Datasheet

LT6015/LT6016/LT6017
15
601567fd
For more information www.linear.com/LT6015
to 0V differential by feedback action of the amplifier.
However, depending
on the configuration of the feedback
around the amplifier, this input resistance can boost noise
gain, lower overall amplifier loop gain and closed loop
bandwidth, raise output noise, with one benevolent effect
in increasing amplifier stability.
In the normal mode of operation (where V
–
< V
CM
< V
+
–1.5V), R
IN
is typically large compared to the value of the
input resistor used, and R
IN
can be ignored (refer to Figure 3).
In this case the noise gain is defined by the equation:
NOISE GAIN ≈ 1+
R
F
R
I
However, when the amplifier transitions into Over-The-Top
mode with the input common mode biased near or above
the the V
+
supply, R
IN
should be considered. The noise
gain of the amplifier changes to:
NOISE GAIN = 1+
R
F
R
I
|| R
IN
+ R
I
||R
F
( )
Likewise the closed loop bandwidth of the amplifier will
change going from normal mode operation to Over-The-
Top operation:
Normal mode:
BW
CLOSED − LOOP
≈
GBW
1+
R
F
R
I
Over-The-Top mode:
BW
CLOSED − LOOP
≈
GBW
1+
R
F
R
I
|| R
IN
+ R
I
||R
F
( )
And output noise is negatively impacted going from normal
mode to Over-The-Top:
Normal mode: (neglecting resistor noise)
e
no
≈ e
ni
• 1+
R
F
R
I
⎛
⎝
⎜
⎞
⎠
⎟
Over-The-Top mode: (neglecting resistor noise)
e
no
≈ e
ni
• 1+
R
F
R
I
|| R
IN
+ R
I
||R
F
( )
⎛
⎝
⎜
⎞
⎠
⎟
Output
The output of the LT6015/LT6016/LT6017 can swing within
a Schottky diode drop (~0.4V) of the V
+
supply, and within
5mV of the negative supply with no load. The output is
capable of sourcing and sinking approximately 25mA.
The LT6015/LT6016/LT6017 are internally compensated
to drive at least 200pF of capacitance under any output
loading conditions. For larger capacitive loads, a 0.22μF
capacitor in series with a 150Ω resistor between the out
-
put and ground will compensate these amplifiers to drive
capacitive loads greater than 200pF.
applicaTions inForMaTion
While it is true that the DC closed loop gain will remain
mostly unaffected (=
R
F
R
I
), the loop gain of the amplifier
has decreased from
A
OL
1+
R
F
R
I
to
A
OL
1+
R
F
R
I
|| R
IN
+ R
I
||R
F
( )
Figure 3. Difference Amplifier Configured for Both
Normal and Over-The-Top Operation
+
–
5V
LT6015
V
IN
V
INCM
R
I
R
F
V
OUT
R
F
R
I
R
IN
601567 F03