LTC6601-2
17
66012f
APPLICATIONS INFORMATION
potential halfway between the V
+
and V
–
pins. Whenever
this pin is not hard tied to a low impedance ground plane,
a high quality ceramic capacitor should be used to bypass
the V
OCM
pin to a low impedance ground plane (see Layout
Considerations). The LTC6601’s internal common mode
feedback path forces accurate output phase balancing to
reduce even order harmonics, and centers each individual
output about the potential set by the V
OCM
pin.
V
OUTCM
= V
OCM
=
V
OUT
+
+ V
OUT
–
2
The outputs (OUT
+
and OUT
–
) of the LTC6601 are capable
of swinging rail-to-rail. They can source or sink up to ap-
proximately 75mA of current. Load capacitances should
be decoupled with at least 25Ω of series resistance from
each output.
The LTC6601 Electrical Characteristics table specifi es an
input referred offset. This specifi cation actually lumps volt-
age offsets due to offset bias currents (I
OS
), and amplifi er
voltage offset into one specifi cation. To refer this specifi ca-
tion to the output, you simply multiply the specifi cation
by the noise gain the LTC6601 is confi gured in:
V
OSODIFF
= 1 + Gain
where Gain is the closed loop gain in the particular fi lter
application:
Gain =
R2
R1
COMPONENT INPUT PIN PROTECTION
All of the LTC6601 pins with the exception of V
+
and V
–
are
protected with steering diodes to either power supply. In
the event that a pin is driven beyond the supply rails, the
excess current should be limited to under 10mA to prevent
damage to the IC.
BIAS Pin
The LTC6601 has a BIAS pin (Pin 3) whose function is to
tailor both performance and power of the LTC6601. The
pin has a Thevenin equivalent impedance of approximately
150kΩ to a voltage source whose potential is 1.15V above
the V
–
supply. This pin has fi xed logic levels relative to V
–
(see the Electrical Characteristics table), and can be driven
by an external source keeping in mind its equivalent input
impedance and equivalent input voltage. If the BIAS pin is
fl oated, care should be taken to control external leakage
currents to this pin to under 1A to prevent putting the
LTC6601 an undesired state.
If BIAS is tied to the positive supply, the LTC6601 dif-
ferential fi lter will be in a fully active state confi gured for
highest performance (lowest noise and lowest distortion).
If the BIAS pin is fl oated or left unconnected, the LTC6601
fi lter will be in a fully active state, with amplifi er currents
reduced and performance scaled back to preserve power
consumption. If the BIAS pin is tied to the most negative
supply (V
–
), the LTC6601 will be placed into a low power
shutdown mode with amplifi er outputs disabled. In this
state, the LTC6601 draws approximately 450µA.
In low power shutdown, all internal biasing current sources
are shut off, and the output pins, OUT
+
and OUT
–
, will each
appear as open collectors with a non-linear capacitor in
parallel and steering diodes to either supply. The turn-on
and turn-off time constant between states are on the order
of 0.4s. Using this function to wire-OR outputs together
is not recommended.
General Design and Usage
As levels of integration have increased and correspond-
ingly, system supply voltages decreased, there has been
a need for ADCs to process signals differentially in order
to maintain good signal-to-noise ratios. These ADCs are
typically supplied from a single supply voltage which
can be as low as 3V (2.7V min), and will have an optimal
common mode input range near mid-supply. The LTC6601
makes interfacing to these ADCs easy, by providing anti-
alias fi ltering, single-ended to differential conversion and
common mode level shifting (translation). Figure 3 shows
a general application of this. The low frequency gain to
V
OUTDIFF
from V
IN
is simply:
V
OUTDIFF
= V
OUT
+
–V
OUT
–
≈
R2
R1
•V
INDIFF
The differential output voltage (V
OUT
+
– V
OUT
–
) is completely
independent of input and output common mode voltages,
or the voltage at the common mode pin. This makes the
