LTC1966CMS8 datasheet(18/32 Pages) LINER

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LTC1966

sn1966 1966fas

APPLICATIO S I FOR ATIO

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

Ω 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 VOOS. 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 85k

Ω 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

Ω

to 285k

Ω, 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

Ω output impedance of the LTC1966

working into a 1

µF capacitor forms a 1st order LPF with

a –3dB frequency of ~1.8Hz. The two filters have 1

µF at

the LTC1966 output for easy comparison with a 1

µF-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

CAVE

µF

OTHER

REF VOLTAGE,

SEE TEXT

200k

681k

0.22

µF

0.22

µF

LT1782

1066 F14

Figure 13. Buffered Post Filter

LTC1966

CAVE

µF

38.3k

169k

0.1

µF

LT1880

1966 F13

OP AMP

LTC1966 VOOS

VIOS

IB/OS • R

TOTAL OFFSET

RB VALUE

ISQ

LT1494

±375µV

±73µV

±648µV

294k

µA

LT1880

±150µV

±329µV

±679µV

SHORT

1.2mA

LT1077

±60µV

±329µV

±589µV

294k

µA

LT2050

±3µV

±27µV

±230µV

SHORT

750

µA

±200µV

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