09/07/08

9

ES7

True RMS-to-DC Converters

Choosing the Averaging Time Constant

The ES7 computes the RMS value of AC and DC signals. At low frequencies and DC,

the output tracks the input exactly; at higher frequencies, the average output approaches

the RMS value of the input signal. The actual output differs from the ideal by an

average (or DC) error plus some amount of ripple.

factors, such as a pulse train with low duty cycle, should have an average time constant

chosen to be at least ten times the signal period.

the RMS converter to settle to within a given percent of the change in RMS level, is set

by the averaging time constant, which varies approximately 2:1 between decreasing and

increasing input signals. In addition, the settling time also varies with input signal

levels, increasing as the input signal is reduced, and decreasing as the input is

increased.

Frequency Response

ES7 utilizes a logarithmic circuit in performing the RMS computation of the input

signal. Table 1 represents the simplified frequency response of the converters from

35mV to 400mV for ES7. Caution must be used when designing RMS measuring

systems so that overload does not occur. The input clipping level for ES7 is ±10V.

Error

RMS

±1%

±3db

35mV

50KHz

1MHz

100mV

200KHz

1MHz

200mV

200KHz

1MHz

400mV

200KHz

500KHZ

Table 1

4