LTC6601-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC6601-1
Linear Technology
'LTC6601-1' PDF : 40 Pages View PDF
LTC6601-1
APPLICATIONS INFORMATION
LTC6601 ideally suited for pre-amplification, level shift-
ing and conversion of single-ended signals to differential
output signals for driving differential input ADCs.
INPUT IMPEDANCE
Calculating the low frequency input impedance of the
LTC6601 depends on how the inputs are driven (whether
they are driven from a single-ended or a differential
source).
Figure 4 shows a simplified low frequency equivalent
circuit of the LTC6601. For balanced input sources (VINP
= –VINM), the low frequency input impedance is given by
the equation:
RINP = RINM = R1
The differential input impedance is simply:
RINDIFF = 2 • R1
For single-ended inputs (VINM = 0), the input impedance
actually increases over the balanced differential case due
to the fact the summing node (at the junction of R1, R2
and R3) moves in phase with VINP to bootstrap the input
impedance. Referring to Figure 4 with VINM = 0, the input
impedance looking into either input is:
RINP
=
RINM
⎛
⎝⎜
1–
1
2
•
R1
⎛ R2 ⎞
⎝⎜ R1+ R2⎠⎟
⎞
⎠⎟
Input and Output Common Mode Voltage Range
The input common mode voltage is defined as the average
of the two inputs:
VINCM
=
VINP
+
2
VINM
The lower limit of the input common mode range is dic-
tated by the ESD protection diodes at the input. While it
is possible for the inputs to swing below V–, the diodes
will conduct if the inputs are taken a diode drop below V–.
The upper limit of the input common mode range varies
as a function of the filter configuration (GAIN), VOCM po-
tential, and whether or not the inputs are single-ended or
differential. While it is possible to exceed the upper limit
of the common mode range, doing so will degrade filter
linearity. Referring to Figure 4, for linear operation, the
summing junction where R1, R2 and R3 merge together
should be prevented from swinging to within 1.4V of the
V+ power supply.
For the general case, the upper input common mode volt-
age limit should be constrained to:
VOCM
•
R1
R1+ R2
+
VINCM
•
R2
R1+ R2
≤
V+
–
1.4V
Or equivalently:
( ) VINCM
≤
⎛
⎝⎜
1+
R1⎞
R2 ⎠⎟
V+ − 1.4V
−
R1
R2
•
VOCM
RINP R1
VINP
VINM
R1
RINM
R2
R3
+
R3
–
R2
VOUT–
VOUTDIFF
VOUT+
VOCM
0.1μF
66011 F04
Figure 4. Input Impedance
The specifications for input common mode range (VINCMR)
are based on these constraints with R1 = R2 = 100Ω, and
VOCM = mid-supply. Substituting the numbers for a single
3V power supply, (V+ = 3V, V– = 0V) with VOCM =1.5V, and
R1 = R2 = 100Ω, into the above equation, the input com-
mon mode range (VINCMR) is between the two limits:
0V ≤ VINCM ≤ 1.7V
which is as is specified for a 3V supply.
66011f
18
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