Question
I just got a question from our customer regarding to the application of AD736.
They are designing a data acquisition card in which the RMS to DC part is used
to measure the rms value. The power supply for AD736 is +/-15V, the input
voltage range is within 5V and the signal frequency is less than 1kHz.
They would like to know how to choose the proper values for the external
components such as Cav Cc and Cf. Is there any equation to follow?
Answer
There is no equation, just the table provided in your email. The averaging
capacitor is connected to a transistor base, and thus dependent on signal
level. Furthermore, the averaging error is inversely proportional to settling
time. I would recommend an empirical approach using an eval board, or I can
recommend a value if the customer could provide a desired minimum and maximum
input level, minimum frequency and maximum rms error. The capacitor value (thus
rms error) matters most at the lower desired operating frequency, and minimum
input amplitude. Figure 16 helps somewhat when estimating the settling times,
but it isn’t accurate because the settling times for rising and falling
transitions are unequal. Falling is about 2x rising.The equation on page 14 is
useful for the low frequency limit. The input at pin1 is 8k +/-20%, the input
at pin 2 is 1012Ω, nearly negligible but a gate-leak resistor may be required
if ac coupling is used, then the resistor value along with the minimum
frequency establishes the capacitor value. In either case, the capacitor value
effect is simply a first order series drop error.One can calculate the 3dB
frequency using 8k and minimum desired frequency to calculate capacitor value.
Bear in mind this filter affects the ripple frequency, which is 2X the minimum
desired frequency.