LT1175-5 データシートの表示（PDF） - Linear Technology
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LT1175-5 Datasheet PDF : 38 Pages
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LTC1966
Applications Information
But with 100µF, the settling time to even 10% is a full 38
seconds, which is a long time to wait. What can be done
about such a design? If the reason for choosing 100µF is
to keep the DC error with a 75mHz input less than 0.1%,
the answer is: not much. The settling time to 1% of 76
seconds is just 5.7 cycles of this extremely low frequency.
Averaging very low frequency signals takes a long time.
However, if the reason for choosing 100µF is to keep the
peak error with a 10Hz input less than 0.05%, there is
another way to achieve that result with a much improved
settling time.
Reducing Ripple with a Post Filter
The output ripple is always much larger than the DC er-
ror, 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 resistor
of a 3rd order Sallen-Key active RC filter. This topology
features a buffered output, which can be desirable depend-
ing on the application. However, there are disadvantages
to this topology, the first of which is that the op amp input
voltage and current errors directly degrade the effective
LTC1966 VOOS. The table inset in Figure 13 shows these
errors for four of Linear Technology’s op amps.
5
LTC1966 6
R1
38.3k
CAVE
1µF
C1
1µF
–
RB
R2
LT1880
169k
+
C2
0.1µF
OP AMP
LTC1966 VOOS
VIOS
IB/OS • R
TOTAL OFFSET
RB VALUE
ISQ
LT1494
±375µV
±73µV
±648µV
294k
1µA
LT1880 LT1077
±200µV
±150µV ±60µV
±329µV ±329µV
±679µV ±589µV
SHORT 294k
1.2mA 48µA
LT2050
±3µV
±27µV
±230µV
SHORT
750µA
1966 F13
Figure 13. Buffered Post Filter
18
A second disadvantage is that the op amp output has
to operate over the same range as the LTC1966 output,
including ground, which in single supply applications is
the negative 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 supply. 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 differential 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.
5
LTC1966 6
R1
200k
CAVE
1µF
OTHER
REF VOLTAGE,
SEE TEXT
C1
R2
0.22µF 681k
–
LT1782
+
C2
0.22µF
Figure 14. DC Accurate Post Filter
1066 F14
1966fb
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