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. Trackand-hold applications are a natural for this family of op
amps. Figure 1 is a track-and-hold circuit that uses a low
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
13
15
A
4
14
1 MCT2†
5
6
VIN
+
–
1/4 LTC201
B
2
1/2 LT1464
7
6
4
5
3
–
8
1/2 LT1464
+
4
V–
2
2
5
2
R1*
†
1 MCT2
1
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"
V+
V–
5
FUNCTION
Track-and-Hold
C1
10nF
POLYSTYRENE
16
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 =
3
DN136 F01
Figure 1. Low Droop Track-and-Hold/Peak Detector Circuit Takes Advantage of the LT1464's 0.5pA Input Bias Current
09/96/136_conv
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.
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
R8
100k
AMP 3 DC OUTPUT (V)
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 logarithmic compression amplifiers. A
PHOTODIODE
–
2
3
– 1.0
– 0.8
– 0.6
– 0.4
– 0.2
0
8
AMP 1
1/2 LT1462
+
4
–5V
10 –9
10 –7
10 –5
PHOTODIODE CURRENT (A)
10 –3
Figure 3. Logging Photodiode Amplifier DC Output
Q1
2N3904
C6
1µF
5V
10 –11
DN136 F03
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
+
8
AMP 4
1/2 LT1462
–
R6
100k
4
– 5V
7
R11
1M
R12
10k
C4
10µF
AC
OUT
R13
10k
DN136 F02
Figure 2. This Logging Photodiode Amplifier Takes Advantage of the LT1462's
1pA Input Bias Current to Amplify the Low Level Signal from the Photodiode’s High Source Impedance
Data Sheet Download
www.linear.com/LT1462
Linear Technology Corporation
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