LTC1966
15
1966fb
applicaTions inForMaTion
For single-ended DC-coupled applications, simply con-
nect one of the two inputs (they are interchangeable) to
the signal, and the other to ground. This will work well
for dual supply configurations, but for single supply
configurations it will only work well for unipolar input
signals. The LTC1966 input voltage range is from rail-
to-rail, and when the input is driven above V
DD
or below
V
SS
(ground for single supply operation) the gain and
offset errors will increase substantially after just a few
hundred millivolts of overdrive. Fortunately, most single
supply circuits measuring a DC-coupled RMS value will
include some reference voltage other than ground, and
the second LTC1966 input can be connected to that point.
For single-ended AC-coupled applications, Figure 9 shows
three alternate topologies. The first one, shown in Figure 9a
uses a coupling capacitor to one input while the other is
grounded. This will remove the DC voltage difference from
the input to the LTC1966, and it will therefore not be part
of the resulting output voltage. Again, this connection will
work well with dual supply configurations, but in single
supply configurations it will be necessary to raise the volt-
age on the grounded input to assure that the signal at the
active input stays within the range of V
SS
to V
DD
. If there
is already a suitable voltage reference available, connect
the second input to that point. If not, a midsupply voltage
can be created with two resistors as shown in Figure 9b.
Finally, if the input voltage is known to be between V
SS
and V
DD
, it can be AC-coupled by using the configuration
shown in Figure 9c. Whereas the DC return path was
provided through Pin 3 in Figures 9a and 9b, in this case,
the return path is provided on Pin 2, through the input
signal voltages. The switched capacitor action between
the two input pins of the LTC1966 will cause the voltage
on the coupling capacitor connected to the second input
to follow the DC average of the input voltage.
For differential input applications, connect the two inputs
to the differential signal. If AC coupling is desired, one of
the two inputs can be connected through a series capacitor.
In all of these connections, to choose the input coupling
capacitor, C
C
, calculate the low frequency coupling time
constant desired, and divide by the LTC1966 differential
input impedance. Because the LTC1966 input impedance
is about 100 times its output impedance, this capacitor is
typically much smaller than the output averaging capaci-
tor. Its requirements are also much less stringent, and a
ceramic chip capacitor will usually suffice.
Output Connections
The LTC1966 output is differentially, but not symmetrically,
generated. That is to say, the RMS value that the LTC1966
computes will be generated on the output (Pin 5) relative
to the output return (Pin 6), but these two pins are not
interchangeable. For most applications, Pin 6 will be tied
to ground (Pin 1), and this will result in the best accuracy.
However, Pin 6 can be tied to any voltage between V
SS
(Pin 4) and V
DD
(Pin 7) less the maximum output voltage
swing desired. This last restriction keeps V
OUT
itself (Pin 5)
within the range of V
SS
to V
DD
. If a reference level other
than ground is used, it should be a low impedance, both
AC and DC, for proper operation of the LTC1966.
Use of a voltage in the range of V
DD
– 1V to V
DD
– 1.3V can
lead to errors due to the switch dynamics as the NMOS
transistor is cut off. For this reason, it is recommended
that OUT RTN = 0V if V
DD
is ≤3V.
Figure 9. Single-Ended AC-Coupled Input Connection Alternatives
+
–
LTC1966
V
DD
V
DD
V
SS
V
DD
V
SS
OR GND
(9a)
C
C
IN1
V
IN
IN2
2
3
LTC1966
(9b)
C
C
R1
100k
R2
100k
IN1
V
IN
V
DD
IN2
2
3
LTC1966
(9c)
C
C
IN1
V
IN
1966 F09
V
DC
IN2
2
3
