DN136 - Micropower Dual and Quad JFET Op Amps Feature pA Input Bias Currents and C-LoadTM Drive Capability

LT1462/LT1463/LT1464/LT1465: Micropower Dual and Quad
JFET Op Amps Feature pA Input Bias Currents and C-Load™
Drive Capability – Design Note 136
Alexander Strong and Kevin R. Hoskins
Introduction
The LT®1462/LT1464 duals and the LT1463/LT1465
quads are the first micropower op amps to offer
picoampere input bias currents and unity-gain stability
when driving capacitive loads. For each amplifier, the
LT1462/LT1463 consume only 28μA of supply current;
the faster LT1464/LT1465, just 145μA. Low supply
current and operation specified at ±5V supplies make
these amplifiers appropriate for portable low power applications. The LT1462/3/4/5 family is especially suited
for piezo transducer conditioning, strain gauge weight
scales, very low droop track-and-holds, wide dynamic
range photodiode amplifiers and other applications that
benefit from pA input bias current. The LT1462/3/4/5
family also exhibits very low noise current, important
to circuits such as low frequency filters. These op
amps allow high value resistors to be used with easily obtainable, low value precision capacitors to set a
filter’s frequency characteristics without compromising
noise performance.
Driving Large Capacitive Loads
Though the LT1462/3/4/5 consume very small amounts
of supply current, they can easily drive 10nF loads while
remaining stable. Instead of increasing their idling current to drive heavy load capacitance, these op amps use
a clever compensation technique to lower bandwidth. As
load capacitance increases, these op amps automatically
reduce their bandwidth by reflecting a portion of the
load capacitance back to the gain node, increasing the
compensation capacitance. Now instead of a 1MHz op
amp trying to drive a large capacitor, a lower bandwidth
op amp is able to drive the load capacitance.
Applications
A benefit of the LT1464/LT1465’s low power consumption is very low junction leakage current, which in turn,
keeps the input bias current below 500fA typically.
Track-and-hold applications are a natural for this family
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and C-Load is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
V+
1/4 LTC201
A
13
15
1 MCT2†
4
5
V+
2
+
1/2 LT1464
6
VIN
B
7
6
4
5
8
3
+
2
6
4
1
VOUT
0.5pA
= 0.05mV/s
10nF
TOTAL SUPPLY CURRENT = 460μA MAX
* R1 = 600Ω FOR ±15V SUPPLIES
R1 = 0Ω FOR ±5V SUPPLIES
† MOTOROLA (602) 244-5768
TYPICAL DROOP =
4
V–
2
5
–
1/2 LT1464
1 MCT2
1
2
R1*
†
–
1/4 LTC201
MODE IN A IN B
Track
0
0
Hold
1
1
Positive Peak Det Reset
0
0
Store
0
1
Negative Peak Det Reset
0
0
Store
1
0
LTC®201 switch is open for logic "1"
14
V–
5
FUNCTION
Track-and-Hold
C1
10nF
POLYSTYRENE
16
3
DN136 F01
Figure 1. Low Droop Track-and-Hold/Peak Detector Circuit Takes Advantage of the LT1464s 0.5pA Input Bias Current
09/96/136_conv
The LT1462/LT1463s low input bias current make it
a natural for amplifying low level signals from high
impedance transducers. The 1pA input bias current
contributes only 0.4fA/√Hz of current noise or 0.4nV/√Hz
voltage noise with a 1MΩ source impedance. A 1MΩ
impedance’s thermal noise of 130nV/√Hz dominates the
op amp’s noise, showing that even with high source
impedances, the LT1462/LT1463 contribute very little
to total input-referred noise. Taking advantage of these
features, Figure 2’s photodiode logging amplifier uses
two LT1462 duals or an LT1463 quad. Here, the photodiode current is converted to a voltage by the first op
amp and D1 and amplified by the first, second and third
R8
100k
logarithmic compression amplifiers. A DC feedback path
comprising R8, R9, C6 and Q1 is active only for no light
conditions. Q1 is off when light is present, isolating the
photodiode from C6. When feedback is needed, a small
filtered current through R8 prevents the op amp outputs
from saturating when no signal is present.
– 1.6
– 1.4
AMP 3 DC OUTPUT (V)
of op amps. Figure 1 is a track-and-hold circuit that uses
a low cost optocoupler as a switch. Leakage for these
parts is usually in the nA region with 1V to 5V across
the output. Since there is less than 0.8mV across the
junctions, the leakage is so small that the op amp’s IB
dominates. The input signal is buffered by one op amp
while the other buffers the stored voltage; this results
in a droop of 50μV/s with a 10nF capacitor.
PHOTODIODE
5V
8
AMP 1
1/2 LT1462
+
3
– 0.8
– 0.6
– 0.4
0
4
–5V
10 –11
10 –9
10 –7
10 –5
PHOTODIODE CURRENT (A)
10 –3
DN136 F03
Figure 3. Logging Photodiode Amplifier DC Output
Q1
2N3904
C6
1μF
–
2
– 1.0
– 0.2
D1
1N4148
R1
100Ω
– 1.2
C1
1nF
1
R2
24k
C2
200pF
5
6
+
AMP 2
1/2 LT1462
–
7
R4
10M
R5
24k
C5
200pF
3
2
AMP 3
1/2 LT1462
D2
1N4148
R3
100k
C3
DC 0.47μF
1 OUT
+
–
5V
R7
10M
D3
1N4148
R9
1M
R10
50k
5
6
+
–
R6
100k
C4
10μF
8
AMP 4
1/2 LT1462
7
4
– 5V
R11
1M
R12
10k
AC
OUT
R13
10k
DN136 F02
Figure 2. This Logging Photodiode Amplifier Takes Advantage of the LT1462s
1pA Input Bias Current to Amplify the Low Level Signal from the Photodiode’s High Source Impedance
Data Sheet Download
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dn136f_conv LT/GP 0996 185K • PRINTED IN THE USA
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© LINEAR TECHNOLOGY CORPORATION 1996
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