Datasheet
18
LTC1966
sn1966 1966fas
APPLICATIO S I FOR ATIO
WUUU
Reducing Ripple with a Post Filter
The output ripple is always much larger than the DC error,
so filtering out the ripple can reduce the peak error
substantially, without the large settling time penalty of
simply increasing the averaging capacitor.
Figure 13 shows a basic 2nd order post filter, for a net 3rd
order filtering of the LTC1966 RMS calculation. It uses the
85kΩ output impedance of the LTC1966 as the first resis-
tor of a 3rd order Sallen-Key active-RC filter. This topology
features a buffered output, which can be desirable de-
pending on the application. However, there are disadvan-
tages to this topology, the first of which is that the op amp
input voltage and current errors directly degrade the effec-
tive LTC1966 V
OOS
. The table inset in Figure 13 shows
these errors for four of Linear Technology’s op amps.
A second disadvantage is that the op amp output has to
operate over the same range as the LTC1966 output, includ-
ing ground, which in single supply applications is the nega-
tive supply. Although the LTC1966 output will function fine
just millivolts from the rail, most op amp output stages (and
even some input stages) will not. There are at least two ways
to address this. First of all, the op amp can be operated split
supply if a negative supply is available. Just the op amp
would need to do so; the LTC1966 can remain single sup-
ply. A second way to address this issue is to create a signal
reference voltage a half volt or so above ground. This is most
attractive when the circuitry that follows has a differential
input, so that the tolerance of the signal reference is not a
concern. To do this, tie all three ground symbols shown in
Figure 13 to the signal reference, as well as to the differ-
ential return for the circuitry that follows.
Figure 14 shows an alternative 2nd order post filter, for a
net 3rd order filtering of the LTC1966 RMS calculation. It
also uses the 85kΩ output impedance of the LTC1966 as
the first resistor of a 3rd order active-RC filter, but this
topology filters without buffering so that the op amp DC
error characteristics do not affect the output. Although the
output impedance of the LTC1966 is increased from 85kΩ
to 285kΩ, this is not an issue with an extremely high input
impedance load, such as a dual-slope integrating ADC like
the ICL7106. And it allows a generic op amp to be used,
such as the SOT-23 one shown. Furthermore, it easily
works on a single supply rail by tying the noninverting
input of the op amp to a low noise reference as optionally
shown. This reference will not change the DC voltage at the
circuit output, although it does become the AC ground for
the filter, thus the (relatively) low noise requirement.
Step Responses with a Post Filter
B
oth of the post filters, shown in Figures 13 and 14, are
optimized for additional filtering with clean step re-
sponses. The 85kΩ output impedance of the LTC1966
working into a 1µF capacitor forms a 1st order LPF with
a –3dB frequency of ~1.8Hz. The two filters have 1µF at
the LTC1966 output for easy comparison with a 1µF-only
case, and both have the same relative Bessel-like shape.
However, because of the topological differences of pole
placements between the various components within the
two filters, the net effective bandwidth for Figure 13 is
slightly higher (≈1.2 • 1.8 ≈ 2.1Hz) than with 1µF alone,
while the bandwidth for Figure 14 is somewhat lower
Figure 14. DC Accurate Post Filter
LTC1966 C
AVE
1µF
5
6
OTHER
REF VOLTAGE,
SEE TEXT
R1
200k
–
+
R2
681k
C1
0.22µF
C2
0.22µF
LT1782
1066 F14
Figure 13. Buffered Post Filter
LTC1966 C
AVE
1µF
5
6
R1
38.3k
–
+
R2
169k
R
B
C2
0.1µF
C1
1µF
LT1880
1966 F13
OP AMP
LTC1966 V
OOS
V
IOS
I
B/OS
• R
TOTAL OFFSET
R
B
VALUE
I
SQ
LT1494
±375µV
±73µV
±648µV
294k
1µA
LT1880
±150µV
±329µV
±679µV
SHORT
1.2mA
LT1077
±60µV
±329µV
±589µV
294k
48µA
LT2050
±3µV
±27µV
±230µV
SHORT
750µA
±200µV