OPA128
®
Difet ® Electrometer-Grade
OPERATIONAL AMPLIFIER
FEATURES
APPLICATIONS
● ULTRA-LOW BIAS CURRENT: 75fA max
● ELECTROMETER
●
●
●
●
●
●
●
●
●
LOW OFFSET: 500µV max
LOW DRIFT: 5µV/°C max
HIGH OPEN-LOOP GAIN: 110dB min
HIGH COMMON-MODE REJECTION:
90dB min
MASS SPECTROMETER
CHROMATOGRAPH
ION GAUGE
PHOTODETECTOR
RADIATION-HARD EQUIPMENT
● IMPROVED REPLACEMENT FOR AD515
AND AD549
DESCRIPTION
The OPA128 is an ultra-low bias current monolithic
operational amplifier. Using advanced geometry
dielectrically-isolated FET (Difet®) inputs, this monolithic amplifier achieves a performance level exceeding even the best hybrid electrometer amplifiers.
Laser-trimmed thin-film resistors give outstanding voltage offset and drift performance.
A noise-free cascode and low-noise processing give
the OPA128 excellent low-level signal handling capabilities. Flicker noise is very low.
The OPA128 is an improved pin-for-pin replacement
for the AD515.
Case (Guard)
8
7
+VCC
–In
2
3
Noise-Free
Cascode
+In
6
Output
Trim
1k Ω
28kΩ
28kΩ
2kΩ
2kΩ
1
Difet® Burr-Brown Corp.
5
Trim
1k Ω
4
–VCC
OPA128 Simplified Circuit
International Airport Industrial Park • Mailing Address: PO Box 11400
Tel: (520) 746-1111 • Twx: 910-952-1111 • Cable: BBRCORP •
©
1986 Burr-Brown Corporation
• Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706
Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132
PDS-653E
Printed in U.S.A. May, 1995
SPECIFICATIONS
ELECTRICAL
At VCC = ±15VDC and TA = +25°C unless otherwise noted. Pin 8 connected to ground.
OPA128JM
PARAMETER
CONDITIONS
MIN
OPA128KM
TYP
MAX
VCM = 0VDC,
RL ≥ 10kΩ
±150
±300
VCM = 0VDC,
RL ≥ 10kΩ
65
MIN
OPA128LM
TYP
MAX
±75
±150
MIN
OPA128SM
TYP
MAX
±40
±75
MIN
TYP
MAX
UNITS
±75
±150
fA
INPUT
BIAS CURRENT(1)
Input Bias Current
OFFSET CURRENT(1)
Input Offset Current
OFFSET VOLTAGE(1)
Input Offset Voltage
Average Drift
Supply Rejection
±260
VCM = 0VDC
TA = TMIN to TMAX
80
NOISE
Voltage: fO = 10Hz
fO = 100Hz
fO = 1kHz
fO = 10kHz
fB = 10Hz to 10kHz
fB = 0.1Hz to 10Hz
Current: fB = 0.1Hz to 10Hz
fO = 0.1Hz to 20kHz
IMPEDANCE
Differential
Common-Mode
VOLTAGE RANGE(4)
Common-Mode Input Range
Common-Mode Rejection
120
±1
30
±1000
±20
±140
90
±100
120
±1
30
±500
±10
±140
90
±32
120
±1
30
±500
±5
±140
90
±32
120
±1
fA
±500
±10
±32
µV
µV/°C
dB
µV/V
92
78
27
15
2.4
4
4.2
0.22
92
78
27
15
2.4
4
3
0.16
92
78
27
15
2.4
4
2.3
0.12
92
78
27
15
2.4
4
3
0.16
nV/√Hz
nV/√Hz
nV/√Hz
nV/√Hz
µVrms
µVp-p
fA, p-p
fA/√Hz
1013 || 1
1015 || 2
1013 || 1
1015 || 2
1013 || 1
1015 || 2
1013 || 1
1015 || 2
Ω || pF
Ω || pF
VIN = ±10VDC
±10
80
±12
118
±10
90
±12
118
±10
90
±12
118
±10
90
±12
118
V
dB
RL ≥ 2kΩ
94
128
110
128
110
128
110
128
dB
(2)
0.5
1
47
3
5
10
0.5
1
47
3
5
10
0.5
1
47
3
5
10
0.5
1
47
3
5
10
MHz
kHz
V/µs
µs
µs
5
µs
±13
±10
100
1000
34
V
mA
Ω
pF
mA
OPEN-LOOP GAIN, DC
Open-Loop Voltage Gain
FREQUENCY RESPONSE
Unity Gain, Small Signal
Full Power Response
Slew Rate
Settling Time, 0.1%
0.01%
Overload Recovery,
50% Overdrive(3)
20Vp-p, RL = 2kΩ
VO = ±10V, RL = 2kΩ
Gain = –1, RL = 2kΩ
10V Step
0.5
Gain = –1
1
5
1
5
1
5
RATED OUTPUT
Voltage Output
Current Output
Output Resistance
Load Capacitance Stability
Short Circuit Current
RL = 2kΩ
VO = ±10VDC
DC, Open Loop
Gain = +1
±10
±5
10
±13
±10
100
1000
34
±10
±5
55
10
±13
±10
100
1000
34
±10
±5
55
10
±13
±10
100
1000
34
±10
±5
55
10
55
POWER SUPPLY
Rated Voltage
Voltage Range,
Derated Performance
Current, Quiescent
±15
±5
IO = 0mADC
0.9
±15
±18
1.5
±5
+70
+125
+150
0
–55
–65
0.9
±15
±18
1.5
±5
+70
+125
+150
0
–55
–65
0.9
±15
±18
1.5
±5
+70
+125
+150
–55
–55
–65
0.9
VDC
±18
1.5
VDC
mA
+125
+125
+150
°C
°C
°C
°C/W
TEMPERATURE RANGE
Specification
Operating
Storage
θ Junction-Ambient
Ambient Temp.
Ambient Temp.
Ambient Temp.
0
–55
–65
200
200
200
200
NOTES: (1) Offset voltage, offset current, and bias current are measured with the units fully warmed up. Bias current doubles approximately every 11°C. (2) Sample
tested. (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) If it is possible for the input voltage to exceed the supply voltage, a series protection resistor should be added to limit input current to 0.5mA. The input devices
can withstand overload currents of 0.3mA indefinitely without damage.
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.
®
OPA128
2
ELECTRICAL (FULL TEMPERATURE RANGE SPECIFICATIONS)
At VCC = ±15VDC and TA = TMIN and TMAX unless otherwise noted.
OPA128JM
PARAMETER
CONDITIONS
MIN
Ambient Temp.
0
TYP
OPA128KM
MAX
MIN
+70
0
TYP
OPA128LM
MAX
MIN
+70
0
TYP
OPA128SM
MAX
MIN
+70
–55
TYP
MAX
UNITS
+125
°C
±170
pA
TEMPERATURE RANGE
Specification Range
INPUT
BIAS CURRENT(1)
Input Bias Current
VCM = 0VDC
±2.5
VCM = 0VDC
1.1
±8
±1.3
±4
±0.7
±2
±43
CURRENT(1)
OFFSET
Input Offset Current
OFFSET VOLTAGE(1)
Input Offset Voltage
Average Drift
Supply Rejection
±2.2mV
±20
VCM = 0VDC
74
VOLTAGE RANGE(2)
Common-Mode Input Range
Commmon-Mode Rejection
0.6
114
±2
0.6
±1mV
±10
80
±200
114
±2
18
±750
±5
80
±100
114
±2
pA
±1.5mV
±10
80
±100
106
±5
±100
µV
µV/°C
dB
µV/V
VIN = ±10VDC
±10
74
±11
112
±10
80
±11
112
±10
80
±11
112
±10
74
±11
104
V
dB
RL ≥ 2kΩ
90
125
104
125
104
125
90
122
dB
RL = 2k
VO = ±10VDC
VO = 0VDC
±10
±5
10
22
±10
±5
10
22
±10
±5
10
22
±10
±5
10
18
V
mA
mA
OPEN-LOOP GAIN, DC
Open-Loop Voltage Gain
RATED OUTPUT
Voltage Output
Current Output
Short Circuit Current
POWER SUPPLY
Current, Quiescent
I = 0mADC
0.9
1.8
0.9
1.8
0.9
1.8
0.9
2
mA
NOTES: (1) Offset voltage, offset current, and bias current are measured with the units fully warmed up. (2) If it is possible for the input voltage to exceed the supply
voltage, a series protection resistor should be added to limit input current to 0.5mA. The input devices can withstand overload currents of 0.3mA indefinitely without
damage.
CONNECTION DIAGRAM
ORDERING INFORMATION
Top View
MODEL
Substrate and Case
Offset
Trim
–In
OPA128JM
OPA128KM
OPA128LM
OPA128SM
8
7 +VCC
1
2
OPA128
PACKAGE
TEMPERATURE
RANGE
BIAS CURRENT,
max (fA)
TO-99
TO-99
TO-99
TO-99
0°C to +70°C
0°C to +70°C
0°C to +70°C
–55°C to +125°C
±300
±150
±75
±150
6 Output
PACKAGE INFORMATION
+In
3
4
5 Offset
Trim
MODEL
–VCC
OPA128JM
OPA128KM
OPA128LM
OPA128SM
ABSOLUTE MAXIMUM RATINGS
PACKAGE
PACKAGE DRAWING
NUMBER(1)
TO-99
TO-99
TO-99
TO-99
001
001
001
001
NOTE: (1) For detailed drawing and dimension table, please see end of data
sheet, or Appendix D of Burr-Brown IC Data Book.
Supply ........................................................................................... ±18VDC
Internal Power Dissipation(1) .......................................................... 500mW
Differential Input Voltage ............................................................... ±36VDC
Input Voltage Range ..................................................................... ±18VDC
Storage Temperature Range .......................................... –65°C to +150°C
Operating Temperature Range ....................................... –55°C to +125°C
Lead Temperature (soldering, 10s) ................................................ +300°C
Output Short Circuit Duration(2) ................................................ Continuous
Junction Temperature .................................................................... +175°C
NOTES: (1) Packages must be derated based on θCA = 150°C/W or θJA =
200°C/W. (2) Short circuit may be to power supply common only. Rating
applies to +25°C ambient. Observe dissipation limit and TJ.
®
3
OPA128
DICE INFORMATION
PAD
FUNCTION
1
2
3
4
5
6
7
8
NC
Offset Trim
–In
+In
–VCC
Offset Trim
Output
+VCC
Substrate
No Connection
Substrate Bias: Isolated, normally connected to common.
MECHANICAL INFORMATION
Die Size
Die Thickness
Min. Pad Size
MILS (0.001")
MILLIMETERS
96 x 71 ±5
20 ±3
4x4
2.44 x 1.80 ±0.13
0.51 ±0.08
0.10 x 0.10
Backing
OPA128 DIE TOPOGRAPHY
None
TYPICAL PERFORMANCE CURVES
TA = +25°C, ±15VDC, unless otherwise noted.
OPEN-LOOP FREQUENCY RESPONSE
POWER SUPPLY REJECTION vs FREQUENCY
140
Voltage Gain (dB)
Gain
100
Ø
80
60
Phase
Margin
∼
≈ 90°
40
–90
–135
Phase Shift (Degrees)
–45
120
20
Power Supply Rejection (dB)
140
–180
0
1
10
100
1k
10k
100k
1M
120
100
80
+PSRR
60
–PSRR
40
20
0
10M
1
10
100
Frequency (Hz)
10k
100k
1M
10M
1M
10M
COMMON-MODE REJECTION
vs FREQUENCY
COMMON-MODE REJECTION
vs INPUT COMMON-MODE VOLTAGE
140
Common-Mode Rejection (dB)
120
Common-Mode Rejection (dB)
1k
Frequency (Hz)
110
100
90
80
120
100
80
60
40
20
0
70
–15
–10
–5
0
5
10
1
15
®
OPA128
10
100
1k
10k
Frequency (Hz)
Common-Mode Voltage (V)
4
100k
TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, +15VDC unless otherwise noted.
BIAS AND OFFSET CURRENT
vs TEMPERATURE
BIAS AND OFFSET CURRENT
vs INPUT COMMON-MODE VOLTAGE
10
Normalized Bias and Offset Current
SM
10pA
IB
1pA
IOS
100
10
1
1
0.1
0.01
–50
–25
0
25
50
75
100
125
–15
–10
Ambient Temperature (°C)
3
3
2
2
1
3
Gain-Bandwidth (MHz)
4
Slew Rate (V/µs)
Gain-Bandwidth (MHz)
4
1
0
–25
0
25
50
75
100
6
2
4
+ Slew
– Slew
1
2
0
0
–50
0
0
125
5
10
20
15
Supply Voltage (±VCC)
Ambient Temperature (°C)
OPEN-LOOP GAIN, PSR, AND CMR vs TEMPERATURE
SUPPLY CURRENT vs TEMPERATURE
140
PSR, CMR, Voltage Gain (dB)
2
Supply Current (mA)
15
GAIN-BANDWIDTH AND SLEW RATE
vs SUPPLY VOLTAGE
GAIN-BANDWIDTH AND SLEW RATE
vs TEMPERATURE
–75
–5
0
5
10
Common-Mode Voltage (V)
Slew Rate (V/µs)
Bias and Offset Current (fA)
100pA
1.5
1
0.5
130
AOL
120
CMR
110
PSR
100
0
–75
–50
–25
0
25
50
75
100
–75
125
–50
–25
0
25
50
75
100
125
Ambient Temperature (°C)
Ambient Temperature (°C)
®
5
OPA128
TYPICAL PERFORMANCE CURVES (CONT)
At TA = +25°C, +15VDC unless otherwise noted.
SMALL SIGNAL TRANSIENT RESPONSE
LARGE SIGNAL TRANSIENT RESPONSE
Output Voltage (mV)
Output Voltage (V)
80
10
0
–10
40
0
–40
5µs
5V
20mV
–80
0
25
0
50
1µs
2
4
Time (µs) 5µs
10
BIAS CURRENT
vs ADDITIONAL POWER DISSIPATION
100pA
15
10pA
KM
Bias Current (fA)
Common-Mode Voltage (±V)
8
Time (µs)
COMMON-MODE INPUT RANGE
vs SUPPLY VOLTAGE
10
5
1pA
100
10
1
0
0
5
15
10
0
20
50
100
150
200
250
300
350
Additional Power Dissipation (mW)
Supply Voltage (±VCC)
FULL-POWER OUTPUT vs FREQUENCY
INPUT VOLTAGE NOISE SPECTRAL DENSITY
1k
30
Output Voltage (Vp-p)
Voltage Density (nV/ Hz)
6
100
10
20
10
0
1
10
100
1k
10k
100k
1k
Frequency (Hz)
100k
Frequency (Hz)
®
OPA128
10k
6
1M
APPLICATIONS INFORMATION
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 leakage and
noise pickup (see Figure 2).
OFFSET VOLTAGE ADJUSTMENT
The OPA128 offset voltage is laser-trimmed and will require
no further trim for most applications. As with most amplifiers, externally trimming the remaining offset can change
drift performance by about 0.3µV/°C for each 100µV of
adjusted effort. Note that the trim (Figure 1) is similar to
operational amplifiers such as HA-5180 and AD515. The
OPA128 can replace many other amplifiers by leaving the
external null circuit unconnected.
Non-Inverting
Buffer
8
2
OPA128
In
OPA128
3
In
7
TO-99 Bottom View
Inverting
6
OPA128
3
3
2
5
±10mV Typical
Trim Range
4
(1)
4 5 6
In
1
–VCC
6 Out
3
+VCC
2
8
2
6 Out
6 Out
OPA128
3
NOTE: (1) 10kΩ to 1MΩ
Trim Potentiometer
(100kΩ Recommended)
7
2
8
1
8
BOARD LAYOUT
FOR INPUT GUARDING
Guard top and bottom of board.
Alternate: use Teflon® standoff
for sensitive input pins.
FIGURE 1. Offset Voltage Trim.
Teflon® E.I. Du Pont de Nemours & Co.
INPUT PROTECTION
Conventional monolithic FET operational amplifiers’ inputs
must be protected 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 –VCC.
FIGURE 2. Connection of Input Guard.
Because of its dielectric isolation, no special protection is
needed on the OPA128. Of course, the differential and
common-mode voltage limits should be observed.
Triboelectric charge (static electricity generated by friction)
can be a troublesome noise source from cables connected to
the input of an electrometer amplifier. Special low-noise cable
will minimize this effect but the optimum solution is to mount
the signal source directly at the electrometer input with short,
rigid, wiring to preclude microphonic noise generation.
Static damage can cause subtle changes in amplifier input
characteristics without necessarily destroying the device. In
precision operational amplifiers (both bipolar and FET types),
this may cause a noticeable degradation of offset voltage and
drift.
TESTING
Accurately testing the OPA128 is extremely difficult due to its
high level of performance. Ordinary test equipment may not
be able to resolve the amplifier’s extremely low bias current.
Static protection is recommended when handling any precision IC operational amplifier.
Inaccurate bias current measurements can be due to:
1. Test socket leakage
2. Unclean package
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 OPA128. To avoid leakage problems, it is
recommended that the signal input lead of the OPA128 be
wired to a Teflon standoff. If the input 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 input leads and
should be connected to a low impedance point which is at the
signal input potential.
3. Humidity or dew point condensation
4. Circuit contamination from fingerprints or anti-static
treatment chemicals
5. Test ambient temperature
6. Load power dissipation
BIFET® National Semiconductor Corp.
®
7
OPA128
500Ω
9.5kΩ
CF
Guard
+15V
7
3
6
OPA128
2
4
5
1
3
Offset Trim
∆Q
100kΩ
pH Probe
RS ≈ 500MΩ
50mV Output
1011Ω
RF
1VDC
Output
8
2
10pF
OPA128
6
Output
eO
e O = – ∆Q/C F
100pF
–15V
8
1011 Ω
Low Frequency Cutoff =
1/(2 π RF CF) = 0.16Hz
FIGURE 4. Piezoelectric Transducer Charge Amplifier.
FIGURE 3. High Impedance (1015Ω) Amplifier.
IB ≈ 100fA
Gain = 100
3
–In
OPA128
2
CMRR ≈ 118dB
6
RIN ≈ 1015Ω
RF
10k Ω
RG
202 Ω
2
RF
10k Ω
3
25k Ω
3
+In
5
25k Ω
2
OPA128
25k Ω
6
25k Ω
6
Output
Burr-Brown
INA105
Differential
Amplifier
1
Differential Voltage Gain = 1 + 2RF /RG
FIGURE 5. FET Input Instrumentation Amplifier for Biomedical Applications.
≈10pF
10kΩ
(1)
1M Ω
2
8
1N914
2
3
OPA606
6
(1)
(1)
Input
1N914
2N4117A
NOTE: (1) Reverse polarity
for negative peak detection.
FIGURE 6. Low-Droop Positive Peak Detector.
®
OPA128
8
3
OPA128
1000µF
Polystyrene
6
Droop ≈ 100µV/s
Output
<1pF to prevent gain peaking.
1010Ω
Silicon Detector Corp.
SD-020-11-21-011
Guard
2
0.01µF
2kΩ
18kΩ
2
Current
Input
8
7
Output
6
OPA128
3
1000MΩ
+15V
0.1µF
3
OPA128
Output
8
5 x 10 9 V/W
0.1µF
6
VO = –1V/nA
4
1010Ω
–15V
Circuit must be well shielded.
FIGURE 7. Sensitive Photodiode Amplifier.
FIGURE 8. Current-to-Voltage Converter.
+5V
109Ω
2
OPA128
Biased
Current
Transducer
3
6
8
3
4
5
10
11
12
INA101HP
1
Output
14
+15V
7
5
VO = 1mV/pA
8
REF101
6
+5V
4
1
FIGURE 9. Biased Current-to-Voltage Converter.
®
9
OPA128