LT6350HMS8#PBF Linear Technology, LT6350HMS8#PBF Datasheet - Page 14

• Manufacturer Part Number: LT6350HMS8#PBF
• Description: IC DIFF CONVERT/ADC DRIVER 8MSOP
• Manufacturer: Linear Technology
• Type: ADC Driverr

Specifications of LT6350HMS8#PBF

• Applications: Data Acquisition
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
LT6350
INPUT AMPLIFIER (OP AMP 1) CHARACTERISTICS
Figure 5 shows a simplifi ed schematic of the LT6350’s input
amplifi er. The input stage has NPN and PNP differential
pairs operating in parallel. This topology allows the inputs
to swing all the way from the negative supply rail to the
positive supply rail. Both differential pairs are operational
when the common mode voltage is at least 1.3V from
either rail. As the common mode voltage swings higher
than V
in PNP differential pair Q1/Q4 drops to zero. Feedback is
maintained through the NPN differential pair Q2/Q3, but
the input stage transconductance, g
factor of 2. A similar effect occurs with I
mon mode voltage swings within 1.3V of the negative rail.
A precision, 2-point algorithm is used to maintain near
constant offset voltage over the entire input range (see
Offset Considerations).
Negative input bias current fl ows into the +IN1 and –IN1
inputs when the input common mode is centered between
the rails. The magnitude of this current increases when
the input common mode voltage is within 1.3V of the
negative rail and only Q1/Q4 are active. The polarity of the
current reverses when the input common mode voltage is
14
+
– 1.3V, current source I
DESD1
DESD3
+IN1
–IN1
V
V
–
–
V
V
D1
+
+
DESD2
DESD4
1
Figure 5. Input Amplifi er (Op Amp 1) Simplifi ed Schematic
saturates, and current
D2
m
Q1
, is reduced by a
2
R3
R1
when the com-
Q2
I
1
Q3
I
2
R2
Q4
R4
Q8
Q5
within 1.3V of the positive rail and only Q2/Q3 are active.
Typical total change in input bias current over the entire
input common mode range is approximately 4μA. These
changes in input bias current will generate corresponding
changes in voltage across the source and gain-setting
resistors. Because the LT6350 input offset current is less
than the input bias current, matching the effective source
and feedback resistances at the input pins will reduce total
offset errors generated by changes in input bias current
and will keep distortion to a minimum.
INPUT AMPLIFIER (OP AMP 1) FEEDBACK
COMPONENTS
When feedback resistors are used to set gain in op amp 1,
care should be taken to ensure that the pole formed by the
feedback resistors and the total capacitance at the inverting
input, –IN1, does not degrade stability. For instance, to
set the LT6350 in a differential gain of +4, R
Figure 3 could be set to 1kΩ. If the total capacitance at
–IN1 (LT6350 plus PC board) were 3pF , a new pole would
be formed in the loop response at 106MHz, which could
lead to ringing in the step response. A capacitor connected
across the feedback resistor and having the same value
R5
D3
V
Q9
+
Q7
Q6
C
+
–
DIFFERENTIAL
GENERATOR
M
V
DRIVE
BIAS
Q10
Q11
V –
V –
V
V
+
DESD6
+
DESD5
OUT1
6350 F05
F
and R
6350fb
G
of