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LTC1968

1968f

the lowpass filter. The input to the LPF is the calculation

from the multiplier/divider; (VIN)2/VOUT. The lowpass

filter will take the average of this to create the output,

mathematically:

APPLICATIO S I FOR ATIO

currents. The power delivered to the load depends on the

firing angle, as well as any parasitic losses such as switch

“ON” voltage drop. Real circuit waveforms will also typi-

cally have significant ringing at the switching transition,

dependent on exact circuit parasitics. For the purposes of

this data sheet, “SCR Waveforms” refers to the ideal

chopped sine wave, though the LTC1968 will do faithful

RMS-to-DC conversion with real SCR waveforms as well.

The case shown is for Θ = 90°, which corresponds to 50%

of available power being delivered to the load. As noted in

Table 1, when Θ = 114°, only 25% of the available power

is being delivered to the load and the power drops quickly

as Θ approaches 180°.

With an average rectification scheme and the typical

calibration to compensate for errors with sine waves, the

RMS level of an input sine wave is properly reported; it is

only with a non-sinusoidal waveform that errors occur.

Because of this calibration, and the output reading in

VRMS, the term True-RMS got coined to denote the use of

an actual RMS-to-DC converter as opposed to a calibrated

average rectifier.

CONTROL

VLOAD

MAINS

VLINE

VTHY

1968 F02a

–

ILOAD

VLINE

VLOAD

VTHY

ILOAD

1968 F02b

Figure 2a

Figure 2b

How an RMS-to-DC Converter Works

Monolithic RMS-to-DC converters use an implicit compu-

tation to calculate the RMS value of an input signal. The

fundamental building block is an analog multiply/divide

used as shown in Figure 3. Analysis of this topology is

easy and starts by identifying the inputs and the output of

and

RMS V

OUT

()

⎛

⎝

⎜

⎞

⎠

⎟

()

⎛

⎝

⎜

⎞

⎠

⎟

()

⎛

⎝

⎞

⎠

()

⎛

⎝

⎞

⎠

() = ()

() =

()

Because V

is DC,

OUT

Figure 3. RMS-to-DC Converter with Implicit Computation

VIN

VOUT

1968 F03

LPF

OUT

()

Unlike the prior generation RMS-to-DC converters, the

LTC1968 computation does NOT use log/antilog circuits,

which have all the same problems, and more, of log/

antilog multipliers/dividers, i.e., linearity is poor, the band-

width changes with the signal amplitude and the gain drifts

with temperature.

How the LTC1968 RMS-to-DC Converter Works

The LTC1968 uses a completely new topology for RMS-to-

DC conversion, in which a ∆Σ modulator acts as the

divider, and a simple polarity switch is used as the multi-

plier1 as shown in Figure 4.

1Protected by multiple patents.

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