LT6221 Linear Technology Corporation, LT6221 Datasheet - Page 15

• Manufacturer Part Number: LT6221
• Description: Single/dual/quad 60mhz, 20v/?s Low Power, Rail-to-rail Input And Output Precision Op Amp
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
severely overdriven, an external resistor should be used to
limit the overdriven current.
The LT6220/LT6221/LT6222’s input stages are also pro-
tected against a large differential input voltage of 1.4V or
higher by a pair of back-to-back diodes, D5/D8, to prevent
the emitter-base breakdown of the input transistors. The
current in these diodes should be limited to less than
10mA when they are active. The worse-case differential
input voltage usually occurs when the input is driven while
the output is shorted to ground in a unity-gain configura-
tion. In addition, the amplifier is protected against ESD
strikes up to 3kV on all pins by a pair of protection diodes
on each pin that are connected to the power supplies as
shown in Figure 1.
Capacitive Load
The LT6220/LT6221/LT6222 are optimized for high band-
width, low power and precision applications. They can
drive a capacitive load up to 100pF in a unity-gain configu-
ration and more for higher gain. When driving a larger
TYPICAL APPLICATIO S
Stepped-Gain Photodiode Amplifier
The circuit of Figure 2 is a stepped gain transimpedance
photodiode amplifier. At low signal levels, the circuit has
a high 100kΩ gain, but at high signal levels the circuit
automatically and smoothly changes to a low 3.2kΩ gain.
The benefit of a stepped gain approach is that it maximizes
dynamic range, which is very useful on limited supplies.
Put another way, in order to get 100kΩ sensitivity and still
handle a 1mA signal level without resorting to gain reduc-
tion, the circuit would need a 100V negative voltage
supply.
The operation of the circuit is quite simple. At low photo-
diode currents (below 10µA) the output and inverting
input of the op amp will be no more than 1V below ground.
The LT1634 in parallel with R3 and Q2 keep a constant
current though Q2 of about 20µA. R4 maintains quiescent
current through the LT1634 and pulls Q2’s emitter above
U
U
U
W
U
capacitive load, a resistor of 10Ω to 50Ω should be
connected between the output and the capacitive load to
avoid ringing or oscillation. The feedback should still be
taken from the output so that the resistor will isolate the
capacitive load to ensure stability. Graphs on capacitive
loads show the transient response of the amplifier when
driving capacitive load with specified series resistors.
Feedback Components
When feedback resistors are used to set up gain, care must
be taken to ensure that the pole formed by the feedback
resistors and the total capacitance at the inverting input
does not degrade stability. For instance, the LT6220/
LT6221/LT6222, set up with a noninverting gain of 2, two
5k resistors and a capacitance of 5pF (part plus PC board),
will probably oscillate. The pole is formed at 12.7MHz that
will reduce phase margin by 52 degrees when the cross-
over frequency of the amplifier is around 10MHz. A capaci-
tor of 10pF or higher connecting across the feedback
resistor will eliminate any ringing or oscillation.
ground, so Q1 is reverse biased and no current flows
through R2. So for small signals, the only feedback path
is R1 (and C1) and the circuit is a simple transimpedance
amplifier with 100kΩ gain.
PHOTODIODE
~4pF
V
S
+
I
PD
Figure 2. Stepped-Gain Photodiode Amplifier
LT6220/LT6221/LT6222
–
+
3.24k
100k
R1
R2
LT6220
V
V
S
S
+
–
30pF
1pF
C2
C1
V
S
= ±1.5V TO ±5V
Q1
V
S
2
–
3
PHILIPS
BCV62
1
4
Q2
R3
33k
V
S
+
R4
10k
LT1634-1.25
V
15
OUT
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