Datasheet

LT6557
9
6557fa
V
+
IN
2.5k
9.1k
6557 F01
I =
V
PIN16
R
SET
Figure 1. Simplifi ed Programmable Input Bias Circuit Diagram
no-signal amplifi er input bias condition according to the
following relationship:
V
Vk
R
BIAS IN
PIN
SET
()
•.
=
16
91
where V
PIN16
= 0.048V typical.
For single 5V supply operation, a 400Ω programming
resistor is generally optimal. In applications that demand
maximum amplifi er linearity, or if external biasing is
preferred (in DC-coupled applications, for example), the
internal biasing circuitry may be disabled by leaving Pin
16 open. With Pin 16 open, input loading is approximately
200kΩ.
Shutdown Control
The LT6557 may be placed into a shutdown mode, where
all three amplifi er sections are deactivated and power sup-
ply draw is reduced to approximately 10µA. When the EN
pin is left open, an internal 40k pull-up resistor brings the
pin to V
+
and the part enters the shutdown mode. Pulling
the pin more than approximately 1.5V below V
+
will en-
able the LT6557 (see Figure 2 for equivalent circuit). The
pull-down current required to activate the part is typically
125µA. In most applications, the EN pin is simply con-
nected to ground (for continuous operation) or driven
directly by a CMOS-level logic gate (see Figure 3 for
examples). Response time is typically 50ns for enabling,
and 1µs for shutdown. In shutdown mode, the feedback
resistors remain connected between the output pins and
the individual ground (or V
–
connected) pins.
V
+
40k
BIAS
CIRCUITRY
EN
Figure 2. Simplifi ed Shutdown Circuit Diagram
LT6557
V
+
V
+
2
(3a) Open Drain or Open Collector
1
EN
DISABLE
LT6557
2
6557 F03
(3b) CMOS Gate with Shared Supply
1
EN
DISABLE
Figure 3. Suitable Shutdown Pin Drive Circuits
Programmable Input Bias
The LT6557 contains circuitry that provides a user-pro-
grammed bias voltage to the inputs of all three amplifi er
sections. The internal biasing feature is designed to mini-
mize external component count in AC-coupled applica-
tions, but may be defeated if external biasing is desired.
Figure 1 shows the simplifi ed equivalent circuit feeding
the noninverting input of each amplifi er. A programming
resistor from Pin 16 to GND (Pin 2) establishes the nominal
APPLICATIONS INFORMATION