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LTC1968

1968f

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 12 shows a basic 2nd order post filter, for a net 3rd

order filtering of the LTC1968 RMS calculation. It uses the

12.5kΩ output impedance of the LTC1968 as the first re-

sistor of a 3rd order Sallen-Key active-RC filter. This topol-

ogy features a buffered output, which can be desirable

depending on the application. However, there are disad-

vantages to this topology, the first of which is that the op

amp input voltage and current errors directly degrade the

effective LTC1968 VOOS. The table inset in Figure 12 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 LTC1968 output, includ-

ing ground, which in single supply applications is the nega-

tive supply. Although the LTC1968 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 LTC1968 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

APPLICATIO S I FOR ATIO

concern. To do this, tie all three ground symbols shown in

Figure 12 to the signal reference, as well as to the differ-

ential return for the circuitry that follows.

Figure 13 shows an alternative 2nd order post filter, for a

net 3rd order filtering of the LTC1968 RMS calculation. It

also uses the 12.5kΩ output impedance of the LTC1968 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 LTC1968 is increased from 12.5kΩ

to 41.9kΩ, 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 12 and 13, are

optimized for additional filtering with clean step re-

sponses. The 12.5kΩ output impedance of the LTC1968

working into a 10µF capacitor forms a 1st order LPF with

a –3dB frequency of ~1.27Hz. The two filters have 10µF

at the LTC1968 output for easy comparison with a

10µF-only case, and both have the same relative Bessel-

like shape. However, because of the topological differ-

ences of pole placements between the various compo-

nents within the two filters, the net effective bandwidth

for Figure 12 is slightly higher (≈1.2 • 1.27 ≈ 1.52Hz) than

with 10µF alone, while the bandwidth for Figure 13 is

Figure 13. DC Accurate Post Filter

Figure 12. Buffered Post Filter

LTC1968

CAVE

10µF

5.6k

24.9k

1µF

10µF

LT1077

1968 F12

OP AMP

LTC1968 VOOS

VIOS

IB/OS • R

TOTAL OFFSET

RB VALUE

ISQ

LT1494

±375µV

±11µV

±1.1mV

43k

1µA

LT1880

±150µV

±48µV

±940µV

SHORT

1.2mA

LT1077

±60µV

±48µV

±858µV

43k

48µA

LTC2054

±3µV

±13µV

±766µV

SHORT

150µA

±750µV

LTC1968

CAVE

10µF

OTHER

REF VOLTAGE,

SEE TEXT

29.4k

100k

2.2µF

LT1782

1068 F13

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