LTC1966CMS8#TR Linear Technology, LTC1966CMS8#TR Datasheet - Page 18

• Manufacturer Part Number: LTC1966CMS8#TR
• Description: IC PREC RMS/DC CONV MCRPWR 8MSOP
• Manufacturer: Linear Technology

Specifications of LTC1966CMS8#TR

• Current - Supply: 155µA
• Voltage - Supply: 2.7 V ~ 5.5 V
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: LTC1966CMS8TR

APPLICATIO S I FOR ATIO
LTC1966
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
85k 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
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
18
LTC1966
5
6
LTC1966 V
TOTAL OFFSET
R
OP AMP
I
B/OS
B
Figure 13. Buffered Post Filter
OOS
V
VALUE
I
IOS
SQ
• R
U
. The table inset in Figure 13 shows
OOS
38.3k
C
1 F
R1
AVE
LT1494
375 V
648 V
294k
1 A
73 V
U
169k
R2
1 F
C1
LT1880
SHORT
1.2mA
150 V
329 V
679 V
200 V
C2
0.1 F
W
LT1077
329 V
589 V
48 A
294k
60 V
–
+
LT1880
LT2050
SHORT
750 A
230 V
27 V
3 V
U
R
B
1966 F13
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 85k 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 85k
to 285k , 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
Both of the post filters, shown in Figures 13 and 14, are
optimized for additional filtering with clean step re-
sponses. The 85k output impedance of the LTC1966
working into a 1 F capacitor forms a 1st order LPF with
a –3dB frequency of ~1.8Hz. The two filters have 1 F at
the LTC1966 output for easy comparison with a 1 F-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 1 F alone,
while the bandwidth for Figure 14 is somewhat lower
LTC1966
Figure 14. DC Accurate Post Filter
5
6
REF VOLTAGE,
SEE TEXT
OTHER
C
1 F
200k
R1
AVE
C1
0.22 F
–
+
LT1782
681k
R2
sn1966 1966fas
C2
0.22 F
1066 F14