Datasheet

LT6233/LT6233-10
LT6234/LT6235
18
623345fc
applicaTions inForMaTion
With the amplifier connected in a gain of A
V
≥ 2, the output
can invert with very heavy overdrive. To avoid this inver-
sion, limit the input overdrive to 0.5V beyond the power
supply rails.
ESD
The LT6233/LT6234/LT6235 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 ow 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 LT6233/LT6234/LT6235 is
equivalent to that of a 225Ω resistor, and for the lowest
possible noise it is desirable to keep the source and feed-
back resistance at or below this value, i.e., R
S
+ R
G
||R
FB
≤ 225Ω. With R
S
+ R
G
||R
FB
= 225Ω the total noise of the
amplifier is:
e
N
= √(1.9nV)
2
+ (1.9nV)
2
= 2.69nV/√Hz
Below this resistance value, the amplifier dominates the
noise, but in the region between 225Ω and about 30k,
the noise is dominated by the resistor thermal noise. As
the total resistance is further increased beyond 30k, the
amplifier noise current multiplied by the total resistance
eventually dominates the noise.
The product of e
N
• √I
SUPPLY
is an interesting way to
gauge low noise amplifiers. Most low noise amplifiers
with low e
N
have high I
SUPPLY
current. In applications that
require low noise voltage with the lowest possible supply
current, this product can prove to be enlightening. The
LT6233/LT6234/LT6235 have an e
N
• √I
SUPPLY
product of
only 2.1 per amplifier, yet it is common to see amplifiers
with similar noise specifications to have e
N
• √I
SUPPLY
as
high as 13.5.
For a complete discussion of amplifier noise, see the
LT1028 data sheet.
Enable Pin
The LT6233 and LT6233-10 include an ENABLE pin that
shuts down the amplifier to 10µA maximum supply cur-
rent. The ENABLE pin must be driven low to operate the
amplifier with normal supply current. The ENABLE pin
must be driven high to within 0.35V of V
+
to shut down
the supply current. This can be accomplished with simple
gate logic; however care must be taken if the logic and the
LT6233 operate from different supplies. If this is the case,
then open-drain logic can be used with a pull-up resis-
tor to ensure that the amplifier remains off. See Typical
Performance Characteristics.
The output leakage current when disabled is very low;
however, current can flow into the input protection diodes
D1 and D2 if the output voltage exceeds the input voltage
by a diode drop.