LT6204 Linear Technology, LT6204 Datasheet - Page 16

• Manufacturer Part Number: LT6204
• Description: (LT6202 - LT6204) Low Power Op Amps
• Manufacturer: Linear Technology

LT6202/LT6203/LT6204
APPLICATIO S I FOR ATIO
16
Input Protection
There are back-to-back diodes, D1 and D2, across the
+ and – inputs of these amplifiers to limit the differential
input voltage to 0.7V. The inputs of the LT6202/LT6203/
LT6304 do not have internal resistors in series with the
input transistors. This technique is often used to protect
the input devices from over voltage that causes excessive
currents to flow. The addition of these resistors would
significantly degrade the low noise voltage of these ampli-
fiers. For instance, a 100
input would generate 1.8nV/ Hz of noise, and the total
amplifier noise voltage would rise from 1.9nV/ Hz to
2.6nV/ Hz. Once the input differential voltage exceeds
tion diodes should be limited to 40mA. This implies 25
of protection resistance per volt of continuous overdrive
beyond 0.7V. The input diodes are rugged enough to
handle transient currents due to amplifier slew rate over-
drive or momentary clipping without these resistors.
Figure 2 shows the input and output waveforms of the
amplifier driven into clipping while connected in a gain of
A
power supply rails, the input transistors will saturate.
When saturation occurs, the amplifier loses a stage of
phase inversion and the output tries to change states.
Diodes D1 and D2 forward bias and hold the output within
0.7V, steady state current conducted though the protec-
V
= 1. When the input signal goes sufficiently beyond the
OV
Figure 2. V
S
= 2.5V, A
U
U
V
= 1 with Large Overdrive
resistor in series with each
W
LT6202/03/04 F02
U
a diode drop of the input signal. In this photo, the input
signal generator is clipping at 35mA, and the output
transistors supply this generator current through the
protection diodes.
With the amplifier connected in a gain of A
can invert with very heavy input overdrive. To avoid this
inversion, limit the input overdrive to 0.5V beyond the
power supply rails.
ESD
The LT6202/LT6203/LT6204 have reverse-biased ESD
protection diodes on all inputs and outputs as shown in
Figure 1. If these pins are forced beyond either supply,
unlimited current will flow through these diodes. If the
current is transient and limited to one hundred milliamps
or less, no damage to the device will occur.
Noise
The noise voltage of the LT6202/LT6203/LT6204 is equiva-
lent to that of a 225 resistor, and for the lowest possible
noise it is desirable to keep the source and feedback
resistance at or below this value, i.e. R
With R
is: e
value, the amplifier dominates the noise, but in the resis-
tance region between 225 and approximately 10k , the
noise is dominated by the resistor thermal noise. As the
total resistance is further increased, beyond 10k, the noise
current multiplied by the total resistance eventually domi-
nates the noise.
The product of e
low noise amplifiers. Many low noise amplifiers with low
e
low noise with the lowest possible supply current, this
product can prove to be enlightening. The LT6202/LT6203/
LT6204 have an e
yet it is common to see amplifiers with similar noise
specifications have an e
For a complete discussion of amplifier noise, see the
LT1028 data sheet.
n
have high I
n
= (1.9nV)
S
+ R
G
||
SUPPLY
R
FB
2
n
+ (1.9nV)
• I
n
, I
= 225 the total noise of the amplifier
SUPPLY
current. In applications that require
SUPPLY
n
• I
2
= 2.7nV. Below this resistance
is an interesting way to gauge
SUPPLY
product of 3.2 per amplifier,
product of 4.7 to 13.5.
S
+ R
V
G
||
2, the output
R
FB
225 .
620234fa