LTC6409
9
6409fa
block DiagraM
applicaTions inForMaTion
Functional Description
The LTC6409 is a small outline, wideband, high speed, low
noise, and low distortion fully-differential amplifier with
accurate output phase balancing. The amplifier is optimized
to drive low voltage, single-supply, differential input analog-
to-digital converters (ADCs). The LTC6409 input common
mode range includes ground, which makes it ideal to
DC-couple and convert ground-referenced, single-ended
signals into differential signals that are referenced to the
user-supplied output common mode voltage. This is ideal
for driving these differential ADCs. The balanced differential
nature of the amplifier also provides even-order harmonic
distortion cancellation, and low susceptibility to common
mode noise (like power supply noise). The LTC6409 can
operate with a single-ended input and differential output,
or with a differential input and differential output.
The outputs of the LTC6409 are capable of swinging from
close-to-ground to 1V below V
+
. They can source or sink
up to approximately 70mA of current. Load capacitances
should be decoupled with at least 10Ω of series resistance
from each output.
Input Pin Protection
The LTC6409 input stage is protected against differential
input voltages which exceed 1.4V by two pairs of series
diodes connected back to back between +IN and –IN.
Moreover, the input pins, as well as V
OCM
and SHDN
pins, have clamping diodes to either power supply. If
these pins are driven to voltages which exceed either
supply, the current should be limited to 10mA to prevent
damage to the IC.
SHDN Pin
The SHDN pin is a CMOS logic input with a 150k internal
pull-up resistor. If the pin is driven low, the LTC6409 pow-
ers down. If the pin is left unconnected or driven high,
the part is in normal active operation. Some care should
be taken to control leakage currents at this pin to prevent
inadvertently putting the LTC6409 into shutdown. The
turn-on and turn-off time between the shutdown and ac-
tive states is typically less than 200ns.
General Amplifier Applications
In Figure 1, the gain to V
OUTDIFF
from V
INP
and V
INM
is
given by:
V
OUTDIFF
= V
+OUT
– V
–OUT
≈
R
F
R
I
• V
INP
– V
INM
( )
(1)
Note from Equation (1), 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 LTC6409 ideally
V
+
V
–
V
+
V
–
–
+
6
–IN
5
V
OCM
4
V
+
V
+
3
SHDN
10
V
–
V
–
9
V
+
V
+
8
V
–
V
–
7
+OUT
2
+IN
1
–OUT
200k
50k
6409 BD
