LTC6362
10
6362fa
APPLICATIONS INFORMATION
Functional Description
The LTC6362 is a low power, low noise, high DC accuracy
fully differential operational amplifier/ADC driver. The
amplifier is optimized to convert a fully differential or
single-ended signal to a low impedance, balanced differ-
ential output suitable for driving high performance, low
power differential successive approximation register (SAR)
ADCs. The balanced differential nature of the amplifier
also provides even-order harmonic distortion cancella-
tion, and low susceptibility to common mode noise (like
power supply noise).
The outputs of the LTC6362 are capable of swinging rail-
to-rail and can source or sink up to 35mA of current. The
LTC6362 is optimized for high bandwidth and low power
applications. Load capacitances above 10pF to ground or
5pF differentially should be decoupled with 10Ω to 100Ω
of series resistance from each output to prevent oscilla-
tion or ringing. Feedback should be taken directly from
the amplifier output. Higher voltage gain configurations
tend to have better capacitive drive capability than lower
gain configurations due to lower closed-loop bandwidth.
Input Pin Protection
The LTC6362 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, all pins have clamping diodes to both power
supplies. If any pin is driven to voltages which exceed
either supply, the current should be limited to under 10mA
to prevent damage to the IC.
SHDN Pin
The LTC6362 has a SHDN pin which when driven to within
0.8V above the negative rail, will shut down amplifier op-
eration such that only 70µA is drawn from the supplies.
Pull-down circuitry should be capable of sinking at least
4µA to guarantee complete shutdown across all condi-
tions. For normal operation, the SHDN pin should be left
floating or tied to the positive rail.
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
( )
Note from the previous equation, 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 LTC6362 ideally
suited for pre-amplification, level shifting and conversion
of single-ended signals to differential output signals for
driving differential input ADCs.
Output Common Mode and V
OCM
Pin
The output common mode voltage is defined as the aver-
age of the two outputs:
V
OUTCM
= V
OCM
=
V
+OUT
+ V
–OUT
2
As the equation shows, the output common mode voltage
is independent of the input common mode voltage, and
is instead determined by the voltage on the V
OCM
pin, by
means of an internal common mode feedback loop.
If the V
OCM
pin is left open, an internal resistor divider
develops a default voltage of 2.5V with a 5V supply. The
V
OCM
pin can be overdriven to another voltage if desired.
For example, when driving an ADC, if the ADC makes a
reference available for setting the common mode volt-
age, it can be directly tied to the V
OCM
pin, as long as
the ADC is capable of driving the 170k input resistance
presented by the V
OCM
pin. The Electrical Characteristics
table specifies the valid range that can be applied to the
V
OCM
pin (V
OUTCMR
).
