
RL = 2 kΩ
RL = 10 kΩ
RL = 100 kΩ
12
50
300
50
200
2000
V/mV
V/mV
V/mV
TMIN to TMAX, RL = 100 kΩ
200
1000
2
V/mV
µV/°C
ISOURCE = 20 µA
TMIN to TMAX
ISOURCE = 2.5 mA
TMIN to TMAX
ISINK = 20 µA
TMIN to TMAX
ISINK = 2.5 mA
TMIN to TMAX
Sink/source, TMIN to TMAX
f = 1 MHz, AV = 1
14.975
14.970
14.80
14.75
14.988
14.985
14.85
14.82
–14.985
–14.98
–14.88
–14.86
±20
100
V
V
V
V
V
V
V
V
mA
Ω
VS = 2.7 V to 15 V
TMIN to TMAX
VO = 0 V
TMIN to TMAX
70
68
Offset Voltage Drift
OUTPUT CHARACTERISTICS
Output Voltage High
Output Voltage Low
Short Circuit Limit
Open-Loop Impedance
POWER SUPPLY
Power Supply Rejection Ratio
Supply Current/Amplifier
DYNAMIC PERFORMANCE
Slew Rate
Full-Power Bandwidth
Settling Time
Gain Bandwidth Product
Phase Margin
Channel Separation
NOISE PERFORMANCE
Voltage Noise
Voltage Noise Density
Current Noise Density
Total Harmonic Distortion
−15
70
66
Typ
ΔVOS/ΔT
VOH
VOL
ISC
ZOUT
PSRR
ISY
SR
BWP
tS
GBP
φo
CS
en p-p
en
in
THD
RL = 10 kΩ, AV = 1
1% distortion, VO = 20 V p-p
VOUT = 0 V to 10 V, to 0.01%
±8
+13
80
–14.975
–14.97
–14.85
–14.8
80
560
625
675
dB
dB
µA
µA
f = 1 kHz, RL = 2 kΩ
2
33
6
2
50
–123
V/µs
kHz
µs
MHz
Degrees
dB
0.1 Hz to 10 Hz
f = 1 kHz
f = 1 kHz
f =10 kHz, VO = 3 V rms, RL = 10 kΩ
2
16
1.1
0.005
µV p-p
nV/√Hz
fA/√Hz
%
Rev. E | Page 4 of 16
Data Sheet
AD824
At VS = 3.0 V, VCM = 0 V, VOUT = 0.2 V, TA = 25°C; unless otherwise noted.
Table 3.
Parameter
INPUT CHARACTERISTICS
Offset Voltage (AD824A−3 V)
Symbol
Test Conditions/Comments
Min
VOS
Typ
Max
Unit
0.2
1.0
1.5
12
4000
10
1013||3.3
mV
mV
pA
pA
pA
pA
V
dB
dB
Ω||pF
20
65
500
250
V/mV
V/mV
V/mV
V/mV
2
µV/°C
2.988
2.985
2.85
2.82
15
20
120
140
±8
±6
100
V
V
V
V
mV
mV
mV
mV
mA
mA
Ω
TMIN to TMAX
Input Bias Current
IB
2
250
2
250
TMIN to TMAX
Input Offset Current
IOS
TMIN to TMAX
Input Voltage Range
Common-Mode Rejection Ratio
CMRR
VCM = 0 V to 1 V
TMIN to TMAX
Input Impedance
Large Signal Voltage Gain
AVO
VO = 0.2 V to 2.0 V;
RL = 2 kΩ
RL = 10 kΩ
RL = 100 kΩ
TMIN to TMAX, RL = 100 kΩ
Offset Voltage Drift
OUTPUT CHARACTERISTICS
Output Voltage High
VOH
VOL
Short Circuit Limit
ISC
ISC
ZOUT
Supply Current/Amplifier
DYNAMIC PERFORMANCE
Slew Rate
Full-Power Bandwidth
Settling Time
Gain Bandwidth Product
Phase Margin
Channel Separation
NOISE PERFORMANCE
Voltage Noise
Voltage Noise Density
Current Noise Density
Total Harmonic Distortion
10
30
180
90
ΔVOS/ΔT
Output Voltage Low
Open-Loop Impedance
POWER SUPPLY
Power Supply Rejection Ratio
0
58
56
PSRR
ISY
ISOURCE = 20 µA
TMIN to TMAX
ISOURCE = 2.5 mA
TMIN to TMAX
ISINK = 20 µA
TMIN to TMAX
ISINK = 2.5 mA
TMIN to TMAX
Sink/source
Sink/source, TMIN to TMAX
f = 1 MHz, AV = 1
2.975
2.97
2.8
2.75
VS = 2.7 V to 12 V,
TMIN to TMAX
VO = 0.2 V, TMIN to TMAX
70
66
SR
BWP
tS
GBP
φo
CS
RL =10 kΩ, AV = 1
1% distortion, VO = 2 V p-p
VOUT = 0.2 V to 2.5 V, to 0.01%
en p-p
en
in
THD
0.1 Hz to 10 Hz
f = 1 kHz
f = 1 kHz, RL = 2 kΩ
f = 10 kHz, RL = ∞, AV = +1
Rev. E | Page 5 of 16
1
74
500
25
30
150
200
600
dB
dB
µA
2
300
2
2
50
–123
V/µs
kHz
µs
MHz
Degrees
dB
2
16
0.8
0.01
µV p-p
nV/√Hz
fA/√Hz
%
AD824
Data Sheet
ABSOLUTE MAXIMUM RATINGS
THERMAL RESISTANCE
Table 4.
Parameter1
Supply Voltage
Input Voltage
Differential Input Voltage
Output Short Circuit Duration to GND
Storage Temperature Range
Operating Temperature Range
Junction Temperature Range
Lead Temperature Range (Soldering 60 sec)
1
Table 5. Thermal Resistance
Rating
±18 V
−VS − 0.2 V to +VS
±30 V
Indefinite
−65°C to +150°C
−40°C to +85°C
−65°C to +150°C
300°C
Package Type
14-Lead SOIC (R)
1
θJA1
120
ESD CAUTION
Stresses at or above those listed under Absolute Maximum
Ratings may cause permanent damage to the product. This is a
stress rating only; functional operation of the product at these
or any other conditions above those indicated in the operational
section of this specification is not implied. Operation beyond
the maximum operating conditions for extended periods may
affect product reliability.
VCC
I5
I6
Q18
R2
Q29
R9
Q21 Q27
Q4
J1
Q6
C3
Q5
J2
Q20
Q19
+IN
R7
Q7
R13
R15
–IN
Q23
Q22
C2
C4
VOUT
Q24 Q25
Q8
C1
Q2
Q3
Q31
I1
R14
I2
R17
I3
Q28
Q26
I4
VEE
Figure 2. Simplified Schematic of 1/4 AD824
Rev. E | Page 6 of 16
00875-002
R12
Unit
°C/W
θJA is specified for the worst case conditions, that is, θJA is specified for device
soldered in circuit board for SOIC package.
Absolute maximum ratings apply to packaged parts unless otherwise noted.
R1
θJC
36
Data Sheet
AD824
TYPICAL PERFORMANCE CHARACTERISTICS
VS = ±15V
NO LOAD
80
60
GAIN (dB)
60
45
90
20
135
0
180
1k
10k
100k
1M
45
90
20
135
0
180
100
10M
1k
10k
100
90
1M
10M
100
90
10
0%
00875-003
10
0%
50mV
1µs
50mV
Figure 3. Open-Loop Gain/Phase and Small Signal Response, VS = ±15 V,
No Load
1µs
Figure 5. Open-Loop Gain/Phase and Small Signal Response, VS = 5 V,
No Load
VS = ±15V
CL = 100pF
80
VS = 5V
CL = 220pF
60
60
GAIN (dB)
40
45
90
20
135
0
180
1k
10k
100k
1M
90
20
135
180
0
–20
10M
1k
10k
FREQUENCY (Hz)
100k
1M
10M
FREQUENCY (Hz)
100
90
100
90
10
0%
50mV
00875-004
10
0%
1µs
Figure 4. Open-Loop Gain/Phase and Small Signal Response, VS = ±15 V,
CL = 100 pF
50mV
1µs
Figure 6. Open-Loop Gain/Phase and Small Signal Response, VS = 5 V,
CL = 220 pF
Rev. E | Page 7 of 16
00875-006
100
45
PHASE (Degrees)
40
PHASE (Degrees)
GAIN (dB)
100k
FREQUENCY (Hz)
FREQUENCY (Hz)
00875-005
100
40
PHASE (Degrees)
40
PHASE (Degrees)
GAIN (dB)
VS = 5V
NO LOAD
80
AD824
Data Sheet
VS = 3V
NO LOAD
60
45
90
20
135
t
PHASE (Degrees)
GAIN (dB)
40
9.950µs
100
90
180
0
10
0%
–20
5V
1k
10k
100k
1M
2µs
10M
FREQUENCY (Hz)
t
100
90
10.810µs
10
0%
00875-007
10
0%
1µs
50mV
5V
Figure 7. Open-Loop Gain/Phase and Small Signal Response, VS = 3 V,
No Load
2µs
Figure 9. Slew Rate, RL = 10 kΩ
100
90
VS = 3V
CL = 220pF
60
00875-009
100
90
VOUT
135
10
0%
100µs
5V
Figure 10. Phase Reversal with Inputs Exceeding Supply by 1 V
180
0
00875-010
90
20
PHASE (Degrees)
45
0.8
–20
0.7
1k
10k
100k
1M
10M
FREQUENCY (Hz)
OUTPUT TO RAIL (V)
0.6
100
90
0.5
0.4
SOURCE
0.3
0.2
SINK
0.1
0%
50mV
1µs
0
1µ
Figure 8. Open-Loop Gain/Phase and Small Signal Response, VS = 3 V,
CL = 220 pF
5µ
10µ
50µ
100µ
500µ
LOAD CURRENT (A)
1m
5m
10m
00875-011
10
00875-008
GAIN (dB)
40
Figure 11. Output Voltage to Supply Rail vs. Sink and Source Load Currents
Rev. E | Page 8 of 16
Data Sheet
AD824
14
COUNT = 60
NUMBER OF UNITS
12
60
40
8
6
10
15
FREQUENCY (kHz)
0
–2.5
20
–2.0
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
2.5
OFFSET VOLTAGE DRIFT (µV/°C)
00875-015
2
20
5
Figure 15. TC VOS Distribution, −55°C to +125°C, VS = 5 V, 0 V
Figure 12. Voltage Noise Density
0.1
10
4
00875-012
NOISE DENSITY (nV/√Hz)
3V ≤ VS ≤ ±15V
150
RL = ∞
AV = +1
VS = 5V, 0V
INPUT OFFSET CURRENT (pA)
125
VS = +3V
THD + N (%)
0.01
VS = +5V
VS = ±15V
0.001
100
75
50
25
20
100
1k
10k
20k
FREQUENCY (Hz)
–25
–60
00875-013
0.0001
0
20
40
60
80
100
120
140
Figure 16. Input Offset Current vs. Temperature
280
100k
COUNT = 860
240
VS = 5V, 0V
INPUT BIAS CURRENT (pA)
10k
200
160
120
80
1k
100
10
–0.4
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
0.4
OFFSET VOLTAGE (mV)
0.5
Figure 14. Input Offset Distribution, VS = 5 V, 0 V
0.1
20
40
60
80
100
120
TEMPERATURE (°C)
Figure 17. Input Bias Current vs. Temperature
Rev. E | Page 9 of 16
140
00875-017
1
40
00875-014
NUMBER OF UNITS
–20
TEMPERATURE (°C)
Figure 13. Total Harmonic Distortion
0
–0.5
–40
00875-016
0
AD824
Data Sheet
120
1k
INPUT VOLTAGE NOISE (nV/√Hz)
80
60
40
20
1k
10k
100k
1M
10M
FREQUENCY (Hz)
1
1
1k
10k
100k
–80
–100
1k
10k
100k
FREQUENCY (Hz)
80
60
40
20
0
10
00875-019
–120
100
100
30
80
80
25
40
40
3V, 0V
20
20
0
0
10k
100k
1M
–20
10M
FREQUENCY (Hz)
OUTPUT VOLTAGE (V)
60
±15V
100k
1M
10M
20
15
10
5
00875-020
60
PHASE MARGIN (Degrees)
100
1k
10k
Figure 22. Power Supply Rejection vs. Frequency
100
100
1k
FREQUENCY (Hz)
Figure 19. THD vs. Frequency, 3 V rms
–20
100
00875-022
POWER SUPPLY REJECTION (dB)
120
–60
OPEN-LOOP GAIN (dB)
100
Figure 21. Input Voltage Noise Spectral Density vs. Frequency
–40
10
10
FREQUENCY (Hz)
Figure 18. Common-Mode Rejection vs. Frequency
THD (dB)
10
00875-021
100
00875-018
0
10
100
Figure 20. Open-Loop Gain and Phase vs. Frequency
0
1k
3k
10k
30k
100k
300k
INPUT FREQUENCY (Hz)
Figure 23. Large Signal Frequency Response
Rev. E | Page 10 of 16
1M
00875-023
COMMON-MODE REJECTION (dB)
100
Data Sheet
AD824
–80
–90
CROSSTALK (dB)
–100
5V
–110
5µs
100
90
1 TO 4
–120
1 TO 3
1 TO 2
–130
10
100
1k
10k
100k
FREQUENCY (Hz)
00875-027
10
0%
00875-024
–140
Figure 24. Crosstalk vs. Frequency
Figure 27. Large Signal Response
2750
10k
VS = ±15V
2500
SUPPLY CURRENT (µA)
100
10
1
0.1
2250
2000
VS = +3V, 0V
1750
1500
1250
100
1k
10k
100k
10M
1M
FREQUENCY (Hz)
1000
–60
00875-025
0.01
10
–20
0
20
40
60
80
100
120
140
TEMPERATURE (°C)
Figure 25. Output Impedance vs. Frequency, Gain = +1
Figure 28. Supply Current vs. Temperature
OUTPUT SATURATION VOLTAGE (mV)
1k
20mV
–40
00875-028
OUTPUT IMPEDANCE (Ω)
1k
500ns
100
90
VS = ±15V
VS = 3V, 0V
100
VOL – VS
10
VS – VOH
1
0.01
00875-026
0.1
1
LOAD CURRENT (mA)
Figure 26. Small Signal Response, Unity Gain Follower, 10 kΩ||100 pF Load
Rev. E | Page 11 of 16
Figure 29. Output Saturation Voltage
10
00875-029
10
0%
AD824
Data Sheet
THEORY OF OPERATION
INPUT CHARACTERISTICS
In the AD824, n-channel JFETs are used to provide a low offset,
low noise, high impedance input stage. Minimum input
common-mode voltage extends from 0.2 V below −VS to 1 V
less than +VS. Driving the input voltage closer to the positive
rail causes a loss of amplifier bandwidth.
The AD824 does not exhibit phase reversal for input voltages up
to and including +VS. Figure 30a shows the response of an
AD824 voltage follower to a 0 V to 5 V (+VS) square wave input.
The input and output are superimposed. The output tracks the
input up to +VS without phase reversal. The reduced bandwidth
above a 4 V input causes the rounding of the output waveform.
For input voltages greater than +VS, a resistor in series with the
noninverting input prevents phase reversal at the expense of
greater input voltage noise. This is illustrated in Figure 30b.
1V
2µs
100
90
Input voltages less than −VS are a completely different story. The
amplifier can safely withstand input voltages 20 V below the
−VS as long as the total voltage from the +VS 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.
OUTPUT CHARACTERISTICS
The unique bipolar rail-to-rail output stage of the AD824
swings within 15 mV of the positive and negative supply
voltages. The approximate output saturation resistance of the
AD824 is 100 Ω for both sourcing and sinking. This can be used
to estimate output saturation voltage when driving heavier
current loads. For instance, the saturation voltage is 0.5 V from
either supply with a 5 mA current load.
For load resistances over 20 kΩ, the input error voltage of the
AD824 is virtually unchanged until the output voltage is driven
to 180 mV of either supply.
10
0%
If the output of the AD824 is overdriven to saturate either of the
output devices, the amplifier will recover within 2 μs of its input
returning to the amplifier’s linear operating region.
1V
(a)
1V
+VS
100
90
GND
0%
Direct capacitive loads 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 6 and Figure 8 show the
pulse response of the AD824 as a unity gain follower driving
220 pF. Configurations with less loop gain, and as a result less
loop bandwidth, will be much less sensitive to capacitance load
effects. Noise gain is the inverse of the feedback attenuation
factor provided by the feedback network in use.
10µs
1V
10
1V
(b)
Figure 31 shows a method for extending capacitance load drive
capability for a unity gain follower. With these component
values, the circuit drives 5,000 pF with a 10% overshoot.
5V
RP
VOUT
00875-030
VIN
+VS
0.01µF
8
Figure 30. (a) Response with RP = 0; VIN from 0 V to +VS;
(b) VIN = −200 V to + VS + 200 mV; VOUT = 0 V to + VS; RP = 49.9 kΩ
100Ω
1/4
VIN
Because the input stage uses n-channel JFETs, input current
during normal operation is positive; the current flows out from
the input terminals. If the input voltage is driven more positive
than +VS − 0.4 V, the input current reverses direction as internal
device junctions become forward biased. This is illustrated in
Figure 10.
Use a current-limiting resistor in series with the input of the
AD824 if there is a possibility of the input voltage exceeding the
AD824
VOUT
0.01µF
4
CL
–VS
20pF
20kΩ
00875-031
GND
positive supply by more than 300 mV or if an input voltage will
be applied to the AD824 when ±VS = 0 V. The amplifier will be
damaged if left in that condition for more than 10 seconds. A
1 kΩ resistor allows the amplifier to withstand up to 10 V of
continuous overvoltage and increases the input voltage noise by
a negligible amount.
Figure 31. Extending Unity Gain Follower Capacitive Load Capability
Beyond 350 pF
Rev. E | Page 12 of 16
Data Sheet
AD824
APPLICATIONS INFORMATION
SINGLE SUPPLY VOLTAGE-TO-FREQUENCY
CONVERTER
Table 6. AD824 In Amp Performance
The circuit shown in Figure 32 uses the AD824 to drive a low
power timer, which produces a stable pulse of width, t1. The
positive going output pulse is integrated by R1 and C1 and used
as one input to the AD824, which is connected as a differential
integrator. The other input (nonloading) is the unknown
voltage, VIN. The AD824 output drives the timer trigger input,
closing the overall feedback loop.
10V
2
6
VREF = 5V
5
3
CMOS
74HCO4
RSCALE **
10kΩ
4
U3B
4
C3
0.1µF
3
U3A
2
1
R1
499kΩ
1%
0V TO 2.5V
FULL SCALE
U1
C1
0.01µF
2%
R3*
116kΩ
AD824
C2
0.01µF
2%
C6
390pF
5%
(NPO)
180 kHz
18 kHz
180 kHz
18 kHz
2 μs
5 μs
270 nV/√Hz
2.2 μV/√Hz
270 nV/√Hz
2.2 μV/√Hz
5µs
OUT1
4
100
90
8
V+
R
6 THR
2 TR
1/4
VS = ±5 V
80 dB
−5.2 V to +4 V
OUT2
U2
CMOS 555
R2
499kΩ
1%
VS = 3 V, 0 V
74 dB
−0.2 V to +2 V
OUT
CV
7 DIS
3
5
10
0%
GND
1
00875-033
C5
0.1µF
U4
REF02
Parameter
CMRR
Common-Mode Voltage Range
3 dB BW
G = 10
G = 100
tSETTLING
2 V Step (VS = 0 V, 3 V)
5 V (VS = ± 5 V)
Noise @ f = 1 kHz
G = 10
G = 100
1V
C4
0.1µF
Figure 33. Pulse Response of In Amp to a 500 mV p-p Input Signal;
VS = 5 V, 0 V; Gain = 10
NOTES
fOUT = VIN/(VREF × t1), t1 = 1.1 × R3 × C6 = 25kHz fS AS SHOWN.
00875-032
* = 1% METAL FILM,
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