Low Cost Precision
Difet
OPERATIONAL AMPLIFIER
Difet
, Burr-Brown Corp.
BIFET
, National Semiconductor Corp.
OPA121
FEATURES
q
LOW NOISE: 6nV/
Hz typ at 10kHz
q
LOW BIAS CURRENT: 5pA max
q
LOW OFFSET: 2mV max
q
LOW DRIFT: 3
V/
C typ
q
HIGH OPEN-LOOP GAIN: 110dB min
q
HIGH COMMON-MODE
REJECTION: 86dB min
Noise-Free
Cascode*
10k
Trim
1
5
3
2
+In
In
Case (TO-99) and Substrate
Output
OPA121 Simplified Circuit
+V
CC
V
CC
8
7
6
4
2k
10k
2k
2k
2k
*Patented
Trim
DESCRIPTION
The OPA121 is a precision monolithic dielectrically-
isolated FET (
Difet
) operational amplifier. Out-
standing performance characteristics are now
available for low-cost applications.
Noise, bias current, voltage offset, drift, open-loop
gain, common-mode rejection, and power supply
rejection are superior to BIFET
amplifiers.
Very low bias current is obtained by dielectric
isolation with on-chip guarding.
Laser-trimming of thin-film resistors gives very low
offset and drift. Extremely low noise is achieved with
new circuit design techniques (patented). A new
cascode design allows high precision input specifica-
tions and reduced susceptibility to flicker noise.
Standard 741 pin configuration allows upgrading of
existing designs to higher performance levels.
APPLICATIONS
q
OPTOELECTRONICS
q
DATA ACQUISITION
q
TEST EQUIPMENT
q
MEDICAL EQUIPMENT
q
RADIATION HARD EQUIPMENT
International Airport Industrial Park Mailing Address: PO Box 11400 Tucson, AZ 85734 Street Address: 6730 S. Tucson Blvd. Tucson, AZ 85706
Tel: (520) 746-1111 Twx: 910-952-1111 Cable: BBRCORP Telex: 066-6491 FAX: (520) 889-1510 Immediate Product Info: (800) 548-6132
1984 Burr-Brown Corporation
PDS-539F
Printed in U.S.A. September, 1993
OPA121
2
NOTES: (1) Sample tested. (2) Offset voltage, offset current, and bias current are specified with the units fully warmed up. (3) Overload recovery is defined as
the time required for the output to return from saturation to linear operation following the removal of a 50% input overdrive. (4) 100
C/W for KU grade.
SPECIFICATIONS
ELECTRICAL
At V
CC
=
15VDC and T
A
= +25
C unless otherwise noted. Pin 8 connected to ground.
OPA121KM
OPA121KP, KU
PARAMETER
CONDITIONS
MIN
TYP
MAX
MIN
TYP
MAX
UNITS
INPUT
NOISE
Voltage, f
O
= 10Hz
(1)
40
50
nV/
Hz
f
O
= 100Hz
(1)
15
18
nV/
Hz
f
O
= 1kHz
(1)
8
10
nV/
Hz
f
O
= 10kHz
(1)
6
7
nV/
Hz
f
B
= 10Hz to 10kHz
(1)
0.7
0.8
Vrms
f
B
= 0.1Hz to 10 Hz
(1)
1.6
2
Vp-p
Current, f
B
= 0.1Hz to 10Hz
(1)
15
21
fA, p-p
f
O
= 0.1Hz thru 20kHz
(1)
0.8
1.1
fA/
Hz
OFFSET VOLTAGE
(2)
Input Offset Voltage
V
CM
= 0VDC
0.5
2
0.5
3
mV
Average Drift
T
A
= T
MIN
to T
MAX
3
10
3
10
V/
C
Supply Rejection
86
104
86
104
dB
6
50
6
50
V/V
BIAS CURRENT
(2)
Input Bias Current
V
CM
= 0VDC
1
5
1
10
pA
Device Operating
OFFSET CURRENT
(2)
Input Offset Current
V
CM
= 0VDC
0.7
4
0.7
8
pA
Device Operating
IMPEDANCE
Differential
10
13
|| 1
10
13
|| 1
|| pF
Common-Mode
10
14
|| 3
10
14
|| 3
|| pF
VOLTAGE RANGE
Common-Mode Input Range
10
11
10
11
V
Common-Mode Rejection
V
IN
=
10VDC
86
104
82
100
dB
OPEN-LOOP GAIN, DC
Open-Loop Voltage Gain
R
L
2k
110
120
106
114
dB
FREQUENCY RESPONSE
Unity Gain, Small Signal
2
2
MHz
Full Power Response
20Vp-p, R
L
= 2k
32
32
kHz
Slew Rate
V
O
=
10V, R
L
= 2k
2
2
V/
s
Settling Time, 0.1%
Gain = 1, R
L
= 2k
6
6
s
0.01%
10V Step
10
10
s
Overload Recovery,
50% Overdrive
(3)
Gain = 1
5
5
s
RATED OUTPUT
Voltage Output
R
L
= 2k
11
12
11
12
V
Current Output
V
O
=
10VDC
5.5
10
5.5
10
mA
Output Resistance
DC, Open Loop
100
100
Load Capacitance Stability
Gain = +1
1000
1000
pF
Short Circuit Current
10
40
10
40
mA
POWER SUPPLY
Rated Voltage
15
15
VDC
Voltage Range,
Derated Performance
5
18
5
18
VDC
Current, Quiescent
I
O
= 0mADC
2.5
4
2.5
4.5
mA
TEMPERATURE RANGE
Specification
Ambient Temperature
0
+70
0
+70
C
Operating
Ambient Temperature
40
+85
25
+85
C
Storage
Ambient Temperature
65
+150
55
+125
C
Junction-Ambient
200
150
(4)
C/W
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user's own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
OPA121
3
ELECTRICAL (FULL TEMPERATURE RANGE SPECIFICATIONS)
At V
CC
=
15VDC and T
A
= T
MIN
to T
MAX
unless otherwise noted.
OPA121KM
OPA121KP, KU
PARAMETER
CONDITIONS
MIN
TYP
MAX
MIN
TYP
MAX
UNITS
TEMPERATURE RANGE
Specification Range
Ambient Temperature
0
+70
0
+70
C
INPUT
OFFSET VOLTAGE
(1)
Input Offset Voltage
V
CM
= 0VDC
1
3
1
5
mV
Average Drift
3
10
3
10
V/
C
Supply Rejection
82
94
82
94
dB
20
80
20
80
V/V
BIAS CURRENT
(1)
Input Bias Current
V
CM
= 0VDC
23
115
23
250
pA
Device Operating
OFFSET CURRENT
(1)
Input Offset Current
V
CM
= 0VDC
16
100
16
200
pA
Device Operating
VOLTAGE RANGE
Common-Mode Input Range
10
11
10
11
V
Common-Mode Rejection
V
IN
=
10VDC
82
98
80
96
dB
OPEN-LOOP GAIN, DC
Open-Loop Voltage Gain
R
L
2k
106
116
100
110
dB
RATED OUTPUT
Voltage Output
R
L
= 2k
10.5
11
10.5
11
V
Current Output
V
O
=
10VDC
5.25
10
5.25
10
mA
Short Circuit Current
V
O
= 0VDC
10
40
10
40
mA
POWER SUPPLY
Current, Quiescent
I
O
= 0mADC
2.5
4.5
2.5
5
mA
NOTE: (1) Offset voltage, offset current, and bias current are measured with the units fully warmed up.
PACKAGE INFORMATION
PACKAGE DRAWING
MODEL
PACKAGE
NUMBER
(1)
OPA121KM
TO-99
001
OPA121KP
8-Pin Plastic DIP
006
OPA121KU
8-Pin SOIC
182
NOTE: (1) For detailed drawing and dimension table, please see end of data
sheet, or Appendix D of Burr-Brown IC Data Book.
CONNECTION DIAGRAMS
Top View
M-Package TO-99 (Hermetic)
Top View
P-Package Plastic Mini-DIP
U-Package Plastic SOIC
8
1
2
7
6
5
3
4
Offset
Trim
Offset
Trim
Output
+V
CC
Substrate and Case
In
+In
OPA121
V
CC
OPA121
1
2
3
4
5
6
7
8
Offset Trim
+V
CC
In
+In
V
CC
Output
Offset Trim
Substrate
TEMPERATURE
MODEL
PACKAGE
RANGE
OPA121KM
TO-99
0
C to +70
C
OPA121KP
8-Pin Plastic DIP
0
C to +70
C
OPA121KU
8-Pin SOIC
0
C to +70
C
ORDERING INFORMATION
ABSOLUTE MAXIMUM RATINGS
Supply ...........................................................................................
18VDC
Internal Power Dissipation
(1)
......................................................... 500mW
Differential Input Voltage ...............................................................
36VDC
Input Voltage Range .....................................................................
18VDC
Storage Temperature Range
M package .................................................................... 65
C to +150
C
P, U packages ............................................................... 55
C to +125
C
Operating Temperature Range
M package ...................................................................... 40
C to +85
C
P, U packages ................................................................. 25
C to +85
C
Lead Temperature
M, P packages (soldering, 10s) ................................................... +300
C
U package (soldering, 3s) ........................................................... +260
C
Output Short-Circuit Duration
(2)
............................................... Continuous
Junction Temperature .................................................................... +175
C
NOTES: (1) Packages must be derated based on
JA
= 150
C/W
(P package);
JA
= 200
C/W (M package);
JA
= 100
C/W (U package).
(2) Short circuit may be to power supply common only. Rating applies to
+25
C ambient. Observe dissipation limit and T
J
.
OPA121
4
1
1k
10
100
10k
100k
1M
10M
Frequency (Hz)
0
20
40
60
80
100
120
140
Voltage Gain (dB)
OPEN-LOOP FREQUENCY RESPONSE
KM
180
135
90
45
Phase Shift (Degrees)
Phase
Margin
65
Gain
1
1k
10
100
10k
100k
1M
10M
Frequency (Hz)
0
20
40
60
80
100
120
140
Common-Mode Rejection (dB)
COMMON-MODE REJECTION
vs FREQUENCY
KM
1
1k
10
100
10k
100k
1M
10M
Frequency (Hz)
0
20
40
60
80
100
120
140
Power Supply Rejection (dB)
POWER SUPPLY REJECTION
vs FREQUENCY
15
10
5
0
+5
+10
+15
Common-Mode Voltage (V)
0.01
10
1
0.1
Bias Current (pA)
BIAS AND OFFSET CURRENT
vs INPUT COMMON-MODE VOLTAGE
0.01
10
1
0.1
Offset Current (pA)
Bias Current
Offset Current
KM
50
25
0
+25
+50
+75
+125
Ambient Temperature (C)
0.01
1k
100
0.1
Bias Current (pA)
BIAS AND OFFSET CURRENT
vs TEMPERATURE
+100
1
10
0.01
1k
100
0.1
Offset Current (pA)
1
10
KM
1
10
100
1k
10k
100k
1M
Frequency (Hz)
1
1k
100
10
Voltage Noise (nV/
Hz)
INPUT VOLTAGE NOISE SPECTRAL DENSITY
KP, KU
KM
TYPICAL PERFORMANCE CURVES
T
A
= +25
C, V
CC
=
15VDC unless otherwise noted.
OPA121
5
15
10
5
0
+5
+10
+15
Input Voltage (V)
2
+2
+1
1
Input Current (mA)
INPUT CURRENTS vs INPUT VOLTAGE
WITH V PINS GROUNDED
0
CC
I
IN
V
Maximum Safe Current
Maximum Safe Current
APPLICATIONS INFORMATION
OFFSET VOLTAGE ADJUSTMENT
The OPA121 offset voltage is laser-trimmed and will require
no further trim for most applications. As with most ampli-
fiers, externally trimming the remaining offset can change
drift performance by about 0.3
V/
C for each 100
V of
adjusted offset. Note that the trim (Figure 1) is similar to
operational amplifiers such as 741 and AD547. The OPA121
can replace most BIFET amplifiers by leaving the external
null circuit unconnected.
*
7
6
2
3
4
5
1
10mV Typical
Trim Range
*10k to 1M
Trim Potentiometer
(100k Recommended)
+V
CC
V
CC
OPA121
FIGURE 1. Offset Voltage Trim.
INPUT PROTECTION
Conventional monolithic FET operational amplifiers require
external current-limiting resistors to protect their inputs
against destructive currents that can flow when input FET
gate-to-substrate isolation diodes are forward-biased. Most
BIFET amplifiers can be destroyed by the loss of V
CC
.
Unlike BIFET amplifiers, the
Difet
OPA121 requires input
current limiting resistors only if its input voltage is greater
than 6V more negative than V
CC
. A 10k
series resistor
will limit input current to a safe level with up to
15V input
levels even if both supply voltages are lost.
Static damage can cause subtle changes in amplifier input
characteristics without necessarily destroying the device. In
precision operational amplifiers (both bipolar and FET types),
TYPICAL PERFORMANCE CURVES
(CONT)
T
A
= +25
C, V
CC
=
15VDC unless otherwise noted.
SMALL SIGNAL TRANSIENT RESPONSE
Time(s)
0
1
2
3
4
5
0
Output Voltage (mV)
+80
+80
+40
+40
LARGE SIGNAL TRANSIENT RESPONSE
Time(s)
0
25
50
+15
0
-15
Output Voltage (V)
OPA121
6
10
5
0
+5
+10
20
10
0
10
20
30
40
50
60
70
80
Input Bias Current (pA)
LF155
OP-15/16/17 "Perfect Bias Current Cancellation"
AD547
OPA121
LF156/157
AD547
LF155
OPA121
LF156/157
A
T = +25C; curves taken from
mfg. published typical data
Common-Mode Voltage (VDC)
this may cause a noticeable degradation of offset voltage and
drift.
Static protection is recommended when handling any
precision IC operational amplifier.
GUARDING AND SHIELDING
As in any situation where high impedances are involved,
careful shielding is required to reduce "hum" pickup in input
leads. If large feedback resistors are used, they should also
be shielded along with the external input circuitry.
Leakage currents across printed circuit boards can easily
exceed the bias current of the OPA121. To avoid leakage
problems, it is recommended that the signal input lead of the
OPA121 be wired to a TeflonTM standoff. If the OPA121 is
to be soldered directly into a printed circuit board, utmost
care must be used in planning the board layout. A "guard"
pattern should completely surround the high-impedance in-
put leads and should be connected to a low-impedance point
which is at the signal input potential.
The amplifier case should be connected to any input shield
or guard via pin 8. This insures that the amplifier itself is
fully surrounded by guard potential, minimizing both leak-
age and noise pickup (see Figure #2).
If guarding is not required, pin 8 (case) should be connected
to ground.
BIAS CURRENT CHANGE
VERSUS COMMON-MODE VOLTAGE
The input bias currents of most popular BIFET operational
amplifiers are affected by common-mode voltage (Figure 3).
Higher input FET gate-to-drain voltage causes leakage and
ionization (bias) currents to increase. Due to its cascode
input stage, the extremely-low bias current of the OPA121
is not compromised by common-mode voltage.
FIGURE 2. Connection of Input Guard.
7
6
8
TO-99 Bottom View
3
OPA121
2
8
6
In
Out
Non-Inverting
3
OPA121
2
8
6
In
Out
Buffer
3
OPA121
2
8
6
In
Out
Inverting
3
2
4 5
6
7
8
1
Mini-DIP Bottom View
3
2
4
1
5
BOARD LAYOUT
FOR INPUT GUARDING
Guard top and bottom of board.
Alternate: use Teflon standoff
for sensitive input pins.
FIGURE 3. Input Bias Current vs Common-Mode Voltage.
TeflonTM E.I. du Pont de Nemours & Co.
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