OPA111 Burr-Brown, OPA111 Datasheet - Page 9

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OPA111

Manufacturer Part Number
OPA111
Description
Low Noise Precision Difet OPERATIONAL AMPLIFIER
Manufacturer
Burr-Brown
Datasheet

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should completely surround the high impedance input 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 3).
If guarding is not required, pin 8 (case) should be connected
to ground.
FIGURE 3. Connection of Input Guard.
NOISE: FET VERSUS BIPOLAR
Low noise circuit design requires careful analysis of all
noise sources. External noise sources can dominate in many
cases, so consider the effect of source resistance on overall
operational amplifier noise performance. At low source
impedances, the lower voltage noise of a bipolar operational
amplifier is superior, but at higher impedances the high
current noise of a bipolar amplifier becomes a serious
liability. Above about 15k , the OPA111 will have a lower
total noise than an OP-27 (see Figure 4).
BIAS CURRENT CHANGE
VERSUS COMMON-MODE VOLTAGE
The input bias current of most popular BIFET operational
amplifiers are affected by common-mode voltage (Figure 5).
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 OPA111 is
not compromised by common-mode voltage.
APPLICATIONS CIRCUITS
Figures 6 through 18 are circuit diagrams of various appli-
cations for the OPA111.
In
In
Board layout for input guarding: guard top and bottom of board.
Alternate: use Teflon
Non-Inverting
3
Inverting
3
2
2
OPA111
OPA111
8
8
®
6
6
standoff for sensitive input pins.
Out
Out
Teflon
®
E. I. Du Pont de Nemours & Co.
In
TO-99 Bottom View
3
2
3
2
OPA111
Buffer
1
4
8
5
6
8
6
7
Out
9
FIGURE 4. Voltage Noise Spectral Density vs Source
FIGURE 6. Pyroelectric Infrared Detector.
FIGURE 5. Input Bias Currrent vs Common-Mode Voltage.
Pyroelectric
–20
Detector
100
80
60
40
20
10
1k
0
1
–15
100
AD547
T
manufacturers' published
typical data.
A
Resistance.
= 25°C; curves taken from
R
–10
S
1k
Source Resistance, R
Common-Mode Voltage (VDC)
E
O
1000M
–5
OP-15/16/17
"Perfect Bias Current Cancellation"
1000pF Polystyrene
OPA111 + Resistor
Resistor Noise Only
OP-27 + Resistor
LF155
10k
Resistor Noise Only
OPA111 + Resistor
2
3
OP-27 + Resistor
1000M
OPA111
OPA111
0
E
O
= e
8
100k
S
( )
N
6
NOTE: Pyroelectric
detectors respond
to rate-of-change
(AC signal) only.
2
5
+ (I
N
R
S
)
1M
2
LF156/157
10
+ 4kTR
BM
OPA111
Output
S
10M
15
®

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