LTC6605-10
14
660510f
Input Impedance
Calculating the low frequency input impedance depends
on how the inputs are driven.
Figure 6 shows a simplifi ed low frequency equivalent
circuit. For balanced input sources (V
INP
= –V
INM
), the
low frequency input impedance is given by the equation:
R
INP
= R
INM
= R1
Therefore, the differential input impedance is simply:
R
IN(DIFF)
= 2 • R1
APPLICATIONS INFORMATION
–
+
R2
V
OUT
–
V
OUT
+
V
OCM
V
OUTDIFF
0.1μF
660510 F06
R1
R
INP
V
INP
V
INM
R1
R2
R3
R3
+
+
–
–
+
–
R
INM
Figure 6. Input Impedance
For single-ended inputs (V
INM
= 0), the input impedance
increases over the balanced differential case due to the
fact that the summing node (at the junction of R1, R2
and R3) moves in phase with V
INP
to bootstrap the input
impedance. Referring to Figure 6 with V
INM
= 0, the input
impedance looking into either input is:
R
INP
= R
INM
R1
1
1
2
•
R2
R1+ R2
Input Common Mode Voltage Range
The input common mode voltage is defi ned as the average
of the two inputs into resistor R1:
V
VV
INCM
INP INM
=
+
2
The input common mode range is a function of the fi lter
confi guration (GAIN), V
INDIFF
and the V
OCM
potential.
Referring to Figure 6, the summing junction where R1, R2
and R3 merge together should not swing within 1.4V of
the V
+
power supply. Additionally, to avoid forward biasing
the ESD protection diodes on the input pins, neither input
should swing further than 325mV below the V
–
power
rail. Therefore, the input common mode voltage should
be constrained to:
V
325mV +
V
INDIFF
2
V
INCM
1+
R1
R2
•V
+
1.4V
()
R1
R2
V
OCM
The specifi cations in the Electrical Characteristics table are
a special case of the general equation above. For a single
3V power supply, (V
+
= 3V, V
–
= 0V) with V
OCM
= 1.5V,
ΔV
INDIFF
= ±0.25V and R1 = R2, the valid input common
mode range is:
–200mV ≤ V
INCM
≤ 1.7V
Likewise, for a single 5V power supply, (V
+
= 5V, V
–
= 0V)
with V
OCM
= 2.5V, ΔV
INDIFF
= ±0.25V and R1 = R2, the
valid input common mode range is:
–200mV ≤ V
INCM
≤ 4.7V
Output Common Mode and V
OCM
Pin
The output common mode voltage is defi ned as the aver-
age of the two outputs:
VV
VV
OUTCM OCM
OUT 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 feedback loop.
If the V
OCM
pin is left open, an internal resistor divider
develops a potential halfway between the V
+
and V
–
volt-
ages. 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
v o l t a g e , i t c a n b e d i r e c t l y t i e d t o t h e V
OCM
p i n , a s l o n g a s t h e
ADC is capable of driving the input impedance presented by
the V
OCM
pin as listed in the Electrical Characteristics table
(R
VOCM
). The Electrical Characteristics table also specifi es
the valid range that can be applied to the V
OCM
pin.
