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LTC1966

sn1966 1966fas

prior generation log/antilog RMS-to-DC converters, whose

averaging time constants are dependent on the signal

level, resulting in excruciatingly long waits for the output

to go to zero.

The shape of the rising and falling edges will be dependent

on the total percent change in the step, but for less than the

100% changes shown in Figure 11, the responses will be

less distorted and more like a standard exponential decay.

For example, when the input amplitude is changed from

100mV to 110mV (+10%) and back (–10%), the step

responses are essentially the same as a standard expo-

nential rise and decay between those two levels. In such

cases, the time constant of the decay will be in between

that of the rising edge and falling edge cases of Figure 11.

Therefore, the worst case is the falling edge response as

it goes to zero, and it can be used as a design guide.

APPLICATIO S I FOR ATIO

Figure 12 shows the settling accuracy vs settling time for

a variety of averaging capacitor values. If the capacitor

value previously selected (based on error requirements)

gives an acceptable settling time, your design is done.

But with 100

µF, 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 100

µF 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 100

µF 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.

Figure 11a. LTC1966 Rising Edge with CAVE = 1µF

Figure 11b. LTC1966 Falling Edge with CAVE = 1µF

TIME (SEC)

100

120

0.1

0.2

0.3

0.4

1966 F11a

0.5

CAVE = 1µF

TIME (SEC)

100

120

0.2

0.4

0.6

0.8

1966 F11b

CAVE = 1µF

SETTLING TIME (SEC)

0.01

0.1

110

0.1

100

1966 F12

C = 100

µF

C = 47

µF

C = 22

µF

C = 10

µF

C = 4.7

µF

C = 2.2

µF

C = 0.47

µF

C = 0.22

µF

C = 0.1

µF

C = 1

µF

Figure 12. LTC1966 Settling Time with One Cap Averaging

LTC1966 Datasheet(PDF) 17 Page - Linear Technology

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