OPA111 Datasheet, PDF (9/12 Pages) Burr-Brown (TI) – Low Noise Precision Difet OPERATIONAL AMPLIFIER

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.
Non-Inverting
Buffer
2
8
6 ^Out
OPA111
2
8
6 ^Out
OPA111
In
3
In
3
Inverting
In
2
6 ^Out
OPA111
3
8
TO-99 Bottom View
4
3
5
6
7
2
8
1
Board layout for input guarding: guard top and bottom of board.
Alternate: use Teflon^® standoff for sensitive input pins.
Teflon^® E. I. Du Pont de Nemours & Co.
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 15kΩ, 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.
1k
OP-27 + Resistor
E_{O OPA111 + Resistor}
Resistor Noise Only
100
R_S
10
1
100
E_O = e_N2 + (I_NR_S)² + 4kTR_S
OPA111 + Resistor
Resistor Noise Only
OP-27 + Resistor
BM
1k
10k
100k
1M
10M
Source Resistance, R_S (Ω)
FIGURE 4. Voltage Noise Spectral Density vs Source
Resistance.
80
T_A = 25°C; curves taken from
manufacturers' published
60 _{typical data.}
40
LF156/157
20
LF155
AD547
0
–20
–15
–10
OPA111
OP-15/16/17
"Perfect Bias Current Cancellation"
–5
0
5
10
15
Common-Mode Voltage (VDC)
FIGURE 5. Input Bias Currrent vs Common-Mode Voltage.
Pyroelectric
Detector
1000pF Polystyrene
1000MΩ
2
8
6
OPA111
3
Output
1000MΩ
NOTE: Pyroelectric
detectors respond
to rate-of-change
(AC signal) only.
FIGURE 6. Pyroelectric Infrared Detector.
®
9
OPA111