LT1783
APPLICATIONS INFORMATION
Supply Voltage
Output
The output of the LT1783 can swing to within 60mV of the
positive rail with no load and within 3mV of the negative
rail with no load. When monitoring input voltages within
60mV of the positive rail or within 3mV of the negative rail,
gain should be taken to keep the output from clipping. The
LT1783 can sink and source over 30mA at 5V supplies,
sourcing current is reduced to 10mA at 3V total supplies
as noted in the Electrical Characteristics.
The positive supply pin of the LT1783 should be bypassed
with a small capacitor (typically 0.1μF) within an inch of
the pin. When driving heavy loads, an additional 4.7μF
electrolytic capacitor should be used. When using split
supplies, the same is true for the negative supply pin.
The LT1783 is protected against reverse-battery voltages
up to 18V. In the event a reverse-battery condition occurs,
the supply current is less than 1nA.
The LT1783 is internally compensated to drive at least
400pF of capacitance under any output loading condi-
tions. A 0.22μF capacitor in series with a 150Ω resistor
between the output and ground will compensate these
amplifiers for larger capacitive loads, up to 10,000pF, at
all output currents.
Inputs
The LT1783 has two input stages, NPN and PNP (see the
SimplifiedSchematic),resultinginthreedistinctoperating
regions as shown in the Input Bias Current vs Common
Mode typical performance curve.
+
For input voltages about 0.8V or more below V , the PNP
Distortion
input stage is active and the input bias current is typically
–40nA. When the input common mode voltage is within
0.5V of the positive rail, the NPN stage is operating and
the input bias current is typically 80nA. Increases in tem-
peraturewillcausethevoltageatwhichoperationswitches
from the PNP input stage to the NPN input stage to move
There are two main contributors to distortion in op amps:
output crossover distortion as the output transitions from
sourcing to sinking current, and distortion caused by non-
linear common mode rejection. If the op amp is operating
inverting, there is no common mode induced distortion.
If the op amp is operating in the PNP input stage (input is
+
towards V . The input offset voltage of the NPN stage is
+
not within 0.8V of V ), the CMRR is very good, typically
untrimmed and is typically 1.8mV.
100dB. When the LT1783 switches between input stages,
there is significant nonlinearity in the CMRR. Lower load
resistance increases the output crossover distortion but
has no effect on the input stage transition distortion. For
lowest distortion, the LT1783 should be operated single
supply, with the output always sourcing current and with
A Schottky diode in the collector of the input transistors,
along with special geometries for these NPN transistors,
allowtheLT1783tooperatewitheitherorbothofitsinputs
+
+
aboveV . Atabout0.3VaboveV , theNPNinputtransistor
isfullysaturatedandtheinputbiascurrentistypically30μA
at room temperature. The input offset voltage is typically
+
the input voltage swing between ground and (V – 0.8V).
+
1.8mV when operating above V . The LT1783 will operate
with its inputs 18V above V regardless of V .
See the Typical Performance Characteristics curves,
“Total Harmonic Distortion + Noise vs Output Voltage
Amplitude.”
–
+
The inputs are protected against excursions as much as
–
10V below V by an internal 1k resistor in series with each
input and a diode from the input to the negative supply.
The input stage of the LT1783 incorporates phase reversal
protection to prevent the output from phase reversing for
–
inputs up to 9V below V . There are no clamping diodes
between the inputs and the maximum differential input
voltage is 18V.
1783fd
11
LINEAR_DIMENSIONS [ Linear Dimensions ]
